Looking for edge detection algorithm

[answer]

I don't understand your problem very well.

You can do a variant of the hough transform for circles. It would be like this

r is the radius of the circle you want
foreach pixel x. y
   sum = 0
    foreach degree D from 0 to 2 PI
      sum += pixelAt(x - r*sin(D), y - r*cos(D))
    peak_image.setpixel(x, y, sum)

Now you have peaks, simply find the peaks and thats the centers of all your circles of radius r. repeat for each radius you want.

It would be better to provide a picture of what you are trying to process.