CHAPTER 1
Dermotomes, Slerotomes, and
Planes
dermatomes, sclerotomes, and planes
Anesthesiologists and algologists must understand the somatic and visceral pathways of the body in order to perform efficient and accurate local anesthetic injections. There is considerable variation in the pathways of both visceral and somatic nerves. Nonetheless, it is useful for the practitioner to have some idea of these pathways. The sensory innervation of the skin is called dermatomes (Fig. 1.1). The sensory
innervation of the bones is called sclerotomes (Fig. 1.2). As a rule, if a nerve
crosses a joint, it usually provides innervation to the capsule and bones of that joint. The sensory innervation of the muscles is called myotomes. In general, the sensory
innervation of the muscles is the same as the motor innervation of the muscles. In most cases, ultrasound for nerve block provides the practitioner a two-dimensional image of the region of interest. In many cases, these cross sections can be described with traditional terminology (Fig. 1.3).
Figure 1 Legend Dermatomes
Figure 2 Legend
Sclerotomes
Figure 3 Legend
Cross-sectional
taxonomy
CHAPTER 2
Pharmacology of Pernineural
Analgesia
Pharmacology of Perineural Analgesia
Koller is credited with the use of cocaine for topical ophthalmic anesthesia (1884), Bier is credited for the use of cocaine for spinal anesthesia (1898), and Braun is
credited for the use of epinephrine as an adjuvant to procaine (1900). As with these first 20 years, there has been tremendous excitement in the past 20 years.
Specifically, novel perineural adjuvants are all off-label for perineural use
(e.g., clonidine, buprenorphine, dexamethasone). However, when combined with
local anesthetics, they appear to be safe in preclinical laboratory models tested after gaining published clinical use (an important exception is midazolam combined with local anesthetics, which significantly worsens in vitro neurotoxicity). Local
anesthetics discussed throughout this atlas (i.e., bupivacaine, ropivacaine,
levobupivacaine [outside the United States], mepivacaine, lidocaine, and
chloroprocaine) can be more extensively reviewed in current textbooks.
Physiology & Pharmacology
Local anesthetics (LA) are weak bases (non-ionized). Only the non-ionized isoform diffuses across the lipid nerve sheath of the peripheral nerve. The onset of action
depends on the amount of drug in the non-ionized form. Because of this, sodium
bicarbonate (NaHCO3) speeds block onset by increasing the LA and local tissue pH’s closer to the pKa of the LA. Conversely, acidosis or infection delays block onset
by decreasing the pH of both LA and local tissue. The ionized isoform acts
intracellularly to block voltage-gated sodium (NaV) channels, thereby inhibiting
impulse conduction and slowing depolarization. The transmembrane electrical
threshold potential is not reached. Therefore, the action potential is not propagated (Fig. 2.1).
In perineural use, LA’s bind NaV channels in the axolemma, preventing NaV
channels from opening, and thus preventing action potentials. Different NaV
channel types have different affinities for La’s, and this becomes important when considering local anesthetic systemic toxicity. Specifically, cardiac NaV channel
subtype 1.5 has remarkable affinity for bupivacaine, and this accounts for the
potential for cardiac arrest with unwanted intravascular injection or absorption of
bupivacaine. Peripheral nerves (NaV 1.7, 1.8, 1.9), dorsal root ganglia (NaV 1.7), and skeletal muscle (NaV 1.4) do not have significant, if any, presence of the NaV 1.5 channel.
It should be clearly understood that the conduction block mechanism of LA (at NaV channels) is not likely the only analgesic mechanism associated with LA. Likewise, LA activity at NaV channels does not contribute to the neurotoxic (i.e., nerve
damage) risks of LA. Nerve damage risks can occur at generally-accepted clinical
doses, via mechanisms unrelated to NaV channel activity. Axonal damage from LA can occur (i) by intrafascicular injection (which is theoretically preventable with
ultrasound-guided regional anesthesia), (ii) with LA in high concentrations, or (iii) in situations of prolonged exposure (such as with a perineural catheter). More
relevant is the potential for disruption of numerous cellular functions. Intracellular calcium levels may play a central role. Specifically, there is disruption of
cytoplasmic calcium signaling, leading to elevations of cytoplasmic calcium, cell membrane calcium influx, and release of calcium from intracellular stores. Excess cytoplasmic calcium leads to neuronal death from activation of kinases and altered energy metabolism. Apoptosis (programmed cell death) is closely linked to these
intracellular calcium alterations. It is critical to acknowledge that there is no
evidence that ultrasound-guided regional anesthesia protects patients from
potentially toxic alterations of intracellular calcium after peripheral nerve block.
The mechanisms of action for clonidine, buprenorphine, dexamethasone, and
midazolam have not been as well-described. Research is needed to determine the
extent to which perineural adjuvants can reduce the overall dose, volume, and
concentration of the LA. Reducing total LA dose should be a primary objective in our subspecialty’s clinical and research agenda, due to the known neurotoxicityof LA.
Clonidine-mediated perineural analgesia is not via agonist activity at the level of the alpha-2 adrenoceptor, but is rather mediated via the hyperpolarization-activated cation current. The same appears to be true for dexmedetomidine (based on in vivo
preclinical models). It should be noted that the alpha-2 adrenoceptor mediates
inflammatory responses of various white blood cells and appears to decrease
perineural inflammation at the LA injection site when alpha-2 agonists are
co-administered with LA.
Although not yet verified, buprenorphine is presumably active
after trans-axonal influx at axon- level opioid receptors and adjacent
axoplasmic G-protein coupling mechanisms, typical of opioid receptors .
A corticosteroid (methylprednisolone) has been described to suppress the
transmission in thin, unmyelinated C-fibers, but not in myelinated A-beta fibers. Whether this same mechanism, or other mechanisms, holds true for perineural
dexamethasone remains to be determined.
Midazolam is profoundly cytotoxic when combined with ropivacaine and
exposed to primary sensory neurons (rat) in culture. Perineural analgesia from
bupivacaine enhanced by midazolam has not been mechanistically determined, with
possibilities including peripheral benzodiazepine receptors or peripheral GABA-A receptors. The author of this atlas chapter does not express or imply the endorsement of a combination of midazolam and LA given as a peripheral nerve block to patients. Restated, the author does not recommend any patient study or off-label treatment strategy that involves the use of combined LA and midazolam for peripheral nerve blocks. In vitro combinations of clonidine, buprenorphine, dexamethasone, and
midazolam are not neurotoxic in cell culture of primary sensory neurons, but
combining midazolam and ropivacaine is profoundly and synergistically neurotoxic.
Figure 1 Legend
Any injected ionized isoform of the local anesthetic is rendered non-
ionized by tissue buffers and/or addition of NaHCO3. The non-ionized
molecule traverses the lipid bilayer membrane. Molecules are then
reprotonated based on equilibration constants, and the LA molecule
ultimately binds into voltage-gated sodium channels when these channels are in an open or inactivated state (there is less affinity of these channels for LA molecules when the channels are in a resting state).
Local Anesthetic & Adjuvant Technique
Drugs are freshly prepared, and all drugs should be preservative-free.
Readers are referred to the recommended local anesthetic (LA) doses for each block described in this atlas. Generally speaking, the adjuvant technique described in this chapter synthesizes clinical experience, clinical research to-date, and the most
relevant and recent in vitro research addressing the relative neurotoxicity of LA
versus adjuvants. Attention will be directed toward the strategic reduction of LA concentrations when perineural adjuvants are used. The multiple adjuvants
described below are based on the in vitro findings that clonidine-buprenorphine did not increase LA neurotoxicity more than did the use of LA alone. Also considered is the in vitro dose-response evidence of dexamethasone worsening LA
neurotoxicity.
The use of clonidine, buprenorphine, or both as perineural analgesic adjuvants is
unequivocally considered off-label, and all associated precautions in patient
education and documentation are suggested. Clonidine is added in doses as low as 0.25 μg/kg, or as high as 1.5 μg/kg. Patients assuming the sitting position for
surgery (e.g., after interscalene block or shoulder surgery) should be dosed at the lowest dose range in this spectrum. Doses of 1 μg/kg clonidine or greater can lead to more likely potential concerns of hypotension, bradycardia, and sedation
(especially in surgical positioning other than supine).
Buprenorphine is added in doses of 1.5-3.0 μg/kg. Doses of 2.5-3.0 μg/kg are
typically reserved for opioid-tolerant patients, and buprenorphine used
perineurally in these doses does not appear to create opioid withdrawal symptoms or reverse systemic opioid analgesia. Doses of 1.5 μg/kg are typically used in patients receiving interscalene nerve blocks. Otherwise, 2 μg/kg is a dose that can
typically avoid opioid-induced side effects (nausea, vomiting, pruritus, etc.).
Multimodal antiemetic prophylaxis, which has been recommended for both general and regional anesthesia, is especially useful when perineural buprenorphine use is planned (e.g., oral perphenazine [8 mg], or IV ondansetron- dexamethasone [4 mg each]).
In the absence of perineural dexamethasone dose studies addressing doses less than 8 mg, this author recommends that if the decision is made to study dexamethasone, or to use dexamethasone off-label in routine clinical care, the dexamethasone dose per nerve should not exceed 1-2 mg until dose-response neurotoxicity is further
elucidated. Dexamethasone in 1-2 mg perineural doses, extrapolated to cell culture and combined with ropivacaine 2.5 mg/mL, clonidine 1 μg/mL, and buprenorphine
3 μg/mL, is likely neither cytoprotective nor cytotoxic when compared to plain
ropivacaine. The use of combined LA and midazolam for peripheral nerve blocks is absolutely not recommended by this author
Summary of Evidence
For this atlas chapter, the author considers clonidine and buprenorphine as
“textbook” drugs, with dexamethasone being more controversial at the time of this writing, based on in vitro data summarized in Table 2.1.Clonidine and buprenorphine are documented to be efficacious as perineural analgesic adjuvants, based on studies involving both placebo controls and systemic controls.
There is no evidence of clonidine efficacy in perineural infusions with ropivacaine in otherwise healthy patients. There are no reports of buprenorphine and/or
dexamethasone perineural infusions, with or without local anesthetics. There are two cases reported describing the perineural combination of clonidine-buprenorphine rendering motor-sparing analgesia in the absence of LA. The emphasis hereafter will be placed on LA combined with one or more perineural adjuvants.
Dexamethasone has been reported as added to LA in perineural doses of 8 mg. This atlas does not express or imply an endorsement for this dose of dexamethasone for perineural use. In bench research, ropivacaine 2.5 mg/mL and dexamethasone in this dose range (extrapolated to cell culture) is not cytoprotective. In cell culture,
dexamethasone 66.6 μg/mL does not significantly alter ropivacaine neurotoxicity.
However, dexamethasone 133.3 μg/mL does significantly worsen ropivacaine-induced neurotoxicity (when compared with plain ropivacaine). In other words, there is an in vitro dose-response curve with respect to higher concentrations of dexamethasone worsening ropivacaine neurotoxicity. It is difficult to extrapolate drug
concentrations in cell culture to single injection perineural dosing. However, in the absence of perineural dexamethasone dose studies addressing doses less than 8 mg, this author recommends that if the decision is made to study dexamethasone, or to use dexamethasone off-label in routine clinical care, that the dose per nerve does not exceed 1-2 mg until dose-response neurotoxicity is further elucidated.
Dexamethasone in 1-2 mg perineural doses, extrapolated to cell culture, and
combined with ropivacaine 2.5 mg/mL, clonidine 1 μg/mL, and buprenorphine
3 μg/mL, is neither cytoprotective nor cytotoxic when compared to plain ropivacaine.
• Is block for surgery (i.e., no general anesthesia [GA] planned)?
• Non-diabetic: 0.5% plus additives
• Diabetic: 0.375% plus additives
Perineural adjuvants
Local anesthetic nerve block concentrations when considering adjuvants;
assumes the long- acting local anesthetic is bupivacaine or levobupivacaine
• Clonidine and buprenorphine can be considered irrespective of diabetes status
• Is block added to GA (i.e., for postoperative pain relief and to reduce intraoperative general anesthetic requirement)?
• Lower extremity (with concerns about weight-bearing)
• Non-diabetic: 0.2% bupivacaine, plus additives
• Diabetic: 0.125% bupivacaine, plus additives
• Upper extremity – if maximum duration is desired
• Non-diabetic: 0.25% - 0.375% bupivacaine, plus additives
• Diabetic: 0.125% - 0.25% bupivacaine, plus additives
• Clonidine - ≤ 75 μg, 0.25-0.50 mcg/kg total patient dose
• If two nerve blocks are planned for the same patient
(e.g., femoral-sciatic), 0.5-1.0 μg/kg total patient dose (split between blocks), for total overall dose ≤ 100 μg
• Lower dose (e.g., 0.25 mcg/kg for a single-block) should be considered for a diabetic patient with sympathetic tone dysfunction
• Buprenorphine
• If opioid-naïve – 1.5 mcg/kg (interscalene) or 2 mcg/kg (other blocks)
• If opioid-tolerant – 3 mcg/kg, max 300 mcg (single-block) or 400 mcg
(2 blocks, e.g., femoral-sciatic)
• Ensure all patients receive the following for antiemetic prophylaxis if
buprenorphine use is planned: perphenazine (preoperative oral 8 mg), ondansetron (4 mg IV), and dexamethasone (4 mg IV) (if not diabetic)
• Dexamethasone
• If complex insulin-dependent (known to be labile and commonly reporting
regular blood sugar values > 200 when “well-controlled”);skip dexamethasone
• If pills only and/or simple, well-controlled, insulin-requiring ;1 mg per nerve block
• If non-diabetic – 2 mg per nerve block, or 1 mg dexamethasone per block
(if 2 blocks, e.g., femoral-sciatic)
Table 1 Legend
Authors’ recommendations for perineural adjuvants for combined
use with local anesthetics in single injection adult nerve blocks
Liposomal Bupivacaine
The FDA recently approved a liposomal bupivacaine for postoperative analgesia by wound infiltration. The pharmacokinetics of liposomal bupivacaine cause a prolonged duration of bupivacaine release. The suspension of bupivacaine in multivesicular
liposomes results in increased drug stability, slow release of bupivacaine, increased time to reach peak plasma concentrations, and an increased elimination half-life. Contact with antiseptics (e.g., chlorhexidine or iodine) should be avoided, seeing as these may alter the lipid layer, resulting in uncontrolled local anesthetic release.
