Medical image dataset
The problem you are most likely having is that you need to search for files in the correct format.  DICOM is what you need to look for.
A few hits:


One example
A library from a DICOM viewer software producer
A library of images from Siemens


You will also need a DICOM viewer.
Image J  is free (non-commercial, no restrictions on use) and can be used under Windows, Mac, and Linux - it does require Java.
There are other viewers available, free as well as commercial.  Just search for DICOM viewers.