Collimator spatial resolution

Pixellated detectors based on cadmium-zinc telluride (CZT) are a promising alternative to NaI-based SPECT cameras. The performance of either system is quantitatively described in terms of resolution and sensitivity. This investigation is based on an analytical model published earlier, describing SPECT collimators according to the NEMA definitions of planar sensitivity and system spatial resolution without scatter. We have extended the model to include the effect of collimator hole shape and orientation as well as collimator and detector penetration. Equations for spatial resolution are derived for NaI and CZT based SPECT systems with hexagonal, round or square parallel-hole collimators and with pinhole collimators. Standard equations for spatial resolution are valid only for square hole collimators along one of the detector axes. In any other direction, spatial resolution is higher. The exact values depend on the hole shape and on the ratio of object-detector distance to collimator length. For pixellated pinhole detectors, similar corrections depending on the ratio of detector pixel size to the diameter of a point source projected on the detector apply. In scintillation cameras, spatial resolution is not only influenced by the average absorption depth in the scintillator, but it is strongly affected by the parallax effect of the collimator hole. Spatial resolution is therefore lower than usually thought. We present a model that describes the performance of current SPECT collimators and permits to optimize future detector systems for high image quality.