The effects of changes in spatial resolution and total number of counts on image quality were investigated for positron and single photon emission computed tomography (ECT) systems. A variety of high contrast phantoms were generated in a computer simulation and count density and spatial resolution were varied independently over a wide range. As system spatial resolution is improved, significantly fewer counts are needed to give images of comparable visual quality. Using 100% object contrast, it was found that the number of counts could be reduced by a factor of four for a 2 mm improvement in spatial resolution over a wide range of parameters. This is due to the fact that image contrast increases rapidly with spatial resolution improvements in high contrast objects such as those used in this simulation and typically encountered in brain and cardiac ECT studies.