Compared to bupivacaine with or without epinephrine, intravenous injection of
liposomal bupivacaine in animal studies has demonstrated improved cardiac and CNS toxicity profiles. Patients receiving liposomal bupivacaine have not experienced
significant EKG (PR, QRS, or QTc) changes, heart rate changes, increased incidence of adverse cardiovascular events, or delayed wound and bone healing. Although
investigations have been published in animal models investigating repeated
injection, administration after other local anesthetics, and perineural injection,
liposomal bupivacaine is not clinically approved for perineural, epidural, or
intrathecal use and should not be administered with other local anesthetics.
Notably, granulomatous inflammation, a foreign body reaction documented to occur with liposomal injection, was the only pathologic change noted histologically in
adipose tissue around the brachial plexus after perineural use.
The FDA approval was largely based on two multicenter, randomized, placebo-
controlled trials. These involved patients undergoing hemorrhoidectomy or
bunionectomy and receiving surgical infiltration with saline or liposomal
bupivacaine at the end of surgery (Table 2.2). The primary outcome for both was
cumulative numeric pain score, measured as area under the curve (AUC). This was significantly lower in the liposomal bupivacaine group than in the placebo group. Opiate consumption was also significantly lower in the liposomal bupivacaine group, and patients were more likely to be opioid-free compared with the patients in the placebo group. Furthermore, in the bunionectomy study, patients in the lipososmal group experienced fewer adverse events than those in the placebo group.
Conversely, two randomized, blinded, controlled trials comparing liposomal
bupivacaine with bupivacaine HCl did not observe the same efficacy (Table 2.2). In patients undergoing mammoplasty or total knee arthroplasy, liposomal bupivacaine or bupivacaine HCl with epinephrine were infiltrated at the end of surgery. The
primary outcome in both studies was mean cumulative pain score with activity,
measured as AUC, and the scores did not differ significantly (Table 2.2). In the breast augmentation study, total opiate consumption was lower in the liposomal bupivacaine group, at 24 and 48 hours postoperatively. However, after total knee arthroplasty,
a difference was not found between the treatment groups’ mean numeric pain scores, opiate consumption, or times to return to normal activities. The lack of a significant difference was attributed to inadequate statistical power. Although a meta-analysis by Dasta et al. found liposomal bupivacaine superior to bupivacaine HCl, it included nine studies from a range of surgeries with a range of dosages, and some only had a placebo comparison.
In summary, compared to a placebo, liposomal bupivacaine appears to be a safe choice for wound infiltration, to decrease opiate consumption, and to improve postoperative analgesia. It continues to be investigated for its effectiveness as compared to
bupivacaine HCl and alternative modes of delivery.
Table 2 Legend
Summary of randomized controlled trials comparing liposomal bupivacaine to bupivacaine HCI or placebo.
CHAPTER 3
The Physics of Nerve Stimulation and Nerve Impedance
Requirements for Nerve Depolarization
Neuronal depolarization by externally applied electrical fields requires a cathodal transmembrane voltage gradient in the range of 6 to 7 mV. Hille, an expert, states, “The conductance changes apparently depend only on voltage and not on the
direction or magnitude of current flow.” An electric field strength of 100 mV/cm is required to achieve such a transmembrane gradient for neurons longer than five times their length constant (approximately 5 × 1
mm). To effect pore opening, this cathodal gradient distorts the voltage-sensing fourth region (S4) of the sodium channel membrane-spanning segments. As positive charges on the S4 region leave the neuron, an outwardly directed gating current
occurs, followed by the inwardly directed current associated with sodium entering the neuron. This gating current is not bulk ion flow associated with the voltage
gradient, but rather the movement in space of the charged moieties on the S4 region.
Coulomb’s Law
Coulomb’s Law of electrostatics defines the force between two immobile charges in space, describing neither current flow nor current effects at a distance.
This relationship states that the force (F, or electric intensity in V/m) between two static charges (q1 and q2) is inversely related to the distance (r) between the charges and the permittivity of free space (ε0). Importantly, F is not current; it is voltage per unit distance. The significant concept emerging from Coulomb’s Law is that
voltage gradients are established without need for charge movement in space, i.e. current flow. The converse is not true; for current to flow charges must move along a voltage gradient. This is a critical concept for understanding nerve stimulation.
A voltage gradient between two points in a bulk conductor of uniform composition causes current flow at right angles to the semi-circular equipotential lines of force as shown in fig. 1.5 No positive charge accumulates in a static position as in the oft depicted example of nerve depolarization in fig. 2, a mixture of electrodynamics and electrostatics. Because there are regions of differing conductivity and paths of
preferential current flow, tissue is best described as non-homogeneous and
anisotropic.
Figure 2 Legend
Depiction of standard nerve stimulation, showing negative charge
accumulation inducing an anodal surround.
Figure 1 Legend
Electric field in a bulk conductor.
Impedance
Tissue voltage/current electrical response is not just resistive as in Ohm’s Law (E=IR); it displays an impedance, comprised of resistive and capacitive components arranged in series and in parallel (RC circuit) depicted in fig. 3. Electrical impedance is a measure of opposition to time-variant electric current. The time function of
impedance is important to alternating current waveforms. Anesthesia nerve
stimulators, with duty cycles (the ratio of the pulse being ‘on’ to that of it being ‘off’) of 1:9999 at 1 Hz with a 100 μs pulse width, do not produce time-variant
waveforms. When such square current pulses are applied to a resistance, square
voltage pulses result, however, when the same current pulse is applied to an
impedance, a voltage charging curve is seen that reflects resistance and reactance. Resistance is fixed opposition to current regardless of time or waveform
considerations, but reactance varies with the waveform, i.e. it reacts to the electrical field. Impedance, resistance, and capacitive reactance are related in eq. 2,
eq. 2
where Z is impedance, f is frequency, and C is capacitance in farads.
Current, when introduced into a parallel RC circuit, initially distributes charge onto capacitive surfaces. Very shortly after current flow initiation, the repulsive action of accumulating charge on capacitive surfaces shunts current through the resistance of the RC circuit. As a progressively larger proportion of the charge flows through the resistance, the voltage differential across the circuit increases in an exponential fashion. When the current pulse is stopped, the voltage falls in an exponential
fashion, the complete curve shown in fig. 4. Direct current cannot flow through a
capacitor; when the capacitive surfaces are fully charged, the current flows
entirely through the resistive limb of the circuit determining the final voltage value. The charging portion of the curve is described by eq. 3,
eq. 3
Direct current cannot flow through a capacitor; when the capacitive surfaces are
fully charged, the current flows entirely through the resistive limb of the circuit
determining the final voltage value. The charging portion of the curve is described by eq. 3,
eq. 4
The shape of this curve, and the resultant impedance characteristics of the total
tissue electrical path, are defined by the individual impedance functions in the path, not just that of the nerve. These multiple resistances and capacitances are related in eq. 5
eq. 5
where C0 – Cn are constants and τ0 – τn are time constants (R x C) for the component impedance sources.6 The initial portion of the charging curve is determined by short time constants and has a steeper slope than the final portion which is determined by longer time constants.
Impedance cont.
There are several sources of impedance in nerve stimulation and important among these are needle factors, e.g., the variable capacitance of the insulated needle
surrounded by conductive tissue. With increasing depth of insertion, according to eq. 2, increasing needle capacitance results in lower system impedance, requiring
greater current outputs to generate adequate voltage gradients for stimulation,
alluded to by Hadzic,7 and modeled by Bashein.8 Impedance versus insertion depth is demonstrated in a saline (fig. 5) and tissue (fig. 6).9 Paresthesia current
requirements at fixed depth with variable impedance are shown in fig. 7, and
demonstrate that developed voltage is the critical factor for stimulation as current requirements varied inversely with impedance.
Total system impedance is also affected by current density at the needle tip/tissue interface. A critical current density for stainless steel, when exceeded, is associated with non-linear capacitive and resistive behaviors shown in figs 8 - 9.10 At a
current output of 0.5mA, the current density at a 22G needle tip will be 77mA/cm2; exceeding the level of non-linear behavior for capacitance and resistance of stainless steel. Thus, current output increases from a stimulator applied to a 22G needle are associated with smaller voltage changes than were the relationship linear.
Cooper’s analysis also explains the importance of the neuronal membrane time
constant for depolarization by externally applied fields shown in fig. 10.3
Spinomotor neuron time constants range from 1.0 -12 msec.11;12 From these data, maximally effective nerve stimulators should have pulse durations up to 5 – 6 msec. Aδ or C fiber related discomfort with longer pulses does not occur since pain is
coded both spatially (fiber class) and temporally (10Hz or greater frequency of
impulses reaching the CNS). Pulses of 1 – 2 Hz generated in these axons do not
result in nociception.13;14
Figure 4 Legend
Charge/discharge curve for an RC circuit with R = 2500Ω,
C = 1.0×10-6 F, and a 34 ms direct current pulse.
Figure 3 Legend
Resistor-capacitor (RC) circuit.
Figure 6 Legend
Impedance vs. depth for a 24G stimulating needle inserted in tissue and
with a 2 kHz sinusoidal waveform applied. Saphenous nerve depth at
7.5 mm by stimulation.
Figure 5 Legend
Impedance vs. depth for a 20G stimulating needle inserted in 0.5 cm
increments in saline with a very large return electrode and a 10 kHz
sinusoidal waveform applied.
Figure 7 Legend
Current requirements for paresthesia after stabilizing a 22G stimulating
needle at paresthesia depth using 0.5 mA of controlled current output.
Return electrode contact areas ranged from 0.25 cm2 to 9.5 cm2
(overlapping data at point 2 for 5 cm and 20 cm electrode separations).
Figure 8 Legend
Capacitance vs. current density.
Figure 10 Legend
Transmembrane potential gradients produced by a 100 mV/cm voltage
gradient reach 6 mV for neurons longer than one length constant
(>1 mm) at a duration of 0.5 time constants (bold lines added).
Figure 9 Legend
Resistance vs. current density.
Nerve Impedance
Neurons are low impedance structures (i.e., long, uninterrupted tubes). In this regard, neurons differ from vessels that contain a large number of cell membranes (e.g., the blood cellular constituents). Once a stimulating needle penetrates the perineurium, current flow partitions equally between intracellular and extracellular fluid over the length constant (~1 mm) of the axons. Impedance to current flow then becomes
the ohmic resistance of these fluids. This is an electrical anisotropicity, or
facilitated conduction pathway. With intraneural sampling, short axonal cell
membrane time
constants lead to higher initial developed voltages for controlled current stimulation (Fig. 3.11). The greater voltage/current ratio may be interpreted as an increased
impedance. However, such a determination performed on the ascending portion of the charging curve reflects neither resistance nor impedance, but simply an
instantaneous voltage/current sample for situations with differing effective time
constants. Time-variant waveforms (i.e., alternating current) are required for
accurate impedance determinations.
Finally, it is clear that controlled current output stimulators cannot accurately create voltage gradients with increasing needle insertion depth. This is due to variable
needle capacitance and current density effects at the needle tip. Only controlled voltage devices (e.g., radiofrequency lesioning stimulators and Grass stimulators) reliably generate the necessary voltage gradients with increasing needle depth, since falling impedance affects their current output but not their voltage. According to Hille and Figure 3.7, the magnitude of current flow is immaterial to depolarization; the voltage gradient is the important factor. Further, local voltage gradients created by controlled current devices are determined by the impedance of the total electrical path, not just of the neuronal cell membrane.
Figure 11 Legend
Intraneural and extraneural charging curves using data from Gabriel and
Prokhorov. The intraneural curve is dashed; the solid curve is extraneural .
Both curves reach their maxima at about 2.5 ms, as shown by Cory. The
dashed vertical line shows developed voltages at a pulse duration of 100 μs.
Note that at 100 μs, the intraneural developed voltage is almost twice the
extraneural voltage.
Figure 7 Legend
Current requirements for paresthesia after stabilizing a 22G stimulating
needle at paresthesia depth using 0.5 mA of controlled current output.
Return electrode contact areas ranged from 0.25 cm2 to 9.5 cm2
(overlapping data at point 2 for 5 cm and 20 cm electrode separations).
CHAPTER 4
Microanatomy of the Peripheral
Nervous System and
Stimulation Thresholds Inside
and Outside the Epineurium
Section 1: Microanatomy - Introduction
The use of ultrasound has provided anesthesiologists with a deeper understanding of the location of peripheral nerves, as well as a powerful tool to anesthetize these nerves. The wavelength of medical ultrasound used for nerve blocks ranges from
0.1-1.0 mm. In most cases, the nerves to be imaged range from 3-15 mm in diameter.
Under optimal conditions, we may, therefore, be able to “look” inside large nerves or a nerve plexus and “see” individual fascicles. Unfortunately, the resolution
provided by ultrasound does not allow us to view the perineurium, a protective sheath that surrounds the fascicles as the nerve root emerges from the lateral recess of the spinal canal and travels toward the periphery as a peripheral nerve. However, in small patients and children, we may be able to image the dura mater where it surrounds the
cauda equina and as the nerve root emerges from the lateral recess in the cervical spine. The latter may be of use when performing cervical transforaminal blocks. Some anesthesiologists may not be familiar with the ultrastructure of nerve roots and nerves and, for this reason, may not appreciate the risks involved with paravertebral nerve blocks, even with ultrasound guidance.
