MRI image segmentation using Watershed

I have tried to segment the ROI of MRI images so many times, but I failed.

I found this example https://ww2.mathworks.cn/help/images/marker-controlled-watershed-segmentation.html to use. But the imregionalmax() is not work.I can't get a binary image include the two main areas. So I want to use its gradient and apply the Watershed() . But it still have many small segmentation areas I want to clean up and just maintain the ROI.HOW can I do? I really don't know how to do ? plz help me. Thanks in advance.