The point spread function (PSF) plays an important role in methods such as single photon emission computed tomography (SPECT). This function is most often approximated using the Gaussian distribution, as a convenient to use function. In this work a geometric model was developed that allows to estimate the intensity distribution of the gamma quantum flux from a point source after passing through the collimator. When developing this model, it was noticed that the approximation of the PSF by the Gaussian function is not always qualitative. So, for example, at distances of 200-300 mm to the source, the approximation by the Gaussian function can be called accurate, in contrast to other cases. Also, a software package based on the Monte Carlo method was developed, which makes it possible to obtain images of a spot from a point source by SPECT. Only a small discrepancy was found between the Monte Carlo spot size and the geometric model data. These discrepancies are related taking into account the distance between the collimator and the detector.