Catastrophic outcomes following paravertebral blocks at the cervical, thoracic, and lumbar levels have been reported, some of which were reversible cases of extensive epidural/subdural block or total spinal anesthesia. Unfortunately, other cases resulted in paraplegia, quadriplegia, or death. The explanation for these catastrophes focused on intracord injection, which complicated the blocks performed on patients under
general anesthesia. Interscalene block has also been blamed for disastrous outcomes. Benumof’s report of four cases of spinal injury after interscalene block attracted much attention and generated a fierce debate on the safety of performing blocks on patients under general anesthesia. The conclusions reached by Benumof, that blocks should not be performed under general anesthesia, were, regrettably, largely
incorrect, particularly in the case of pediatric patients. These tragic outcomes, like many others, were most likely caused by intra-root injection and had nothing to do with the fact that the patients were under general anesthesia when the blocks were placed. Benumof’s conclusions were largely based on a misunderstanding of the
microanatomy of the connective tissue framework of the nervous system. All these cases had two things in common, however: root level nerve blocks and thin,
relatively sharp needles.
Our understanding of the microanatomy of the peripheral nervous system is
already over 130 years old. Key and Retzius in 1876 (Richardson’s stain) and
Horster and Whitman in 1931 (trypan blue) studied the spread of intraneurally
injected solutions. In 1948, French repeated this work with radiopaque contrast
medium in dogs. In 1952, Moore used methylene blue-stained exocaine, and, in 1978,
Selander used radioactive local anesthetic with fluorescent dye to study the
microanatomy of the sciatic nerve in rabbits. In the interval, electron microscopy was used toconfirm what was already known.According to conventional teaching
(including many modern anatomy and anesthesia textbooks), the cerebrospinal
fluid (CSF) originates in the choroid plexus, is discharged into the cerebral
ventricles, and exits through the foramina of Luschka and Magendie. The CSF then gathers in the cisterns at the base of the brain, where it flows to the villi or
pacchionian bodies, then into the peripheral venous circulation.
In 1948, Hassin proposed a new depiction of how the CSF courses through the
circulation.
In summary:
– The CSF is the extracellular fluid of the brain and spinal cord.
– The circulation of the CSF also involves the Virchow-Robin spaces that sur round the arterioles in the brain. These spaces form the blood-brain barrier.
– Absorption of the CSF is not through the villi or pacchionian bodies only,
but also through the perineurial spaces of the cranial nerves and spinal roots.
– The CSF acts as the “lymph fluid” of the central nervous system and
carries away waste.
– There is no central force per se that drives the CSF into the circulation.
The cardiac cycle causes expansion and contraction of the brain and
spinal cord, which are encased in a rigid compartment. During systole,
the entire brain and spinal cord expand, and pressure in the CSF increases.
Following a pressure gradient, the CSF flows from the central space out
into the perineurial spaces of the cranial and spinal nerve roots.
Plexus Trunks and Spinal Roots
The distal roots and trunks of the plexuses should be seen as transitional areas where the fascicles are no longer ensheathed by the perineurium. At the trunk level, the perineurium is split into septa (Fig. 4.4). Functionally, the trunks should be
regarded as a zone between peripheral nerves with clearly defined fascicles and rigid perineuria, and the root area is where perineurial septa are joined to form the dura mater. The perineuria of peripheral nerves, therefore, are continuations of the dura mater. The axons inside roots are no longer protected by the perineurium, and the tissue fluid inside the roots is the cerebrospinal fluid (Fig. 4.4). When Bigeleisen
injected 2 mL of India ink over 2 seconds into the C5 nerve root of a fresh cadaver, the pressure exceeded 30 psi. The tracer stained the subarachnoid space at the C5 level and traveled distally into a fascicle of the posterior cord (Fig. 4.1).
The mesothelial cells of the arachnoid membrane become hyperplastic at the point where the nerve leaves the spinal cord, and they form a cuff around the roots, just after they penetrate the dura mater. Beyond this cuff, no tissue that is recognized as arachnoid can be seen. The connective tissue framework of the peripheral nervous
system, therefore, appears to arise entirely from a continuation of the perineurium, starting at the dura mater. As the nerve progresses peripherally, it is further
subdivided by perineural septa until each fascicle eventually has its own perineural sheath.
In a series of seminal papers, Maoyeri quantified the extent of neural and
non-neural tissue in the nerve root and peripheral nerves of the brachial plexus and sciatic nerve (Figs. 4.5-6). In particular, he found that the percent of neural tissue within the nerve was greatest at the nerve root and diminished in the more distal parts of the nerve (Tables 4.1-2). This most likely gives rise to the well-known
dictum that the more distal a nerve block is placed, the safer it is the block. At the level where the supraclavicular block is performed, the deep cervical fascia is
adherent to the epineurium on the superior surface of the plexus (Fig. 4.4b). Some practitioners refer to this tissue as the plexus sheath. Puncture and injection of this sheath invariably
result in an intraneural injection. Injection outside this fascial layer rarely results in a clinical block. In contrast, the inferior surface of the plexus is covered only by the epineurium. Thus, injection deep to the plexus often results in a satisfactory block as the local anesthetic diffuses through the epineurium.
Figure 1 Legend
• A: Needle and probe position during injection.
• B: Ultrasound scan at C5 showing intraneural injection.
• C: Cadaver dissection in the infraclavicular region after injection of C5.
• D: Cross section through spine at C5 after injection of C5.
Figure 2 Legend
Cross section of peripheral nerve.
MEDIAL
LATERAL
Figure 3 Legend
Radiocontrast injection into the
fascicle of a dog sciatic nerve.
ANTERIOR
POSTERIOR
Table 1 Legend
Neural and non-neural ratios in the brachial plexus.
Figure 3 Legend
Radiocontrast injection into the
fascicle of a dog sciatic nerve.
Figure 5 Legend
Microanatomy of the brachial plexus.
- m1: anterior scalene muscle
- m2: middle scalene muscle
- a: subclavian artery
- v: subclavian vein
- L: lung
- r: rib
- c: clavicle
- ST: superior trunk
- MT/IT: combined middle and
inferior trunks
- BP: brachial plexus
- omo: omohyoid muscle
- PC: posterior cord
- LC: lateral cord
- MC: medial cord
- S: scapula
- sb: subscapularis muscle
- pma: pectoralis major muscle
- pmi: pectoralis minor muscle
- aa:
- av:
- cn:
- mc: musculocutaneous nerve
- ml: lateral root of median nerve
- mm: medial root of median nerve
- rn: radial nerve
- u: ulnar nerve
Figure 6 Legend
Microanatomy of the sciatic nerve.
- AM: adductor magnus
- AB: adductor brevis
- AL: adductor longus
- BF: biceps femoris
- BFL: biceps femoris long head
- F: femur
- FA: femoral artery
- FV: femoral vein
- G: gracilis
- GM: gluteus maximus
- GMed: gluteus medius
- IGA: inferior gluteal artery
- IGV: inferior gluteal vein
- IO: internal obturator muscle
- PA: popliteal artery
- PV: popliteal vein
- PM: piriformis
- RF: rectus femoris
- S: sartorius
- SM: semimembranosus
- SN: sciatic nerve
- ST: semitendinosus
- TL: tensor fasciae latae
- VL: vastus lateralis
- VIM: vastus intermedius
- VM: vastus medialis
- AM: adductor magnus
- AB: adductor brevis
- AL: adductor longus
- FA: femoral artery
- FV: femoral vein
- IGA: inferior gluteal artery
- IGV: inferior gluteal vein
Practical Matters & Clinical Consequences
With the recent increase in ultrasound utilization, it has become apparent to
practitioners that nerves can range from hypoechoic to hyperechoic in appearance. In most cases, the closer the nerve lies to the spine, the more likely it is to be
hypoechoic. With the insight of microanatomy, this is easy to understand. At the root level, there are mostly axons interspersed with cerebrospinal fluid, which, similar to the fluid within the axons, is hypoechoic. In the periphery, there are stroma and fat outside the perineurium, both of which are hyperechoic. The perineurium itself, which divides the nerve into fascicles, is also hyperechoic. This architecture gives rise to the honeycomb appearance of peripheral nerves.
Recent ultrasound work on axillary nerve blocks has also confirmed our clinical
impressions. Intraneural injections have no bad consequences as long as the injection is deep to the epineurium but outside the perineurium. Intrafascicular injections are difficult when blunt needles are used. Nonetheless, the work of French showed that contrast medium injected into fascicles lingered for up to five weeks. This finding makes the occasional numb finger that we sometimes encounter in our patients
several weeks after an interscalene or supraclavicular block, more comprehensible.
Injections at the root and trunk levels in some individuals should be regarded as
epidural injections because theinjection is made directly outside the dura or
peridural space. Thus, all the time-tested rules for epidural injections should also apply to root level or paravertebral injections. This includes the avoidance of thin, sharp needles. Instead, the use of large, blunt needles for continuous and single
injections should be the norm. Fractionation of the dose, with frequent aspiration for vascular and intrathecal injection, should also be performed. Perhaps it would also be wise to ollow the anticoagulation guidelines for epidural placement.
All the tragic cases previously referred to could reasonably be attributed to intra-root injections by relatively thin, sharp needles that can easily penetrate the dura. These blocks were performed with needles that should never be used for an epidural block, yet were used for a form of epidural block - the paravertebral or paraspinal block. The differences in the ultimate clinical presentations were merely a function of the volume, rate, and pressure of the injection. All or most of these tragic outcomes may have been avoided by the use of large bore Tuohy needles, test dosing, and ultrasound. Comprehension of the functional microanatomy would have gone a long way toward understanding what was happening to these unfortunate patients.
Sadly, many cases of total spinal anesthesia and permanent nerve damage continue to occur because of a lack of understanding of the ultrastructure of nerves. Because
these injuries have previously been reported in the literature, as well as because of
the risk of monetary and legal punishment to practitioners and hospitals, these
additional cases no longer reach the standard anesthesia community as case reports. It is the authors’ hope that the information in this chapter will diminish this problem.
Section 2: Nerve Stimulation Thresholds
Historically, peripheral nerve blocks were performed via the landmark technique. Based on the assumed location of nerves relative to palpable bones, muscles, and vessels, a needle was “blindly” inserted through the skin. When a paresthesia was elicited in the distribution of the nerve, the needle tip was presumed to have made contact with the nerve, and local anesthetic was injected. In contrast, if a
dysesthesia was elicited in the distribution of the nerve, penetration into the nerve was suspected. Practitioners were advised to withdraw the needle until the
dysesthesia resolved to avoid intraneural injection and nerve injury.
In the 1960’s, the first nerve stimulation-assisted nerve blocks were performed by Greenblatt and Denson and were later popularized in the 1980’s by Raj. The
technique involved “blindly” inserting an insulated electrical stimulating needle through the skin via surface landmarks until a contraction was observed in the
muscle innervated by the nerve. Motor contraction due to stimulation of the nerve at stimulation currents of 0.3-0.5 mA was proposed to suggest that the needle tip was in close proximity to the nerve and that local anesthetic administration at these
settings would induce successful regional anesthesia. Sustained motor contraction at less than 0.3 mA, however, was assumed to indicate intraneural needle placement, and clinicians were warned to withdraw the needle to avoid nerve damage.
With the advent of ultrasound and the ability to sonographically visualize the
needle-to-nerve relationship, prior theories came into question. Investigators began to study the symptomatology and motor responses of various placements of the
needle relative to the nerve. In 2006, Bigeleisen demonstrated under direct
ultrasound imaging that needle-to-nerve contact does not generate paresthesia.
Rather, it was only when the needle breached the epineurium that paresthesia
occurred. Moreover, Bigeleisen discovered that this phenomenon does not occur in all patients. In many patients, clear sonographic evidence showed that epineurium penetration did not reliably cause paresthesia, and, surprisingly, intraneural needle placement rarely caused dysesthesia in sedated patients. Investigators began studies of combined ultrasound and stimulation-guided nerve blocks of the brachial plexus and sciatic nerve and found that stimulation thresholds of 0.5 mA frequently
resulted in penetration of the epineurium. Stimulation thresholds of 0.2mA or less almost always resulted in intraneural placement of the needle.
For practitioners who wish to avoid intraneural injection, reevaluation of the
stimulation threshold is necessary for successful, yet extraneural needle placement.
Ultrasound guidance for most superficial blocks provides adequate resolution to
visualize the needle adjacent to the nerve, but it cannot determine whether the needle is within a fascicle once the epineurium has been breached. With the administration of an appropriate volume of local anesthetic and enough time for the block to set,
conditions to provide surgical anesthesia can usually be met when the needle is seen
outside the nerve. When deep blocks are performed, however, and ultrasound
imaging is less accurate, stimulation remains the technique of choice. If a surgical block is desired, sustained muscle contractions at a stimulation threshold of 0.4 mA are advised, with the understanding that many of these blocks will be
subepineurial. But if the objective is to provide analgesia, stimulation thresholds of 1.0 mA or greater are usually sufficient to provide analgesia with the likelihood of extraneural needle placement. Regardless of the intent or technique used,
intrafascicular injections, heralded by injection pressures greater than 1 atm or
significant dysesthesia on injection, should be terminated immediately, and the
needle repositioned.
