In SPECT imaging, stationary systems have been developed to avoid several of the practical issues associated with traditional SPECT systems, based on rotating gamma-cameras. Stationary SPECT systems are, however, usually designed in such a way that the scanner needs to be much larger than the object to be imaged, limiting them to imaging small objects. To overcome this problem, we have recently proposed an innovative design for a highly compact stationary SPECT system [1], using a collimator with parallel-holes whose direction depends on their axial position. To accurately reconstruct an object using this system, we need to take into account its complex collimator geometry and the fact that the collimator and detectors are very close to the image-space, which is not considered in traditional parallel-hole collimator modeling. In this paper we describe an analytical method to accurately model the projection of an image voxel onto the detector. We then validate the method with Monte Carlo simulations.