h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Aren't antiparticles used in CAT scans... I'm sure I read something about it when temping at PPARC.

a CAT scan stands for "Computed Axial Tomography' which is nothing more than a method of controlling and analyzing a standard x-ray (part of the electromagnetic spectrum) using a computer (the technique can be done by hand as well but is much more difficult and time consuming, and the computer is capable of forming nice 3-D images).

What you are thinking of is a PET scan (Positron Emission Tomography), in which a radioactive substance is attached to some bodily substance (glucose, water, etc.) and injected into the body. Depending on the 'tag', the radioactive substance will congregate in a specific area of the body. As the radioactive material decays positrons are released, which quickly collide with an electron releasing a high-energy gamma ray, which travels through the body tissue and into a gamma camera. Because the marker is attached to bodily chemicals, the PET scan can give doctors a good idea of how a particular bodily process is functioning.

Hope this helps.