CHAPTER 5
Ultrasound Equipment
Equipment
Whether blocks are conducted in the operating room, the preoperative holding area, or a formal block area, patients should have an intravenous catheter initiated,
monitors placed, and supplemental oxygen delivered prior to the block (Fig. 5.1). The induction of regional anesthesia has risks similar to those of general anesthesia. For this reason, a block cart stocked with equipment for regional block should also
contain appropriate equipment and drugs for resuscitation in the event of an
anesthetic catastrophe (Fig. 5.2). Judicious sedation and analgesia, along with a kind bedside manner, will prepare most patients for regional anesthesia but will also avoid a depth of anesthesia that precludes feedback to the anesthesiologist (Fig. 5.3). The
following ought to be present:
- Intravenous catheter and balanced salt solution
- Blood pressure monitoring
- Pulse oximetry monitoring
- Electrocardiographic monitoring (necessary for patients with cardiac disease
or dysrhythmia)
- Supplemental oxygen by face mask or cannula
- Medications for anxiolysis (e.g., midazolam, propofol, dexmedetomidine) and
analgesia (e.g., fentanyl)
- Equipment for airway management (e.g., laryngoscopes, endotracheal tubes,
bag and face mask, oral airways, laryngeal mask airways)
- Pharmaceuticals for resuscitation (e.g., propofol, epinephrine, intralipid 20%,
vasopressin, amiodarone)
The skin is prepared in a sterile fashion, and local anesthetic is injected
subcutaneously at the site at which the block needle is to be inserted. The ultrasound probe is covered with a sterile, transparent membrane for single shot blocks, and the anesthesiologist dons sterile gloves. Most authors recommend use of short-bevel needles, as it appears to be more difficult to penetrate the perineurium with this type of needle (although clinical outcome data are lacking). The authors recommend the following:
- Antiseptic for skin preparation
- Transparent, sterile membrane to cover ultrasound probe
- Lidocaine in a 3 or 5 mL syringe with a 27- or 30-gauge needle for skin anesthesia
- Short-bevel block needle (length appropriate to depth of nerve)
Figure 2 Legend
Equipment for airway management and resuscitation.
Figure 1 Legend
Equipment for monitoring, suction, and supplemental oxygen.
Figure 3 Legend
Nurse assisting with sedation and positioning.
Equipment cont.
The local anesthetic solution utilized is dependent upon the anesthesiologist’s
intent. Blocks intended for rapid onset and short duration may be conducted with mepivacaine, lidocaine, or chloroprocaine, whereas longer-acting blocks require
ropivacaine or bupivacaine. While they significantly reduce the duration of
long-acting agents, mixtures add little to speed onset of block. Although it is
axiomatic that injection must not proceed when injection pressures are high, it is not yet clear whether pressure monitoring influences the occurrence of nerve injury in the clinical setting. The following are necessary:
- Appropriate volume of desired local anesthetic solution
- Desired additives for local anesthetic (e.g., clonidine, dexamethasone,
buprenorphine, epinephrine)
- Peripheral nerve stimulator, attached to block needle and patient
- Pressure monitoring device in-line with injectate system, if desired
The choice of an ultrasound system depends upon the resources and needs of the user. The system should be portable, with high definition and a choice of probes to allow imaging of both superficial (high-frequency probe) and deep (low-frequency probe) nerves (Fig. 5.4). The following should be present:
- Portable, high-fidelity ultrasound machine
- Ultrasound gel, preferably sterile
- Ultrasound probe appropriate for block
For catheter insertion, a higher degree of sterility is necessary, since catheters
are indwelling devices with relatively high rates of colonization. This mandates
the use of sterile gowns, masks, and drapes, as well as the catheter system itself
(Fig. 5.5). The choice remains up to the individual anesthesiologist as to whether
peripheral nerve stimulation is utilized for confirmation of the target nerve, and, if so, whether a stimulating or non-stimulating catheter is preferred. The authors
suggest the following:
- Perineural needle/catheter set (stimulating or non-stimulating)
- Sterile drapes
- Sterile gown, mask, and gloves
- Sterile adhesives to hold catheter in place
- Sterile, transparent membrane to cover catheter site
- Peripheral nerve stimulator, if desired.
.
Figure 5 Legend
Equipment for sterile procedures.
Figure 4 Legend
Ultrasound platform.
CHAPTER 6
Principles of Sonography
Principles of Sonography
The frequencies of medical ultrasound range from 2 MHz to 20 MHz. The speed of sound in tissues is about 1500 m/sec. The average wave length of the ultrasound beam in this band is less than 1 mm. This limits the use of ultrasound to structures that are 1 mm in diameter or larger (Fig. 6.1). Most nerves of interest range in size from 2 mm to 10 mm. Veins and arteries of interest are typically 3 mm to 15 mm.
Many factors contribute to the quality and resolution of the ultrasound image. In general, higher frequency probes generate higher resolution images. Unfortunately, the signal strength or intensity of high frequency ultrasound waves (10 MHz to 20 MHz) is rapidly attenuated in tissue. Thus, high frequency probes are best suited for structures less than 4 cm deep from the skin. For deeper structures, probes
oscillating between 2 MHz and 5 MHz are more useful (Fig. 6.2). The speed of sound in air and tissues varies from 300 m/sec in air to 4000 m/sec in bone. In soft tissues, blood, and cystic fluid, the speed of sound is about 1500 m/sec. Most ultrasound
platforms assume an average speed for beam propagation of about 1500 m/sec.
Because the speed of sound differs slightly in different types of soft tissues, small imaging
artifacts may occur as the beam passes from one tissue type to another. This occurs most frequently when the beam passes from soft tissue through blood or cystic fluid.
The ultrasound beam may be refracted as it passes through tissue. When this occurs, a nerve or other organ may appear at a different anatomical location than its actual site. This is the same phenomenon responsible for the apparent bending of your
forearm when you place it in a bucket of water (Fig. 6.3). Notice that, in Figure 6.3b, the needle appears to be bent as it enters the fluid-filled cyst. Fat globules below the skin, in the muscle, and around nerves are about 1 mm in diameter. These globules (Fig. 6.4a) serve as scattering and diffraction sites for the incident and reflected
ultrasound beam and cause a speckled appearance in the image (Figs. 6.4bc). This is called specklation. For these reasons, obese patients can be very difficult to image (Fig. 6.5).
The image of a nerve on ultrasound is very sensitive to the angle of incidence of the beam relative to the nerve. This angle of incidence of the beam relative to the nerve is referred to as the angle of insonation. Sometimes changing the angle of insonation by only a few degrees can bring the nerve into focus. Diffraction and scattering, as well as the heterogeneous three-dimensional structure of the nerves and their
surrounding tissues, are thought to cause this phenomenon. An analogy is shown in Figure 6.6a. Here, light is shown on a three-dimensional icon. Varying the direction of the incident light beam by only a few degrees results in a radically different
shadow in any direction. To obtain the optimal ultrasound image, the image should be centered on the screen by sliding or rotating the probe on the patient’s skin.
For deep structures, compressing the tissue with pressure may improve the image. Once these maneuvers have been completed, toggling the probe will produce the best image (Fig. 6.6).
Modern platforms allow the user to adjust the brightness (also known as the gain) of the entire image or of more superficial (near field) and deep (far field) structures. Increasing the gain makes the entire image whiter. Increasing the gain too much
creates a snowy background in which all the structures become indistinguishable. In general, the gain should be set so that most of the background is black and only the
structures of interest, such as nerves and vessels, are easily seen. Many machines have an automatic gain control which adjusts the gain for the user.
Modern machines also allow the user to adjust the contrast, formally known as the dynamic range compression. Increasing the dynamic range compression makes the white images whiter and the black images blacker (Fig. 6.7). This may bring the
edges of anatomic structures into better view. Decreasing the dynamic range
compression makes everything in the image a more homogeneous grey.
All machines allow the user to adjust the depth to which the probe penetrates.
Whenever possible, the depth should be set to the shallowest setting at which all the structures of interest are imaged. Generally, the focal zone should encompass the area of greatest interest in the image. The near focal zone has the greatest resolution and is referred to as the Fresnel zone. The deeper focal zone (far field) is referred to as
the Fraunhofer zone. This zone has poorer resolution. Some machines come with a preset focal zone, but most machines allow the user to adjust the depth of the focal zone or to set multiple focal zones by adjusting the time delays between transducer elements in the probe (Fig. 6.8).
Arteries can usually be distinguished by their pulsatile nature. Veins can be
distinguished by their compressibility. Pressing on the skin with the probe will
usually cause the vein to collapse. Color flow Doppler imaging can also be used to identify and distinguish arteries and veins. Blood flowing perpendicular to the probe is colored black (Figs. 6.9ab). By convention, blood flowing toward the probe is
colored red, and blood flowing away from the probe is colored blue (Fig. 6.9c).
Velocity gates can be set to measure the flow velocity of vessels. High velocities usually signify arteries, while low velocities usually signify veins.
.
Figure 2 Legend
• A: Equation for beam attenuation as a function offrequency.
- r: tissue depth
- n: frequency
- μ: amplitude attenuation constant
• B: Beam attenuation as a function of frequency.
Figure 1 Legend
Relationship between wavelength and resolution.
Figure 4 Legend
• A: Cross section through the axilla.
- 1: axillary vein
- 2: basilic vein
- 3: wall of axillary artery
- 4: nerve fascicles
• B: Scattering from an irregular surface.
• C: Diffraction of ultrasound beam causing specklation.
Figure 3 Legend
Reflection and refraction of ultrasound beam.
A
B
C
Figure 5 Legend
Schematic and ultrasound through the axilla.
- 1: axillary artery
- 2: axillary vein
- 3: biceps muscle
- 4: coracobrachialis muscle
- 5: branch of musculocutaneous nerve
- 6: humerus
- 7: median nerve
- 8: ulnar nerve
- 9: radial nerve
Figure 6 Legend
• A: Relationship between beam direction and image.
• B-G: Different probe movements. Notice how the nerve in Figure
6Gi disappears with toggling in Figure 6Gii.
Figure 7 Legend
• A: Schematic for contrast adjustment
(dynamic range compression) of the supraclavicular fossa.
• B: Normal contrast.
• C: High contrast.
• D: Low contrast.
A
C
D
B
Figure 8 Legend
• A: Near field and far field beam dispersion and
diffraction.
• B: Use of variable time delays to change the focal zone.
• C: Supraclavicular fossa in focus.
• D: Supraclavicular fossa out of focus.
C
A
D
B
Figure 9 Legend
• A, B: Schematic and ultrasound of the infraclavicular fossa.
- 1: axillary artery
- 2: lateral cord
- 3: posterior cord
- 4: medial cord
• C: Color flow Doppler.
- Red: flow toward the probe
- Blue: flow away from the probe
B
A
C
Principles of Sonography cont.
Transducer elements can be arranged in linear or curved arrays (Fig. 6.10). Linear
arrays create rectangular images and are most useful for superficial structures. Curved arrays create wedge-shaped images and are most useful for deeper structures. When a curved array is used, the beam disperses laterally, resulting in a lower resolution than that of a linear array. In a phased array, the transducer elements are in a linear arrangement, but the elements fire in sequence, creating a phase delay between each element. The net result is a wedge-shaped image from a set of linear transducers.
Because this signal is averaged, its resolution is also lower than a standard linear array.
The array in a transducer can also be programmed to fire with multiple phase delays. This creates a simple twodimensional tomogram called a compound beam image. This signal can be processed to highlight nerves (Fig. 6.11a). Because the beam comes from many directions, the image formed is less sensitive to the angle of incidence. Thus, even though transducer alignment may not be optimal, a recognizable image may still be produced (Fig. 6.11b). The resolution in compound beam mode is
blurry when compared to the resolution in a standard linear or curved array without this type of signal averaging.
Most probes have transducers that emit the highest amplitude of their ultrasound wave at a specific fundamental frequency. Higher harmonics of this frequency are also emitted at lower a mplitudes (Fig. 6.12). By listening for the echo at these
higher harmonic frequencies, image resolution can be enhanced. Because the
harmonics have very low amplitudes, only transducers that have sufficient power output can be used for harmonic imaging. This type of image enhancement is referred to as tissue harmonic imaging. It is very useful in cardiac imaging, but it has not yet been shown to be useful in nerve imaging. Compound beam imaging negates most of the advantages of tissueharmonic imaging.
Modern probes operate in pulsed mode. The transducer elements in the probe are used as both transmitters and receivers. The transducer elements emit a short ultrasound burst and then wait for the echo before emitting another ultrasound burst. This allows the probe to be smaller than a continuous wave probe, in which there are separate
emitters and transducers (Fig. 6.13).
When a needle is inserted into tissue perpendicular to the ultrasound beam, it is a good reflector and it is easy to image. Ghosts on the deep side of the needle are caused by continued needle vibration after the ultrasound wave strikes the needle (Fig. 6.14). These reverberations return to the receiver later than the first volley of echoes.
Consequently, they are seen as deeper in the tissue. In some cases, it may be
necessary to insert the needle nearly parallel to the beam in order to reach the
targeted organ. In this case, most of the echo is lost and the needle image is much fainter (Fig. 6.15).
Above the collar bone, nerves are usually dark (hypoechoic) (Fig. 6.16). Nerves
located below the collar bone are usually white (hyperechoic) (Fig. 6.5b). The
reasons for this dichotomy are not known but are thought to be related to the depth of the nerves, the amount of fat around the nerves, and the relative amounts of fat and stroma within the nerves themselves. On ultrasound cross sections, nerves are round, hypo- or hyperechoic, reticulated structures.When imaged along their long axis, nerves appear as linear, hypo- or hyperechoic streaks. Bones are hyperechoic
(Fig. 6.5b) and usually very bright white. Arteries and veins are black unless color flow Doppler imaging is used (Fig. 6.9). However, when the transducer is
perpendicular to the blood flow, arteries and veins will still be black, even with
color flow Doppler imaging.
Most nerves have some fasciae around them. There is usually a potential space
between the fascia and the epineurium. When a needle punctures the fascia, local
anesthetic can usually be deposited between the fascia and the nerve (Fig. 6.17a). This creates a black (hypoechoic) ring around the nerve. In some cases, the fascia adheres to the epineurium or is missing. In these cases, the needle may puncture the nerve and the nerve will swell as the local anesthetic is injected (Fig. 6.17b).
