[en] One of the problems posed by the astigmatic collimator is an accurate and effective description of its imaging properties. Since the image of an object can change drastically with its position relative to the collimator, the traditional method of describing a collimator in terms of its resolution and sensitivity is inadequate and must be extended to include the effects of source position. The authors have developed the generalized collimator transfer function (GCTF) to describe the non-stationary effects of collimator hole pattern on imaging. This concept is ideally suited to deal with the non-stationary effects of convergent and astigmatic collimation. The performance of a stationary imaging system can be described mathematically in terms of its point source response function (PSRF). The PSRF describes the image resulting from a point source located anywhere in front of the camera. the underlying assumption is that if the image of a point source is known, then the image of any extended source distribution can be reconstructed by convolution of the point source response function with the extended source distribution. For stationary systems the imaging processes is assumed to be shift-invariant. However, the authors goal is to analyze systems that are not necessarily shift-invariant. In order to accommodate such systems, they introduce the generalized point source response function (GPSRF). During the past year they have applied the GPSRF to generate SPECT images of analytical phantoms constructed as a collection of Gaussian source distributions