How to handle outside of ROI without any pixel

[Ricus]

Dear users, Recently,I had processed a stack of images.However, there is a problem puzzling me,that is how to handle outside of image's ROI without any pixel. eg; Some particles are inlayed one image with rectangular edge, I aim to count the numbers of particles in my circle ROI not in rectangular area. PS: If there is not only one image ,but a stack?