Artifacts are inherent in ultrasound imaging. Typical artifacts mentioned above
include refraction, diffraction (specklation), and reverberation. Another common
artifact is called a side lobe artifact. When the beam is formed, most of the output is focused in front of the transducer, but a small amount of the beam is transmitted as side lobes (Fig. 6.18). Figure 6.18a shows a transducer in a cup of water. Figure 6.18b shows side lobes arising from multiple reflections (yellow and purple arrows in Figure 6.18a) at the corner of the cup and mirroring of the bottom (“mb” in
Figure 6.18b). Side lobes can also be reflected back to the transducer by tissue that is lateral to the main beam. These artifacts will appear in the image as if they arose from tissue in front of the transducer.
The imaging algorithm of the machine assumes that the later echoes return to the transducer, the deeper the tissues from which they arise (far field). Because the beam is attenuated as it travels into deeper tissues, these echoes are weaker, and the
resulting far field image is faded relative to the near field image. The algorithm
compensates for this by brightening the far field image. This is called time gain
compensation (Fig. 6.19). In some cases, the tissue in the near field may be a blood
vessel or cyst. When this occurs, the beam passes through the blood or water with little attenuation. When this beam strikes a strong reflector in the far field, such as
a vessel wall, its echo is minimally attenuated. This echo, which arises from the far field, is brightened by the algorithm even though it has only been minimally
attenuated by the near field blood vessel or cyst. When this occurs, the tissue deep to the cyst or vessel may appear artificially bright. In some cases, time gain
compensation may cause an artifact deep to a vessel to appear like a nerve, even though no nerve exists deep to the vessel. In Figure 6.19, the brachial plexus,
axillary artery, and axillary vein are real. The area inside the blue outline looks like nerve tissue when, in fact, it is a time gain compensated artifact. The deep vessel wall appears hyperechoic due to time gain compensation.
Shadowing occurs when a strong reflector prevents the beam from penetrating the reflector. Strong reflectors include bone, fasciae, and calcified tissues. Notice the shadowing (hypoechoic areas) deep to the transverse processes in Figure 6.20.
Mirror artifacts occur when the beam encounters strong reflectors deep to the
structure of interest (Fig. 6.21). Echoes which reverberate between the tissue of
interest and the strong reflector will return to the transducer at a later time. These later echoes are imaged as an artifactual mirror image deep to the true structure (Fig. 6.21b).
Figure 10 Legend
Transducer types.
Figure 11 Legend
Compound beam imaging (sonotomography)..
Figure 12 Legend
Frequency band in transducer.
Figure 14 Legend
Reflection from needle at perpendicular angle of insonation.
Figure 13 Legend
Signal processing modes.
A
B
Figure 15 Legend
Reflection from needle at oblique angle of insonation.
A
Figure 16 Legend
Ultrasound scan of nerve roots.
- 1: transverse process
- 2: nerve root
- R:
A
B
B
Figure 18 Legend
Ultrasound of cup holding water, showing side lobe effect.
- s: side
- b: bottom
- mb: mirror bottom
- arrows: side lobe effect
Figure 17 Legend
Ultrasound scan of musculocutaneous nerve.
- 1: local anesthetic around nerve
- 2: local anesthetic injected into nerve
A
B
Figure 19 Legend
Ultrasound of infraclavicular plexus, showing time gain artifact.
- 1: time gain artifact
- 2: axillary vein
- 3: axillary artery
- 4: brachial plexus.
Figure 21 Legend
Principle of mirroring.
- A: Subclavian artery
- B: Mirror artifact of the subclavian artery, which appears deep to
the pleura, which is a strong reflector
A
Figure 20 Legend
Ultrasound of the lumbar transverse
process.
Noticethe black shadow deep to the bone of the transverse process.
B
Principles of Sonography Technical Tips
- Stimulation with a peripheral nerve stimulator is not necessary if the
operator is certain of the nerve’s identity on ultrasound. When a stimulator
is used, it should only be used to confirm the target nerve. It can then be
turned off. Because the local anesthetic is injected around or into the
target nerve under ultrasound visualization, there is no need to “titrate”
the current to the twitch.
- It is challenging to keep the needle perfectly parallel to the long axis of
the transducer. Frequent fine adjustment of the transducer may be
necessary, along with switching the line of site of the operator from the
ultrasound screen to the site of needle insertion. Some practitioners prefer
to use a needle guide to help keep the needle aligned with the ultrasound beam.
- As local anesthetic is injected, each increment should cause visible
expansion of the tissues at the tip of the needle. This provides evidence
that the tip of the needle is not intravascular.
- If all the local anesthetic solution seems to accumulate on only one side of
the target nerve, the needle should be gently advanced or moved to
another site around the nerve in order to allow accumulation of the
solution around the entire nerve (the “halo” effect). This minimizes block
set-up time. It should be clear that each aliquot of local anesthetic
injected should cause distension of the tissues at the tip of the needle.
It is not necessary to restimulate as the needle is moved around the nerve,
but the patient should be assessed for pain in the region of the nerve.
- As with any block guided by the nerve stimulator, mild injection paresthe sia may occur during injection of local anesthetic in ultrasound-guided blocks. This should be differentiated from the severe pain which is more
likely indicative of intrafascicular injection.
- Time gain compensation may make it difficult to see the posterior cord or
radial nerve deep to the axillary artery.
- Shadowing will make it difficult to image nerves deep to bones or thick
fasciae. When this occurs, it is useful to image the nerve where the nerve
lies superficial or lateral to the bone. For example, the sciatic nerve can
be easily visualized from the lateral, posterior, or medial approach, but
not with the transducer anterior to the femur.
CHAPTER 7
Three- and Four-
Dimensional Ultrasound in
Neuraxial Anesthesia
3D/4D Image Acquisition & Display
In two-dimensional (2D) ultrasound imaging, a 3D appreciation of the anatomy is built in the sonographer’s mind by examining structures in multiple 2D views or
sections (acquired by movement of the probe). Abdominal and obstetric
low-frequency (2 to 5 MHz) curvilinear transducers are recommended for neuraxial imaging in adults. Low frequencies improve imaging of deeper structures, and curved probes provide a wider field of view. These 2D ultrasound probes typically consist of a single line of up to 256 piezoelectric crystals, which examine a 2D area and
produce a B-mode image. The two dimensions are the depth and width of the
imaging field. In contrast, static 3D ultrasound captures a volume of data that may then be digitally manipulated and displayed without movement of the probe. 3D
imaging adds an elevation dimension to the depth and width of 2D imaging in order to describe a 3D volume of data. Each data element, or voxel, represents the
echogenicity of that point in the 3D volume of tissue.
For the purposes of neuraxial ultrasound, there are two methods that 3D/4D
ultrasound systems use to examine a 3D volume.
- Mechanically steered arrays: In these systems, a standard curvilinear transducer with a single line of piezoelectric crystals is contained within a fluid-filled housing. The transducer is mechanically steered within the housing and swept over the area to be examined, building a volume of data (Fig. 7.1a). The time taken to acquire one volume of data is the time taken for the transducer to cover the sweep angle.
- 2D phased array transducers: In contrast to conventional transducers that
contain a line of piezoelectric crystals, these transducers contain a 2D array of
crystals. From this array, the ultrasound beam is electronically swept through the volume being examined, resulting in a quicker examination. These transducers
require more piezoelectric crystals. An example is the Philips X6-1 xMATRIX Array transducer, which contains 9,212 elements (Fig. 7.1b).
Both techniques produce data covering a pyramidal volume (Fig. 7.2). Realtime
3D (RT3D), or 4D ultrasound, simply refers to repeated acquisition of 3D ultrasound volumes over time, producing a real-time examination. The image resolution and the dimensions of the volume of interest (imaging depth, sector width, and sweep angle) are key determinants of the frame rate in 4D ultrasound imaging.
The data acquired during a 3D scan can be examined on the ultrasound machine or offline on a workstation. The two main post-processing techniques relevant for
displaying neuraxial images from a 3D dataset are multiplanar reconstruction (MPR) and 3D rendering.
Figure 1 Legend
• A: Image acquisition from a mechanically steered transducer.
• B: Image acquisition from an electronically steered transducer.
Figure 2 Legend
The volume of data acquired during threedimensional imaging
demonstrating standard imaging planes.
Figure 3 Legend
A three-dimensional sonogram of the right paramedian L3/4 vertebral interspace.
• A: Longitudinal paramedian image.
• B: Axial image.
• C: Coronal image.
• 3D: Three-dimensional reconstruction.
Figure 4 Legend
Line drawings of the relevant anatomy shown in Figure 7.3.
3D Rendering Techniques
There are numerous computational techniques to display 2D images of 3D datasets. The majority of 3D techniques involve ray casting, in which an imaginary ray is passed from the viewing screen through the dataset.
Surface rendering, or shaded surface display, uses algorithms to determine the
surfaces in the dataset and represent them as shaded 2D polygons arranged in 3D space. The software can then apply lighting effects to the model.
Maximal intensity projection passes rays through the dataset. The point of maximal intensity along that ray is displayed and creates x-ray-like images. This rendering technique is well suited to CT angiography, where the point of maximal intensity is the contrast-filled lumen of a vessel. Minimal intensity projection is a similar
technique except that black or empty voxels are rendered as solid, and white voxels are shown as empty. This is useful for demonstrating the shape or volume of an
echo-free structure.
Volume rendering is the most frequently used 3D ultrasound technique. The software is used to pass a virtual ray from the viewer through the 3D volume of data. When the ray passes through an echogenic voxel, the point is displayed on the screen. In vivo, surfaces are most easily recognized when surrounded by echolucent fluid. The structures best displayed by 3D ultrasound using surface rendering are heart valves surrounded by blood and fetuses surrounded by amniotic fluid. How the structures are displayed depends heavily on a variety of user-defined controls.
Acquisition and Optimization of 3D/4D Imaging of the Spine
A 3D/4D study of the spine begins with an optimal 2D image, and all the usual image optimization procedures must be followed prior to 3D/4D imaging. A poor-quality 2D image usually results in a much worse 3D image. The structure of interest should be positioned in the center of the 2D image, and the probe frequency, depth, focal zone, gain, compression, and gray scale should be optimized. Adjusting compression may improve the contrast of the image. Though less important in MPR images, the time gain compensation can be adjusted to decrease gain from nearer structures that produce near-field clutter in 3D-rendered images. Near-field clutter can also be
removed by cropping during post-processing. To reduce artifacts, techniques such as frequency compounding, spatial compounding, and tissue harmonic imaging may be employed. However, while these techniques may improve the image, they do so at the cost of frame rate.
Elevation compounding, or volume contrast imaging, is an additional option
(available with some 3D systems) that can be used to improve image quality in 2D imaging. This is a 3D technique that uses a small sweep angle (elevation) to image a limited number of adjacent 2D slices, essentially producing a thick 2D image. The software compares the adjacent slices to remove speckles and enhance the
signal-to-noise ratio.
The 3D examination can proceed by acquisition of a single 3D volume and subsequent postprocessing, real-time MPR imaging, or real-time 4D rendering. The terminology and methods of acquiring these images vary between manufacturers.
Acquisition of a Single 3D Volume
A single 3D volume can usually be acquired quickly and without much adjustment. A region-of-interest box may be used to limit the unnecessary data recorded and
reduce the time taken. Some systems allow the user to switch to an MPR mode, which displays at least two perpendicular planes. Using the two perpendicular 2D images, the three dimensions of the volume of interest can be defined (height, width, and
elevation). After the volume of interest is defined, a single 3D volume can be
acquired including all the relevant anatomy. The image shows the MPR data after
acquisition.
It is difficult to produce the reconstructed image shown here (Fig. 7.3) in the
majority of circumstances. To render structures such as boneclearly, the image must be manipulated to show only the echogenic bone and remove other tissues. Numerous controls are available in 3D volume rendering to achieve this goal and are illustrated in images. However, a high-quality image is rarely achieved because the overlying tissues are not echo-free.
Threshold refers to the minimal echo intensity required for a voxel to render as a solid point. Increasing the threshold removes less intense echoes and reveals deeper structures.
Opacity (whose inverse is transparency) is the degree to which structures close to the viewer obscure structures that are farther away. Decreased opacity (or increased transparency) allows deeper structures to be seen through more superficial
structures. High levels of opacity produce images similar to surface rendering, while low levels of opacity are useful for seeing bright, deeper structures that would
otherwise be obscured. Window width (contrast) and level (brightness) should be
optimized during 2D imaging but can be further adjusted during volume rendering and determine the intensity of each displayed pixel.
Smoothing performed by rendering algorithms can alter the average gray values
between nearby voxels to make surfaces appear smoother and to potentially improve the anatomical representation.This results in loss of detail but may make the image easier to interpret. Reducing smoothing makes surfaces appear more textured.
Even with all these adjustments, however, it is still difficult to render the vertebrae using ultrasound.
Real-Time MPR Imaging and Real-Time 4D Rendering
MPR or xPlane imaging modes are more useful in neuraxial imaging, both during the
scan-and-mark procedure and for needle insertion. Plane A is positioned as in the 2D
examination, and the position of plane B can be adjusted to give the perpendicular view. An acoustic window can be assessed in two planes at once, which is potentially useful in the thoracic spine, where the windows can be small. The two planes can be seen without changing probe position, decreasing the time taken for the examination.
Compared to 2D imaging, the frame rate is reduced with MPR imaging. The frame rate reduction is greater with mechanically steered arrays than phased array
(electronically steered) probes. The size of the region of interest is an important
determinant, as the frame rate is inversely proportional to the time taken to acquire one volume of data. The sweep angle should be minimized, particularly for
mechanically steered transducers. Decreasing the resolution of the image may
improve the frame rate, but it decreases image quality and probably needle visibility.
While very useful in echocardiography, 4D rendering has a limited role in neuraxial
imaging because of the poor-quality rendering of 3D structures surrounded by
echogenic tissues.
