Gian Pablo wanted to turn an MRI of a crocodile skull into a 3D model and print it out on a Makerbot 3D printer; the process was complex, but produced a very good result; Pablo's documented the process so you can repeat it and improve on it:
The crocodile skull data is derived from a CT scan of a real skull. It is very detailed, preserving information about the internal volumes of the skull. That makes it fragile. I quickly realized that I was going to have to slice it into two parts in order to be able to print it without support. Cutting support off a fragile piece can be very difficult!Printing complex organic shapes with a Makerbot « MakerBot Industries
The first step is optional: the model is very high resolution, more than will really be noticeable in the print. If you want, open the model in Meshlab, and use Quadratic Decimation to reduce the number of polygons to 50% or 25% of the original number. This will make things faster. However, I had no issues slicing the model in Skeinforge at full resolution, though it took longer.
The next step is to get it into the right orientation. You can do this in ReplicatorG, which works fine, but since I was going to use Netfabb Studio for the rest of the process, I just used that.