You might be thinking of a CAT scan.
MRI = Magnetic Resonance Imaging. Used to be called Nuclear Magnetic Resonance. You put something in a strong magnetic field, the nuclei in the material align their magnetic moments to the field. When you remove the magnetic field, the nuclei relax and give up some photons. The photons give information about the material.
There's no ionizing radiation in MRI scans. There's just a very big magnetic field (and that makes lots of people nervous).
In CAT scans (Computerized Axial Tomography), lots of X-ray slices are made and a computer assembles the information and gives cross-sectional information.
Google gives lots more information. :-)
Cheers,
Scott.