Real-Time 4D Needle Insertion for Neuraxial Anesthesia
The landmark-guided midline and paramedian needle techniques offer the most direct approaches to the epidural and subarachnoid spaces. Real-time, ultrasound-guided combined spinal epidural procedures were first described via an out-of-plane
technique with the needle inserted in the midline and the probe in a paramedian
position. For in-plane needle visualization, the needle may be inserted using a
paramedian approach.
We performed a feasibility study on epidural insertions in embalmed cadavers using real-time 4D ultrasound with a mechanically steered transducer. We attempted to
identify the needle using 3D rendering but found that the echogenicity of the
paraspinal musculature completely obscured the 16-gauge Tuohy needle.
The speckles from the erector spinae muscle appeared as a solid mass of tissue in 3D
rendering, and the needle could not be recognized. We found that needle visibility was best in the primary imaging plane (plane A or x) during MPR imaging. Needle visibility elsewhere was poor. Compared to 2D imaging, 4D MPR produces inferior image resolution, frame rate, and needle visibility.
As a result of this experience, we observed that the potential future use of 4D
ultrasound is likely to involve real-time MPR. With the exception of an additional perpendicular plane displayed on the ultrasound screen, the technique is very
similar to the real-time 2D paramedian in-plane technique. This technique improves the orientation of the operator, thereby offering a significant potential advantage.
The primary imaging plane is positioned over the lamina in a longitudinal
paramedian position such that the intrathecal space and posterior border of the
vertebral body are visible. The perpendicular plane (plane B or y) can then be used to medially or laterally adjust the angle of the transducer so that the probe is aimed more medially over the lamina, nearer the base of the spinous process. The needle is then
inserted in-plane under the inferior end of the probe, similarly to the 2D technique. The needle can then be passed in-plane toward the target. In half of the approaches in our study, medial or lateral adjustments were made to the needle path as a result of the information derived from the perpendicular MPR plane. However, at the time of writing, no publications have discussed the clinical use of this technique.
Challenges for 3D/4D Ultrasound in Neuraxial
Anesthesia and Potential Solutions
There are numerous challenges to be overcome prior to the use of 3D ultrasound in neuraxial anesthesia.
Image Quality and Frame Rate: Compared to 2D ultrasound, both MPR and 3D
rendering yield inferior image resolution, which adversely affects anatomical
clarity and needle visibility. Also, mechanical transducers have a much lower frame rate, as the transducer must sweep through the entire volume to generate images. Lower frame rates affect real-time needle insertion when it becomes difficult to
recognize the movement of tissues used to identify needle location. Minimizing the volume of interest and reducing the sweep angle result in higher frame rates. As
matrix transducer technology improves and it becomes possible to image in multiple planes at high frame rates, these issues of suboptimal image quality and frame rate will be overcome.
Needle Visibility and Path Length: Neuraxial blocks are relatively deep, typically 4 to 6 cm but sometimes significantly more. The 4D ultrasound probes also have
relatively large footprints. With a larger probe footprint and significant target depth,
the needle must be inserted farther from the center of the probe in order to achieve
good needle visualization using an in-plane technique, even when obesity is not a factor. As a result, the needle follows a longer paramedian path during real-time
ultrasoundguided needle insertion. Even 16-gauge Tuohy needles can prove difficult to see, and tissue movement must be relied upon to determine needle location.
Needle visibility may be improved in the future with echogenic Tuohy needles. For spinal anesthesia, a needle-through-needle combined spinal epidural technique is likely to improve needle visibility and manage the flexibility of long spinal needles. However, such difficulties may be overcome by stereotactic ultrasound guidance, where
needle location is displayed on-screen independent of needle visibility.
Challenges for 3D/4D Ultrasound in Neuraxial
Anesthesia and Potential Solutions cont.
Assistance and Identifying the Epidural Space: Performing ultrasound-guided neuraxial anesthesia involves several physical tasks, including holding the probe, inserting the needle, applying the loss of resistance technique, feeding the catheter, and controlling the ultrasound machine. It is impractical for all of these tasks to be performed by one clinician alone. The simplest solution is to have a scrubbed
assistant hold the ultrasound probe while the anesthesiologist performs the
procedure.
There are several alternative methods for identifying the epidural space that may be
suitable for ultrasound-guided techniques. The hanging drop technique may be used, as well as mechanical devices like the Episure AutoDetect LOR Syringe (Indigo Orb; Irvine, CA, USA) or the Epidrum (Exmoor Innovations; Somerset, UK). Saline
(positioned in a bag above the patient and connected to the Tuohy needle with sterile tubing) will begin to flow when the epidural space is entered, demonstrating a loss of resistance. However, there is little literature on this technique. In all the
aforementioned techniques, the primary operator watches the ultrasound screen while an assistant observes for loss of resistance. Alternatively, an assistant may perform the loss of resistance technique at a distance from the primary operator by using low compliance extension tubing, such as that used for invasive pressure monitoring. New techniques such as optical spectroscopy are also potential methods.
Challenges for 3D/4D Ultrasound in Neuraxial
Anesthesia and Potential Solutions cont.
Ultrasound Gel: The composition of ultrasound gels is often unknown but may
include low concentrations of propylene glycol or glycerol, both of which are known to have neurolytic properties at high concentrations. If ultrasound gel somehow comes into contact with the needle, it is risky to introduce the gel into the epidural or
intrathecal spaces. To avoid this, small amounts of gel should be used with
particular care to ensure that the needle does not contact it, or alternative ultrasound
conduction media may be considered, such as repeated application of saline to the skin.
Training and Certification: Guidelines are only recently being developed for
training in ultrasound-guided regional anesthesia. It has been suggested that
experience of 40 cases is required to attain competence with 2D scanning of the
lumbar spine. Neuraxial ultrasound-guided procedures are technically demanding and, as 4D ultrasound is even more cognitively and technically challenging, may
require substantially more experience. As the role of 4D ultrasound remains
undefined, the training requirements for performingprocedures are unknown.
Clinician Resistance: Landmark-guided neuraxial anesthesia techniques are well
established, and 2D ultrasound is not required for successful anesthesia in the
majority of patients.
Cost: 3D ultrasound equipment is substantially more expensive than 2D but, in time, is likely to become more readily available.
CHAPTER 8
Anatomic Anomalies of
Ultrasound
Anatomic Anomalies of Ultrasound
Ultrasound-guided regional anesthesia has provided the regional anesthesiologist with the ability to visualize pertinent nerves and local anesthetic spread. One
consequence of this development is the ability to visualize anatomic variations and
sonopathology.
These findings may impact performance of regional anesthesia and lead to an altered
anesthetic technique. There is currently some debate about the role of regional
anesthesiologists in diagnosing pathologic abnormalities while performing
ultrasound-guided nerve blocks. While this question has important medicolegal and clinical implications, delving into this issue would be beyond the scope of this
chapter. Nonetheless, practitioners generally agree that knowledge of the common anatomical variations is essential to the performance of effective and safe regional anesthesia.
In order to appreciate the anatomic variants, it is necessary to first discuss some of the common anatomic artifacts that frequently confuse novices. Then some of the
common anatomic variants may be examined. Finally, it is possible to highlight some
of the more common sonopathologic findings that may be encountered while
performing ultrasound-guided regional anesthesia.
Anatomic Artifacts
There are various anatomic artifacts that one may encounter while performing a
regional anesthetic block. Anatomic artifacts occur when tissue structures are
confused with each other. These are usually structures that appear similar to nerves. Some common examples include tendons, muscles, lymph nodes, and blood vessels.
Tendons: Tendons may be confused with nerves that have a hyperechoic appearance.
This confusion most commonly occurs while performing sciatic nerve blocks at the
popliteal fossa, or during selective nerve blocks in the arm and forearm. The normal
sonographic image of a tendon is comprised of groups of hyperechoic dots
scattered across a hypoechoic background. Nerves often have fascicles with
hypoechoic centers (Fig. 8.1a), which may also be apparent in a long axis view (Fig.
8.1b). In Figure 8.1b, notice the epineurium of the nerve and the hypoechoic
fascicles within the nerve. The primary distinction between tendons and nerves is
that tendons have hyperechoic, continuous fibrils, while nerves have fascicles.
These fascicles have hyperechoic borders (caused by the perineurium) and
hypoechoic centers (caused by the axons). Tendons may also show greater
anisotropy than nerves, showing a greater change in echogenicity when the probe
is toggled. Moreover, a tendon should originate from a muscle or be attached to a
bone. This can be seen by scanning along the length of the structure in question.
Finally, tendons should move in the image when the appropriate muscle or joint is
contracted (Figs. 8.1cdef).
Vasculature: Larger blood vessels are not usually mistaken for nerves. In fact, most
regional anesthesiologists use vascular landmarks to help identify many nerves.
Smaller blood vessels are easier to confuse with nerves, especially in the short axis
view. This problem is frequently encountered while performing brachial plexus
nerve blocks (Figs. 8.2-3). If in doubt, Color flow Doppler is invaluable in helping to distinguish small blood vessels from nerves.
Lymph nodes: Lymph nodes may be flat or oval and either hypo- or hyperechoic. Lymph nodes can be differentiated from nerves by their lack of fascicles. They are most commonly seen in axillary, femoral, sciatic, and interscalene blocks (Fig. 8.4).
Muscle: Muscle structure is either fusiform or pennate. A fusiform muscle has
parallelly oriented fascicles that converge towards the tendon at the end of the
muscle. In relation to its aponeurosis or tendon, a pennate muscle has fascicles that are more
feather-like in appearance. Muscles have a relatively hypoechoic appearance, with
some hyperechoic structures (connective tissue or fat) embedded in the muscle.
Some practitioners liken this appearance to a “starry night.” The ratio of hypoechoic
to hyperechoic muscle is based on the ratio of connective tissue to fascicles within
each muscle. In the long axis, muscles may have a feather-like appearance (pennate
muscle), and these muscles are usually well organized. Intramuscular tendons and
aponeuroses are best imaged in the short axis. It may be difficult to distinguish
muscles from nerves when performing gluteal or popliteal sciatic nerve blocks
(Fig. 8.5). Placement of local anesthetic may enable the practitioner to distinguish
between the two. Also, small muscles may have the appearance of small vessels or
nerves. Use of a nerve stimulator can usually allow the practitioner to differentiate
nerves from muscles.
Figure 1 Legend
• A: The median nerve at the wrist with hypoechoic fascicles. Following the nerve and tendon while having the patient flex his second finger allowed us to confirm the identity of the nerve.
- FCR Tendon: flexor carpi radialis
- MN: median nerve
• B: The median nerve in the long axis. Notice the epineurium and the perineurium around the hypoechoic axons.
• C: The median nerve proximal to the wrist. The fascicles are clearly seen, which
distinguishes it from the PL (palmaris longus tendon) medial to the nerve.
• D: Tracing the median nerve distally toward the wrist.
• E: The median nerve at the wrist. Note the difficulty distinguishing the nerve from the tendons surrounding it. With close examination, you can still make out the fascicles of the median nerve.
• F: A long-axis view of the median nerve and one of the tendons of the wrist. The
epineurium, seen as a bright hyperechoic line, is clearly made out. It is a little
difficult to identify the hyperechoic fascicles of the nerve, but they are clearly
distinguishable from the hyperechoic continuous fibrils seen in the tendon.
- FPL: flexor pollicis longus
- MN: median nerve
- PL: palmaris longus
- t: tendon
A
B
Figure 2 Legend
Cross section through the axilla.
- BP:brachialplexus
- SCA:subclavianartery
- SCM:sternocleidomastoidmuscle
Figure 4 Legend
After repositioning the probe, it becomes clear that the structure is the transverse cervical artery, which divides the
brachial plexus in two.
BP: brachial plexus
LN:
CA:
SCA: subclavian artery
SCM: sternocleidomastoid muscle
TCA: transverse cervical artery
- TCA:transversecervicalartery
SCM
Figure 3 Legend
Supraclavicular brachial plexus. The small structure circled in red seems to be a division of the brachial plexus. However, on closerinspection,itopensintothe subclavianartery.
- BP:brachialplexus
- SCA:subclavianartery
- SCM:sternocleidomastoidmuscle - TCA:transversecervicalartery
Figure 5 Legend
While performing sciatic blocks it may be difficult to distinguish between the nerve and the surrounding muscle. Placement of local anesthetic may enable you to
distinguish between the two.
ST: semitendinosus muscle
PA:
N: nerve
Anatomic Variation by Region
There have been numerous examples of anatomic variations that have been described while performing ultrasound-guided regional anesthesia. In one study, nearly half of the patients scanned had a variation of the expected interscalene anatomy. Awareness of the prevalence of the anatomic variation is important. It is also important to
systematically scan patients and remember the anatomic relationship of muscles, bones, vessels, and nerves.
Interscalene Brachial Plexus:
Intramuscular plexus: In this case, a part of, or the entire brachial plexus is located within one of the scalene muscles, as opposed to between them. This has been
described for both the supraclavicular and interscalene blocks (Fig. 8.6).
Supraclavicular Brachial Plexus:
Rainbow plexus: The brachial plexus is arranged in a rainbow shape around the
subclavian artery. This can be seen while performing supraclavicular blocks
(Fig. 8.7). Divided plexus: This is commonly seen while performing supraclavicular blocks. In the authors’ experience, the most common cause is the transverse cervical artery (Fig. 8.3). Another possible cause is the presence of a cervical rib.
Axillary Brachial Plexus:
Nerves or blood vessels in an unexpected distribution: This is commonly seen while performing the IS or axillary nerve blocks. There have been countless anatomical variations described for the location of the nerves while performing axillary nerve blocks (Fig. 8.8).
Femoral Nerve:
Femoral nerve lies outside the correct fascial plane.
Popliteal Sciatic Nerve:
Inability to visualize the common peroneal nerve.
Common peroneal and posterior tibial nerves join together to form the sciatic nerve on top of each other, as opposed to side-by-side (Fig. 8.9).
