I am not a medical professional, but (unforunately) have had some issues necessitating some medical imaging over the past year or so. I recently had an MRI, and they gave me a CD with the image files. OsiriX was great way for me to study the images. It worked superbly in 2-d and 3-d modes, and has many useful and intuitive features.

I haven't used other programs of a similar nature, but this one worked so very well for me that I didn't have any motivation to look elsewhere.