For finding the contour of the image

Hello everybody! Wish you all a good day! I have use "meanshift" to get some images.Just like:

And

Or

here we can see clearly we have a similar circle in the image. Could you please tell me how to keep or just identify it (like the following 3 images without the lighter parts!):

We can see that these images always have some connected edge with the meaningless parts, which causes the problem.