Figure 6 Legend
Intramuscular plexus. The brachial plexus is located in the scalene muscle.
Notice how distinguishing muscle from nerve can be difficult.
BP: brachial plexus
SM: scalene muscle
SCA: subclavian artery
SCM: sternocleidomastoid muscle
Figure 7 Legend
The brachial plexus is seen almost
surrounding the subclavian artery, similar to the shape of a rainbow. Most commonly you will only find one division of the
plexus extending above the subclavian
artery.
-BP: brachial plexus
-SCA: subclavian artery
SCM: sternocleidomastoid muscle
Figure 8 Legend
Axillary plexus with multiple branches of the axillary artery. The smaller
arterial branches may be confused for nerves.
-AA: axillary artery
-AV: axillary vein
-M: median nerve
-MC: musculocutaneous nerve
-R: radial nerve
-U: ulnar nerve
Figure 9 Legend
A: Femoral nerve lateral to the femoral artery with the appearance of a femoral nerve deep to the femoral artery.
-artery: femoral artery
-vein: femoral vein
B, C: Sciatic nerve blockade at the popliteal fossa. The common peroneal nerve joins on top of the posterior tibial nerve as opposed to lateral to it.
-CP: common peroneal nerve
-PT: posterior tibial nerve
-TN:
Sonopathology of Anatomic Structures
There are numerous pathologic findings that may be seen while performing
ultrasound-guided regional nerve blocks. These include abnormalities of the viscera,
vasculature, and nerves.
Vascular Sonopathology: Due to the prevalence of atherosclerosis in the US
population, the most common abnormality seen is atherosclerotic plaques.
Atherosclerotic plaques are usually calcified, and they are hyperechoic. A dropout shadow may be observed deep to the plaque. Plaques with high lipid contents may be echolucent (Figs. 8.10-11). The thickness of the intima and media can be used to quantify the degree of disease as well as the associated risk of stroke or
myocardial infarction. Plaques are most commonly seen in the carotid artery during interscalene blocks, or the femoral artery during femoral nerve blocks. Another
pathological finding is a deep venous thrombus (DVT). A fresh DVT has variable echogenicity, is usually non-compressible, and appears as a dilated homogeneous circle (Fig. 8.12). Chronic DVT’s may shrink and have a hyperechoic and
heterogenous appearance (Fig. 8.13). Color flow Doppler may confirm lack of blood flow.
Arterial Dissections, Aneurysms, or Pseudoaneurysms: A dissection is caused by an intimal tear leading to a false chamber or passage in the wall of the artery. In this setting, color flow Doppler usually shows a flow between the arterial lumen and the passage in the wall of the artery. This is most commonly seen in the aorta. In smaller arteries, the etiology is usually iatrogenic, due to previous arterial
cannulation. An aneurysm is the ballooning of an artery due to structural weakening of the artery. The ultrasound image will show an irregularly shaped, fusiform
dilation of the artery (Fig. 8.14). A pseudoaneurysm occurs as a result of arterial
injury leading to a hematoma in congruity with an artery. Doppler examination shows blood flow between the hematoma and the artery (Fig. 8.15).
Nerves: Ultrasound is currently being used as a tool to diagnose neuropathology, including neural tumors (Figs. 8.16abc), traumatic disruptions, neuritis, and
neuropathies (Fig. 8.17). Identification of neural pathology is difficult and usually requires scanning the nerve in long and short axis views. Enlarged fascicles and loss of the hyperechoic epineurium may signal acute neuropathy. Patients with chronic neuropathy may have enlarged nerves with thickened epineuria. A contralateral scan for comparison is necessary. When new neuropathology is seen, it is possible that the regional technique should be abandoned and the patient should be sent for a more formal examination.
Thyroid: The thyroid is imaged during stellate ganglion block and frequently during interscalene block. The normal thyroid is usually hypoechoic and has a
homogeneous ground glass appearance. Anechoic circles usually signify blood
vessels but may represent thyroid pathology. Doppler imaging can help define these. “If you see something, say something,” and refer the patient for more definitive
testing (Figs. 8.18-19).
Figure 10 Legend
Short-axis view of the carotid artery
showing a large hyperechoic plaque.
- CA:carotidartery
- IJ:internaljugularvein
Figure 12 Legend
Acute deep vein thrombosis of the right vein. Even with significant pressure the vein was incompressible and enlarged.
- AA:axillaryartery
- AV:axillaryvein
- BP:brachialplexus
- PM:pectoralismajormuscle - - PMI:pectoralisminormuscle
Figure 11 Legend
Long-axis view of the carotid artery.
Note how the plaque completely
obstructs flow through the carotid and
how the lumen of the carotid appears to be occluded secondary to atherosclerosis.
Figure 14 Legend
An aneurysm of the popliteal artery.
Note how the artery appears elongated
and nonuniform in shape.
Figure 13 Legend
• A: Incompressible left internal jugular
vein in a patient with a chronic deep vein thrombosis. Maximal force was
applied, and the vein was still incompressible. The vein also has the classic
heterogeneous appearance of a chronic deep vein thrombosis.
- CA: carotid artery
- IJ: internal jugular vein
- SCM: sternocleidomastoid muscle
• B: The right internal jugular vein of the patient. Note how the right internal jugular vein is of normal size and anechoic appearance.
Figure 15 Legend
• A:Apseudoaneurysmoftherightpoplitealartery.The characteristic
to-and-fro between the artery and the pseudoaneurysm is seen in this image.
• B:Athrombosis.
Figure 16 Legend
A: Long-axis view of the ulnar nerve in the forearm proximal to the schwannoma. The epineurium and intact nerve fascicles are easily identifiable.
B: Long-axis view of the schwannoma (1) in the ulnar nerve. Note the compression and distortion of the nerve fascicles and the increased diameter of the nerve.
C: A coronal T2 view of the schwannoma. Note how closely this
correlates with the sonographic images.
B
A
Figure 18 Legend
Normal thyroid tissue as seen in the short axis. This is similar to the view that you would obtain while performing a stellate ganglion block.
CA: carotid artery
IJ: internal jugular vein
Figure 17 Legend
The sciatic nerve just proximal to the popliteal fossa.
A: The normal sonographic appearance of the sciatic nerve.
B: The sciatic nerve in a patient with diabetic neuropathy.
Note the increased overall size of the nerve and the presence of large and irregular hypoechoic fascicles.
A
Figure 19 Legend
A large, heterogeneous hypoechoic mass of the left lobe of the thyroid. This mass was discovered incidentally and was
causing significant right tracheal
deviation.
CA: carotid artery
B
CHAPTER 9
Ultrasound Simulator Training
Ultrasound Simulator Training
Developing competence in ultrasonography and applying the results to clinical care is a complex process. It requires psychomotor skills and aptitude for optimal image window acquisition and interpretation. Once an optimal image window is acquired and correctly interpreted, the information needs to be correctly applied to patient care. The opportunity cost of training health care providers on ultrasonography is extremely high. Optimal training requires (1) a qualified instructor; (2) trainees; (3) an ultrasound machine; and (4) patients with a variety of anatomic norms, variations, and abnormalities. All of these elements must come together in one physical space and time, and the process must repeat with a new patient presenting these normal and variable conditions over an extended period of time. Because some clinical
presentations are rarely encountered, it could take months to years before a care
provider is able to competently scan a sufficient number of patients with certain conditions. The inability to train on sufficient numbers of variant cases is a
recognized impediment to ultrasound competency.
Many of the currently used training methods have significant limitations.
Traditionally, these consisted of clinical bedside teaching and attending hands-on training courses. Simple phantom models were developed quickly, along with didactic videos available on the Internet and from commercial vendors. These were followed by interactive virtual reality simulators and then high-fidelity ultrasound simulator workstations. Each of these training devices has a different cost-benefit ratio.
Examples of simple phantoms, virtual reality simulators, and high-fidelity
simulators are described within the chapter.
Training Devices
High-fidelity Ultrasound Simulators: High-fidelity simulators require relatively expensive, large modules that may require the user to visit a simulation center. Most devices with good haptic feedback cost between $30,000 and $100,000.
Interactive Virtual Reality Simulators: Interactive virtual reality simulators
usually employ a portable computer and some type of joystick or artificial ultrasound probe that allows the user to perform an ultrasound exam on a virtual human. Costs begin at about $5,000. Most of these simulators do not allow the user to perform
realistic procedures on the virtual phantom, but they can provide good image
acquisition skills. The user will have to receive other types of training to learn to image the needle and place it next to the nerve. The SonoSim Ultrasound Training Solution combines the features of a fully interactive ultrasound simulator with the scalability of online learning and assessment (Figs. 9.1-2). The simulator has the
core technology to capture scanned images with corresponding transducer movements in three-dimensional space during original data capture, and embed the acquired datasets into a laptop-based training interface. Additionally, it contains a cognitive task-training feature that enables ultrasound-guided needle-based procedure training (e.g., regional nerve blocks).
Physical Mannequins: Blue Phantom and MiniSim mannequins are commercially available at prices ranging from $700 to $10,000. These phantoms provide the user an introduction to standard nerve blocks. These simulators are most useful for novices learning elementary probe manipulation and scanning, as well as needle imaging and guidance. An example of a femoral nerve mannequin is shown in Fig. 9.3. Although the ultrasound images that can be generated are not very realistic, they teach the relative locations of bones, vessels, and nerves (Fig. 9.4).
Figure 3 Legend
The yellow strip indicates the
approximate location of the femoral nerve, and medial to this lie the artery and vein, respectively. The black screen signals the user with a flashing light and a tone when the user touches the virtual nerve with an electrostimulation needle. The two ports at the caudal end of the simulator allow the user to attach a
syringe to the femoral artery and vein. A separate user can cause these vessels to pulse while the trainee scans the nerves and vessels. An example of the ultrasound image is shown in Figure 9.4.
Figure 1 Legend
SonoSim demonstrating the use of a
virtual probe with underlying structures and the associated ultrasound.
Figure 2 Legend
SonoSim demonstrating a virtual probe on the skin and the associated
ultrasound.
Figure 4 Legend
Ultrasound image of the LifeTech mannequin in
Figure 9.3.
Training Method
Volumetric Description of Virtual Patient: Popular sources include computed
tomography (CT), magnetic resonance imaging (MRI), and even computer-aided
design. Our data come from the Visible Human dataset. In November 1994 the Center for Human Simulation (CHS) at the University of Colorado Medical School delivered the Visible Human Male (VHM) to the National Library of Medicine (NLM) as part of a contract under the Visible Human Project. In November 1995 it delivered the Visible Human Female. The Visible Humans represent complete submillimeter visual anatomic descriptions of a male and female. The voxel (volume element) resolution of the male is 1/3 millimeter in x and y (orthogonal coordinates in the axial plane) and 1 millimeter in the axial direction. The female has the same x and y resolution with three times the axial resolution, giving her 1/3 millimeter voxels. The full-color voxel anatomy data are supplemented with full-body CT and MRI images. The CHS has since cut multiple specimens having 1/10 millimeter resolution in all three
directions (Fig. 9.5).
Segmentation and Classification: ToLTech, working in conjunction with the CHS, has been developing fundamental tools that are necessary for both visual and
haptic interaction with the Visible Human and similar datasets. These fundamental tools provide the foundation for robust anatomically accurate simulators. A major part of this effort deals with segmenting and classifying the data. Together, ToLTech and the CHS have developed multiple strategies for assigning to each voxel a number that can be related to a specific structure. We refer to this three-dimensional
augmentation as the alpha channel. These methods fundamentally produce borders of structures through hand drawing, automatic edge detection, or surface splines. The tissue within the border is then assigned an identifier. Figure 9.6 shows a surface spline being used to outline a kidney in a single image of the VHM. ToLTech and the CHS have worked together to segment and classify the entire VHM. Although the
effort to refine the data is ongoing, most, if not all, structures that have a
three-dimensional extent of over a millimeter are now in the database. The alpha channel has become the foundation for ToLTech’s three-dimensional display
– both haptic and graphic – of individual anatomic structures.
Figure 5 Legend
Cryomicrotome of the ankle. This is a zoomed-in image taken from a foot and ankle that were cryosectioned at 1/10 millimeter.
Figure 6 Legend
Marking and numbering a cross section of the kidney.
Training Method cont.
Simplified Physics: In 2000 ToLTech developed algorithms to create simulated
ultrasound from the VHM alpha channel. The basic idea was to send rays out from the simulated probe through virtual tissue and determine the expected energy return. The method assumed that the information contained in a clinical ultrasound could be closely approximated by superposition of the following:
-Impedance mismatch between structures,
-Attenuation of signal based on the normal of the interface compared with
the position of the transmitter/receiver,
-Energy loss proportional to material attenuation, and
-Statistically described angle-independent texture associated with anatomic structures.
It also assumed that noise would dominate any other effects seen in the image. ToLTech currently associates impedance, sound velocity, and attenuation with blood, muscle, air, connective tissue, water, fat, and lung, bone, brain, and nerve tissue. Creating virtual ultrasound from the VHM data brings the following attributes:
-High enough resolution to simulate today’s ultrasound, which allows the
user to seamlessly interrogate all areas of interest.
-A segmented and classified foundation for the simulation, which allows us
to provide interactive interrogation of the tissues displayed in the
simulated ultrasound (e.g., the user can place a cursor on the simulated
ultrasound and have the structure identify itself). This is important for
feedback-based mentoring and testing.
-Immediate rendering of deformations of the data due to the probe or
anatomic motion.
-The ability of simulated ultrasound to be combined with the virtual
anatomy, allowing for the practice of ultrasound-guided needle insertion.
Modifying Posture: ToLTech and the CHS have collaborated to develop off-line techniques for altering the posture of the virtual patient. These techniques utilize finite element modeling (FEM) to deform tissues and tetrahedra (three-dimensional constructions using triangles) to volumetrically describe the virtual patient.
