How can I find small (3-10 pixel radius) 'unclear' circles in an image?

I'm trying to find circular objects in a microscopy image. I originally was using imfindcircles with no problem,but the magnification has changed, now the objects I want to find are much more numerous, and generally about 3-10 pixels across, and not of the greatest contrast.

I've attempted to threshold out the background, but that hasn't helped yet.

Is there a more rigorous method of finding circular objects?