A Lucite phantom was constructed with voids cut in long cylinders, short cylinders and spheres in order to quantitate the effect of partial volume averaging on detectability. Typical scans of varying contrast revealed that the long cylinders cinsistently can be detected to a smaller size than either short cylinders or spheres of the same diameter. The authors feel that low contrast detectability must involve the influence of beam profile on partial volume contrast losses for spherical objects.