PV-Wave
PV-WAVE from Visual Numerics. It is commonly used for creating 3D renderings of voxel based data, and stacks of MRI images as you describe could be displayed with PV-WAVE, using the Z-Buffer, Ray tracer, or iso-surface renderer.
Contact:
Visual Numerics, Inc., 6230 Lookout Rd., Boulder, CO 80301 Phone: 303-530-9000 Fax: 303-530-9329.