Detect small particles within/inside a bigger one

Hi every one,

I would like to detect particles from the below image (black background, grey particles). I manually highlighted some particles in the second image, where one can see small particles staying on top of a bigger one.

u.tif (526.0 KB)

u_particles.tif (526.0 KB)

I have done particle analyse with ImageJ before (using the plugin"Analyze Particles") but never had to deal with this kind of images. I observe that an approach starting with a binarization based on pixel intensity level will not work because all the smallest particles on top of bigger ones will disappear in the

Not knowing what should be done in this case, I am asking for your suggestion on a method and/or a tool (imagej or not) that would solve this problem. Even being able to detect only smallest particles help because I can count the biggest ones manually.

Thank you a lot for having read this topic and probably spending some minutes thinking about it