Extrinsic landmarks detection from images ? [closed]

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I am working on a problem of registration of a particular type of medical imaging. To do this, I have to extract extrinsic markers automatically from the images.

Take this image as an example:

I have to detect those white little cicles in the bottom, knowing that the color is not always the same.

I used fastNlMeansDenoising and blur to denoise image, then I've tried some edge detection techniques, in particular Canny edge detector, I used trackbars to try to find the best thresholds, but i can't find commun ones that will work on all images.