Since I have access to an MRI unit and always wanted a copy of my own brain (you never know when it might come in handy....), I acquired a 3D dataset from a scan of my brain. I loaded the DICOM data set onto a proprietary workstation and segmented the brain from the surrounding tissue/skull/vessels, etc. I then loaded the file into ZBrush for a lot of cleaning up. It was shelled inside of ZBrush, then exported as an .stl. The first version was printed at 80% actual size using SLA (shapeways). The second (blue) one I printed at full size using my own 3D printer in a semi-transparent PET-G material.
Below are the ZBrush screen-shots and the final printed product: