This work proposes a novel approach to model the collimator response in SPECT. The approach consists of projecting the activity volume on a high number of virtual projection planes that are then averaged with an angular point spread function. It was motivated by the new possibilities offered by GPU for 3-D projection/backprojection. This approach also allows to model a wide range of SPECT imaging systems. Results show that reconstruction using our resolution modelling method is consistent with standard blurring. As an example, we show how to implement a convergent collimator response.