Actually NIH Image does quite a bit for 3D and 4D image processing. The power
of the program is in its flexible macro language. When run on a G-3 Mac its
amazingly fast. Look at the MRI macro set for reslicing image stacks, and the
projection command (under the stacks menu). You can import 16 bit images at 1K
x 1K and manipulate RGB (24 bit) 3 image stacks to produce overlays. Also of
interest would be Object Image (FTP://simon.bio.uva.nl);  an off shoot of NIH
image, and Image-J (Wayne's new Java implementation will run on Unix, Macs,
Windows) can be found by searching the Web. Mailing lists are available for
Image -J discussions.

All are available for free. Can't beat the performance to price.
Start there and see if you need more.

George A. Peeters M.D. M.S.
Solamere Technology Group
Salt Lake City, UT
801 322-2645