You may find useful the following collection of 100 MRI datasets,
that you can download from these collections in MIDAS:

   http://www.insight-journal.org/midas/community/view/21

"Designed Database of MR Brain Images of Healthy Voluteers"
Size : 5 Gb

Contributed by:
Elizabeth Bullitt, Keith Smith and Weili Lin.
University of North Carolina at Chapel Hill

See
Bullitt E, Zeng D, Gerig G, Aylward S, Joshi S, Smith JK, Lin W, Ewend
MG (2005) Vessel tortuosity and brain tumor malignancy: A blinded study.
Academic Radiology 12:1232-1240
as an example of how this kind of healthy image database can be used,