Identify best matching image(s) to a Mask (grayscale) image

Hi All, I am trying to explore different ways to use one image as a mask (grayscale image with correct green icon) and loop through a folder of images to find scores of best matching images. The only inclusion is the capillary tube structure, i.e., the round circle. The cells are ignored since some might or might not have cells. If needed or easier, I can find an image without cells to use for the mask image. The size of the tube cannot be used either, since some settings can cause the tube to appear smaller or larger in the image. The unwanted images will have a white halo inside the tube or a bigger halo outside the tube. There of course might not be a perfect match, hence the preference for getting a list of scores/correlation values. What might be some methods to look at for such problems? Thank-you, Mathew