I am trying to do segmentation on fusion of 2 image sets(PET-CT and mpMRI). PET-CT is full body and MRI is pelvic. With the help of Landmark Registration module I am able to overlap the two images. I prefer mpMRI is fixed and CT is moving. As mp MRI show great anatomical detail, I need it to be the fixed one. But when I try to segment my ROI on the fusion image, the result of the segmentation in 3d is far off from where I expect it to be. Is there any suggestion on how to overlap the segmentations also.