Unicode characters not displaying correctly in IE

I'm using signwriter in a client's site, to generate images for a product (like 'type your message'), but I found out, that even though it works fine in Chrome/Firefox/Safari, the images get generated with broken text in IE.

The image is generated fine, but the greek characters are not displayed correctly.

Any ideas?

In the 2 images, you can see how the image is generated in chrome and how it is created in IE.
The code I use to create the images is

 if (isset($_GET['bg'])){
      $signwriter->foreground = $_GET['col'];
      $signwriter->background = $_GET['bg'];
      $signwriter->fontsize = $_GET['tsize'];
  }

$signwriter->imagetype = 'jpeg';
$signwriter->width = 260;
$signwriter->height = 70;
$signwriter->textalign = 'center';

  $text = $_GET['mytext'];

  $output_string =  signwriter_image($text, $signwriter);