Volume rendering is applicable to medical scans or anything that returns data about the nature of a volume of space and now there's an open source program freely available and its results are interactive and amazing.
Exposure Render is an open source Direct Volume Rendering Application that is now available in an official version 1. It runs under Windows XP or later and needs a CUDA compatible GPU. You can download a compiled version ready to run or the full source code.
Volume rendering is particularly applicable to rendering medical scans or anything that returns data about the nature of a volume of space - usually as a set of 2D slices. See Chapter 39 of GPU Gems for an overview of volume rendering. Color and opacity values are assigned to each point in space, lighting and a camera position is set up and then the scene is rendered usually via a clever ray tracing algorithm - direct volume rendering - or by constructing a mesh approximation. The big problem with direct volume rendering is that it is computationally expensive.
Exposure Render uses the GPU and progressive Monte Carlo ray tracing fast enough to be interactive. It allows you to specify any number of shaped light sources and it uses a "real world" camera complete with lens and aperture. It implements complex materials and both surface and volumetric scattering.
The bottom line is that the quality of the rendering is impressive and at interactive speeds.
It is not difficult to see how this rendering engine could be of value in medicine and scientific imaging in general but it is also simply amazing viewing. It opens the possibility of realtime interactive imaging using nothing but off-the-shelf and low cost graphics hardware.
If you can't spare the time to download and play with sample data, have a look at the videos: