The effect of CT scanner geometry on spatial resolution, contrast sensitivity, and patient dose are considered and used to determine the limiting factor or factors for a particular scanner. To this end analytical expressions for the components of the system modulation transfer function (MTF) are identified, and a Geometrical Figure of Merit is introduced based on the information capacity per rad. This formalism is used to compare four commercial CT units representing the main types presently in use. Improvements in scanner geometry are suggested which will improve this Figure of Merit for a given design. A new geometry, which would be relatively inexpensive to manufacture, is introduced and considered in the context of the formalism presented. Such a geometry utilizing state of the art components is shown to be superior to present and improved versions of CT scanners currently available. © 1979 SPIE.