The tetrahedra individually obey laws of deformation and interpenetration, so the aggregate then describes the overall deformation of the virtual patient. This creates a set of transforms, one for each tetrahedron from the original space to the deformed space. One common use for the transforms takes the vertices associated with the
polygonal surfaces of the virtual patient and creates the vertices for the altered
posture. This can be done either statically, creating one posture for the simulation, or by loading the information into splines used to dynamically alter the posture in real-time. Figures 9.7-8 show the VHM in postures altered to correspond to
laterocollis and anterocollis. The images were taken from a simulator used to train neuromuscular injection techniques. To simulate ultrasound relative to the altered posture, we run the simulated acoustic rays through the altered volumetric
description and use the transforms to define their paths in the original dataset, where the acoustic properties are defined for the individual voxels.
Figure 6 Legend
Marking and numbering a cross section of the kidney.
Figure 8 Legend
Shift of a virtual patient to an anterior head tilt.
CHAPTER 10
Advanced Ultrasound-Guided
Needle Technology
Advanced Ultrasound-Guided Needle Technology
Ultrasound-guided techniques have become the standard of practice in regional
anesthesia. Multiple studies have shown that using ultrasound improves both the practicality and the efficacy of performing most types of nerve blocks. One
important feature that the ultrasound provides is needle tip visibility, which may lead to fewer complications and a more accurate anesthetic application than the
traditional nerve stimulation technique. As such, various innovations have been
developed in the hopes of maximizing needle visibility. These technologies, which will be discussed further in the chapter, include echogenic needles, needle guides, guidance positioning systems, and digital enhancement software.
Echogenic Needles – Clinical Data
Though clinical trials comparing echogenic needles to standard needles are still few and far between, studies generally show that echogenic needles have better
visibility, are less affected by the angle of insertion, and can be utilized to more
rapidly reach the target, especially for deeper nerves. However, there are still
limitations that should be considered. For example, most of these studies were done in nonliving models, such as cadavers or phantoms, which do not necessarily translate directly to the use of echogenic needles in living humans. Furthermore, the sample sizes of these studies are small, and none of the studies measured important clinical outcomes such as the rate of successful blocks, the difference in complication rate, and the cost efficiency.
Nevertheless, echogenic needles appear to be a step forward in regional anesthesia, and some clinical trials are currently in progress to better assess these outcomes. One such trial is a randomized clinical comparison evaluating the use of the Pajunk Sonoplex versus a standard Pajunk non-echogenic needle in sciatic nerve blocks for knee replacements. As more of these trials are performed, we come to understand more about the actual clinical efficacy of the echogenic needle.
Echogenic Needles: General Concepts
The visibility of an object imaged with ultrasound depends on how well it reflects the incoming ultrasound wave back to the probe sensors. Snell’s law requires that the angle of incidence (insonation) equal the angle of reflection (backscattering) for a flat, hard surface. Thus, maximal reflection occurs when the direction of the wave is perpendicular to the reflecting surface (Fig. 10.1a). By convention, this angle of
insonation is defined as zero. When using linear probes, the needle shaft must be
parallel to the surface of the probe in order to get a strong reflection from the
surface of a smooth noodle. In clinical practice, however, the angle of insonation usually varies from 20 to 70 degrees during ultrasound-guided nerve block. In other words, when the probe is flat on the skin, the needle makes an oblique angle with the surface of the probe (Fig. 10.1b), preventing the sound wave from being reflected back to the probe.
In contrast, an echogenic needle has an engraved pattern in the needle shaft that
maximizes ultrasound wave reflection back towards the probe. Two types of
echogenic etchings are in common use. The first type is a dihedral groove etched into the needle surface (Fig. 10.2a). This type of etching works well if the incoming wave is a plane wave aligned with the shaft of the needle. In most cases, the groove is etched continuously around the shaft of the needle, similarly to the thread of a screw (Fig. 10.3). In this case, the dihedral reflector actually becomes a universal three-dimensional surface reflector. The depth and period of the groove can be
manufactured to optimize the reflection from sound waves of different frequencies. The other type of reflector is a tetrahedral reflector that has a four-sided intermittent etching in the shaft of the needle (Fig. 10.2b). In this case, the tetrahedral etching is the universal three-dimensional reflector, and the needle has a triangular pattern etched into its surface (Fig. 10.4).
Figure 1 Legend
A: Drawing of an ultrasound beam perpendicular to a needle.
B: Drawing of an ultrasound beam oblique to a needle.
Figure 2 Legend
A: Dihedral echogenic etching in a needle.
B: Tetrahedral echogenic etching in a needle.
A
B
Echogenic Needles: Examples
Life-Tech EchoBright: This needle has a continuous dihedral, V-shaped groove etched into its surface (Fig. 10.3). This design has been shown to produce a constant, even reflection intensity when the angle of insonation varies over a range of 0 to 80 degrees (Fig. 10.4). It is also available in an insulated model so that it can be used with an electrostimulator.
Pajunk Sonoplex: This needle has a linear pattern of tetrahedral reflectors spaced out circumferentially around the shaft (Fig. 10.5). This arrangement allows for
reflection and imaging at a wide range of insertion angles and bevel orientations
(Fig. 10.6). Similarly to a standard insulated block needle, the Sonoplex can be
connected to a nerve stimulator and used for a single injection application or
continuous infusion. Different tip heads are also available (Fig. 10.5).
Figure 3 Legend
Image of a dihedral needle.
Echogenic Needles: Examples
Life-Tech EchoBright: This needle has a continuous dihedral, V-shaped groove etched into its surface (Fig. 10.3). This design has been shown to produce a constant, even reflection intensity when the angle of insonation varies over a range of 0 to 80
degrees (Fig. 10.4). It is also available in an insulated model so that it can be used with an electrostimulator.
Pajunk Sonoplex: This needle has a linear pattern of tetrahedral reflectors spaced out circumferentially around the shaft (Fig. 10.5). This arrangement allows for
reflection and imaging at a wide range of insertion angles and bevel orientations
(Fig. 10.6). Similarly to a standard insulated block needle, the Sonoplex can be
connected to a nerve stimulator and used for a single injection application or
continuous infusion. Different tip heads are also available (Fig. 10.5).
Figure 5 Legend
Image of a tetrahedral needle.
Types of Needle Guides
Mechanical Needle Guides: Mechanical guides force the needle to remain aligned with the probe by using either an in-plane approach or an approach in which the
needle is inserted transverse to the long axis of the probe. For nerve blockade, the in-plane guide is most useful (Figs. 10.7a, 10.7b). The guide keeps the needle in-line with the probe even when the user cannot see the exact insertion point of the needle relative to the probe (Fig. 10.7c). CIVCO makes these guides for many probes. The reusable red bracket that attaches to the probe costs about $400, and the green
disposable guide costs $15 (Figs. 10.7ab). Using about $4 for both the red bracket and the green guide, these needle guides can also be designed with simple CAD
software and manufactured in a fully disposable form using a three-dimensional
printer (Fig. 10.7c).
• Combined Electromagnetic Needle Guides: These guides track the needle in
virtual space by combining a mechanical needle guide with an electromagnetic
guidance system. The guides produce a virtual image of the needle on the ultrasound image, allowing for very accurate placement of the needle tip relative to the nerve. Two such needle guides that are currently in the market today are the Soma AxoTrack and CIVCO eTRAX.
- Soma AxoTrack: The Soma AxoTrack utilizes magnetic coupling to detect needle position. The system has two main components. The first is a
specially designed ultrasound probe that features a track of magnetic
sensors and a needle guide (Figs. 10.8ab). The second component is a
custom needle equipped with a magnetic ring in its shaft (Fig. 10.8d).
As the needle moves forward and backward, the distance between the
magnetic ring and the needle tip is constant. This magnetic ring
activates different magnetic sensors in the probe, thus indirectly
providing the location of the tip (Fig. 10.8c). This system has been
used for central venous access, cyst aspiration, arthrocentesis, biopsy,
and nerve blocks.
- CIVCO eTRAX: The CIVCO eTRAX has an electromagnetic transmitter
in the needle tip (Fig. 10.9a). The signal transmitted from the tip is
detected by a sensor in the ultrasound probe and is used to generate a
real-time virtual image of the needle on the screen (Fig. 10.9b). The
system has a needle bracket that locks the needle in place to ensure
proper in-plane alignment (Fig. 10.9c). In addition to nerve blocks,
this system has been used for ablations, biopsy, drainage, aspiration,
therapeutic delivery, and vascular access.
Figure 4 Legend
Ultrasound image of a dihedral
echogenic needle in a pork cadaver.
Figure 7 Legend
• A, B: Drawings of an articulating needle guide.
• C: Picture of an articulating needle guide.
A
Figure 6 Legend
Ultrasound image of a tetrahedral
needle in a pork cadaver.
B
C
Types of Needle Guides
Mechanical Needle Guides: Mechanical guides force the needle to remain aligned with the probe by using either an in-plane approach or an approach in which the
needle is inserted transverse to the long axis of the probe. For nerve blockade, the in-plane guide is most useful (Figs. 10.7a, 10.7b). The guide keeps the needle in-line with the probe even when the user cannot see the exact insertion point of the needle relative to the probe (Fig. 10.7c). CIVCO makes these guides for many probes. The reusable red bracket that attaches to the probe costs about $400, and the green
disposable guide costs $15 (Figs. 10.7ab). Using about $4 for both the red bracket and the green guide, these needle guides can also be designed with simple CAD
software and manufactured in a fully disposable form using a three-dimensional
printer (Fig. 10.7c).
Combined Electromagnetic Needle Guides: These guides track the needle in
virtual space by combining a mechanical needle guide with an electromagnetic
guidance system. The guides produce a virtual image of the needle on the ultrasound image, allowing for very accurate placement of the needle tip relative to the nerve. Two such needle guides that are currently in the market today are the Soma AxoTrack and CIVCO eTRAX.
- Soma AxoTrack: The Soma AxoTrack utilizes magnetic coupling to detect
needle position. The system has two main components. The first is a
specially designed ultrasound probe that features a track of magnetic
sensors and a needle guide (Figs. 10.8ab). The second component is a
custom needle equipped with a magnetic ring in its shaft (Fig. 10.8d).
As the needle moves forward and backward, the distance between the
magnetic ring and the needle tip is constant. This magnetic ring
activates different magnetic sensors in the probe, thus indirectly
providing the location of the tip (Fig. 10.8c). This system has been used
for central venous access, cyst aspiration, arthrocentesis, biopsy, and
nerve blocks.
- CIVCO eTRAX: The CIVCO eTRAX has an electromagnetic transmitter in
the needle tip (Fig. 10.9a). The signal transmitted from the tip is
detected by a sensor in the ultrasound probe and is used to generate a
real-time virtual image of the needle on the screen (Fig. 10.9b). The
system has a needle bracket that locks the needle in place to ensure
proper in-plane alignment (Fig. 10.9c). In addition to nerve blocks, this
system has been used for ablations, biopsy, drainage, aspiration,
therapeutic delivery, and vascular access.
B
B
Figure 9 Legend
Picture of a locking needle guide with a needle that emits an electromagnetic signal
and a GPS electromagnetic system that tracks the needle.
Figure 8 Legend
Picture of a needle guide with a locking track that stabilizes
the needle, and a virtual needle image.
Types of Needle Guides cont.
GPS-guided Needle Guides: This type of needle guide uses a GPS to produce a
virtual image of the needle, as well as an electromagnetic system to superimpose the virtual needle image on the ultrasound image. It consists of transmitting units and a receiver but does not require a mechanical guide to keep the needle in-line with the probe. For this reason, the user has much more freedom in choosing how he or she wishes to insert the needle relative to the probe. This type of unit usually costs about $10,000 for the transmitter and receiver. Specialized needles– costing about $50 each–are also required.
- A basic GPS system is shown in Fig. 10.10. By knowing the distance
from each transmitter (A, B, and C in Fig. 10.10), the receiver
(green dot) can deduce its location via a process known as triangulation.
Utilizing this mechanism, one can build a GPS needle locator by
embedding a sensor in the needle and creating a transmitter to emit the
signals. One such system is the UltraSonix GPS, which is composed of a
GPS transmitter unit (the dark gray device in Fig. 10.11) and GPS sensors
on both the needle (tip of the yellow cable in Fig. 10.11) and the
ultrasound probe. Using both of these sensors, the needle location can
be projected on the screen, along with its position in relation to the
probe (Fig. 10.12).
Figure 10 Legend
Illustration of a basic GPS system. The location of the receiver (green dot)
can be inferred using triangulation by knowing the location of three
different transmitters (A, B and C).
Figure 11 Legend
Picture of an electromagnetic needle guide
that does not require a locking stabilizer.
Figure 12 Legend
Ultrasound images of the electromagnetic
GPS guide shown in Figures 10.10-11.
Needle Guides – Benefits and Advancements
Clinical Data: As with echogenic needles, not many studies have been performed comparing the use of needle guides against standard methods. One study by Ball et al. compared the use of a standard physical needle guide in placing central venous catheters via the long-axis approach to the standard short- and long-axis freehand techniques. Two objective outcomes were concluded: (1) compared to both freehand techniques, the needle guide significantly improved the amount of time the needle was visible; but (2) compared to the long-axis freehand technique, the needle guide did not decrease the amount of time until the puncture of the target vessel was made, and it actually took more time than the short-axis freehand technique. However, this is somewhat expected, as any long-axis approach is technically more difficult than the short-axis approach. In theory, the benefits of this particular needle guide may apply to the use of the guide in peripheral nerve blocks.
Needle Enhancement Software: In addition to the hardware advancements discussed so far, Sonosite has created a software algorithm that can digitally enhance a
standard needle image without affecting the appearance of background structures (Fig. 10.13). This software can be downloaded to some of the newer Sonosite
ultrasound machines, but the detail of the algorithm is proprietary.
Figure 13 Legend
Needle image enhancement software.