Circular Selection

Hey Guys

I have looked everywhere in google but cant find a thing.

I need to add a circular selection to my web app to select an inner and outer diameter of an image to basically measure a "Donut".

A selection tool that seems quiet cool is at http://deepliquid.com/projects/Jcrop/demos.php

Unfortunitly that is a square selection tool.

I know you can propably do it with jQuery but to be honest I wouldnt know where to start.

I have attached an image of what I am looking to achive.

Thanks for the help guys.