CAT scanners have an X-ray source, and a linear detector array mounted opposite each other on a ring that spins continuously as you are moved through the scanner. The position of the source and each detector element are known to a high degree of precision for each and every moment of the scan. Thus each detector signal at any given moment is the density of your body along a line between those two points.

All of the data becomes a set of simultaneous linear equations. The computer solves these equations and generates a 3 dimensional array of voxels (volume pixels), which are then saved as your CAT scan data.

The viewer software can then show a view through any arbitrary plane, adjusting contrast, etc.