Collimator Angulation Error and Its Effect on SPECT

Angulation errors in collimator septa degrade tomographic resolution in SPECT due to mispositioning of events in the projection data. These errors should be small compared to other contributions to SPECT system resolution, such as intrinsic camera and collimator resolution, gantry alignment, and algorithmic smoothing. We describe a radioisotope technique to measure septal angulation errors over the entire collimator surface to +/- 0.05 degrees (RMS). A simple computer simulation is used to estimate a tolerance level, based on a 1% degradation in reconstructed resolution, of 0.25 degrees (RMS) and 0.6 degrees (max.) for collimator septal deviations. We also point out two methods for collimator linearity correction during SPECT reconstruction.