display dynamic image URL using xsl-fo external-graphi

Hi,
My requirement is to display a dynamic image using XSL-FO
external-graphics tag.
The source of the image is a servlet URL:

For eg:
http://10.89.17.26:8081/myapp/servlet/acs/dynamicImage?arg=0sOH6mFP%2Bve
f3Jr%2BbDwfj8qdlFQtHaP9%2BaGhIynH%2Bw%2FSY1Z1DoiUhj3O6HXUi16BZP9MOwQyCSM
tOYtUQbM%2F8MF1dgg1i3R1%2FkorW%2BbzRtipJpVlBUTpt4YCYKanyh3JOx5kv4%2B7Txy
LwRa62YhV2kiQMgRE%

If I paste this URL to browser's URL (and with my Application sever on),
it displays the image correctly.

I am trying to display using below code snippet:
<fo:external-graphic src="url('$dynURL')">
 </fo:external-graphic>

How do I ensure that it can be displayed in PDF using xsl-fo?
Please help.

Regards,
Arvind