print path to imagecache preset?

Hi, I'm using thickbox to display larger images in my node.tpl. I need to add the path to the image cache preset into a links href within my node.

I've printed my image out in the usual way including the class name for Thickbox to work, shown below:

<?php
print '<a href="'.base_path().$item['filepath'].'" class="thickbox">'.theme('imagecache', 'product_thumbnail', $node->field_product_line[0]['filepath']).'</a>';
?>

Thanks for any help