Here are medical imaging datasets I am aware of:
Neuroimaging (see http://www.nitrc.org for others)
OASIS http://www.oasis-brains.org/
ADNI http://adni.loni.ucla.edu/ (huge dataset, requires application)
OpenfMRI http://openfmri.org/
EEG http://eeg.pl/epi
Some other applications, example CT Colonography http://www.acrin.org/