Segmenting circle-like shapes out of Binary Image

Question

I have several binary images and my task is to segment circle-like shape. The circles are not perfect rounded circle, but all of them will look like circle. Here are some exampl

Answer 1

Do you know the radii of the disks you are looking for? If yes, morphological openings (erosion then dilation) would be straightforward, and very fast. The result using Mathematica:

Opening[img, DiskMatrix[15]]

If not, as other proposed, computing the contour image and then using the Hough transform would be a method worth pursuing. The image just above shows the contour image.