Retina and non-retina backgrounds with Sass

mobile.scss

@mixin mobile-sprite($selector, $image){
  #{$selector} {
    background-image: image-url('mobilenormal/' + $image);
  }
  @media only screen and (-webkit-min-device-pixel-ratio: 2){
    #{$selector} {
      $image_path: 'mobilenormal/' + $image;
      background-image: image-url('mobileretina/' + $image);
      background-size: image-width($image_path) image-height($image_path); 
    }
  }
}

@include mobile-sprite('.header', 'header.png');