The geometrical limitation of the spatial resolution inherent in a CT scanner has been analyzed. In order to assess this limitation, an equation for calculating the theoretical attenuation profile optical transfer function (OTF) from the geometrical dimensions of the system was derived. Four commercially available CT units were studied which are representative of the three basic design types in use. For each unit the OTF was calculated and the results are presented. Using this information in conjunction with the pixel size and data sampling frequency of the attenuation profiles, an analysis of the factors limiting the spatial resolution of each unit was made. In addition, subtle effects on the image quality of a scan due to the geometry of the data-taking mechanics are discussed and correlated with the observations of experimental comparisons. A suggestion is made for the optimal design of a CT unit that will encompass the conclusions of this analysis. © 1977 SPIE.