Image analysis of circular object

I've a image with a circular object within it. I am interested only in the circular object for further analysis. I've tried several approaches to get the pixel for that circular object. The first approach executed was to equate every matrix outside the circular object to zero but I had a concern, i couldn't zero the edges since my original picture was in square. My other approach is trying to write a matlab code that will extract the circular object from the image but I didn't know how.

I'll appreciate if anyone can be of help.

I look forward to your reply