[en] This paper reports on the point response resolution of a gamma camera which deteriorates with increased distance from the face of the collimator. This results in reconstruction artifacts that are seen as shape distortion and density non-uniformity. For parallel, fan, and cone beam geometries, iterative reconstruction algorithms have been developed which eliminate these artifacts by incorporating the three-dimensional spatially varying geometric point response into models for the projection and backprojection operations which also model photon attenuation. The algorithms have been tested on an IBM 3090-600S supercomputer. The iterative EM reconstruction algorithm is 50 times longer with geometric response and photon attenuation models than without modeling these physical effects. The authors demonstrate an improvement in image quality in the reconstruction of projection data collected from a single photon emission computed tomography (SPECT) imaging system. Using phantom experiments, the authors observe that the modeling of the spatial system response imposes a smoothing without loss of resolution