Difficult circular objects detection

Hi! I'm new to image processing and Matlab. I'm working on an image processing project which consists of detecting circular objects in a set of images similar to the one below (liposomes in glucose solution). I used Circular Hough transform in order to detect circles in the image but I cannot manage to detect also circles with an inner gray level similar to the one in the background (let's call them "lighter"). In addition, there are some noisy artifacts in the image that are detected as circular objects. I tried to improve the image by using a sharpening filter to get the boundaries of the lighter circles more visible, but I failed as the artifacts got sharpened too; I also tried contrast improvement obtaining the same result. I thought to compute a mean/blurring operation in order to make less relevant the artifacts but in this way I also delete the lighter circles.

Do you have any suggestions? Thanks

Answer 1

[Rahul punk]

https://www.mathworks.com/matlabcentral/fileexchange/70220-rahulpunk?s_tid=prof_contriblnk