iOSでTwitterのアイコンを取得して表示する

AccountsViewController.swift

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        println("AccountsViewController#tableView cellForRowAtIndexPath called.")
        let cell: UITableViewCell = UITableViewCell(style: UITableViewCellStyle.Default, reuseIdentifier: "Cell")
        // ユーザIDを表示する
        cell.textLabel?.text = accounts[indexPath.row].username
        // プロフィール画像を取得して非同期で表示させる
        var q_global: dispatch_queue_t = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        var q_main: dispatch_queue_t  = dispatch_get_main_queue();
        cell.imageView?.image = nil;
        dispatch_async(q_global, {
            
            var ac: AnyObject = self.accounts[indexPath.row]
            let userid = ac.valueForKeyPath("properties.user_id")! as String
            let requestAPI = NSURL(string: "https://api.twitter.com/1.1/users/show.json")
            println("twitter userid=\(userid)")
            
            let req = SLRequest(forServiceType: SLServiceTypeTwitter,
                requestMethod: SLRequestMethod.GET,
                URL: requestAPI,
                parameters: ["user_id": userid])
            req.account = ac as ACAccount
            let slHandler: SLRequestHandler = {(response, urlResponse, error) in
                if let jsonObj: AnyObject = NSJSONSerialization.JSONObjectWithData(response, options: NSJSONReadingOptions.MutableLeaves, error: nil) {
                    var responseJson = jsonObj as NSDictionary
                    let profImgUrl = responseJson["profile_image_url"] as String
                    var imageData: NSData? = NSData(contentsOfURL: NSURL(string: profImgUrl)!)
                    if var image: UIImage = UIImage(data: imageData!) {
                        dispatch_async(q_main, {
                            cell.imageView?.image = image;
                            cell.layoutSubviews()
                        });
                    }
                    self.prof[userid] = profImgUrl
                } else {
                    println("jsonArray is nil")
                }
            }
            req.performRequestWithHandler(slHandler)
            
        })
        return cell;
    }