Measure the circumference of a vessel using imageJ

I have images of stained vessels and want to calculate the wall thickness. I know I can use the freehand tool in image J to calculate the areas of the inner and outer circles and calculate the thickness of the wall that way. However I would like to know if there is an automated way to trace the circumferences on ImageJ. I have far too many images and individually tracing them all is taking far too long.

Any help would be much appreciated!!!

Picture

Many thanks guys