how to select all threshold region from a elliptical region

[eweiren2002]

Dear all, I am a new user of imageJ. I have a ROI selection problem with the using of imageJ. I want to select all the threshold covered region from a pre-draw elliptical region or free hand region. I tried to search the answer from the previous posts, however, I cannot find the way I expect. Can you help me. Thanks. wei