The influence of penetrating radiation on collimator performance

Geometric and penetration responses of a multihole focusing collimator to a point source have been determined using a digital computer and a ray tracing technique. Results are presented as point and line spread functions for collimated rays and penetrating rays, and as plane sensitivities and modulation transfer functions derived from the line spread functions. Responses to a 141Ce line source have been measured for a lead collimator and for a cadium collimator of practically identical dimensions. A comparison of measured and computed responses shows that while computed penetration approximates measured penetration reasonably well, there is a measurable contribution from scattered rays. Plane sensitivities and the modulation transfer function derived from the computed responses are shown. The effects of penetration are compared with predictions made by other investigators. This analysis of collimator performance provides information not previously available and should be useful for evaluating collimators of existing or proposed design.