In computer assisted tomography [for human subjects], the slice being imaged is not a uniform slab because the X-rays are not parallel, the source and detector may not have equal dimensions, the source collimator may produce a penumbra region on the detector and in some machines 2 slices are scanned simultaneously from a common focal spot. One method of measuring slice thickness, uniformity and overlap is with a phantom containing an Al strip at a 45.degree. angle. This provides a direct display, in the viewing plane, of the slice profile. Measurements were made on 7 new tomographic scanners and photographs and plots of slice geometry were presented. Slice thickness should be defined as the full width at half maximum sensitivity, but that this measure should not necessarily be used as the desirable increment between scans.