Uniformity correction using non-uniform floods

Calibration and quality control (QC) of gamma cameras with fixed pinhole collimators presents unique challenges since the usual assumption of uniform flood intensity at the surface of the detector is violated. For the GE Discovery NM 530c*, a fast cardiac SPECT system with multiple pinholes and CZT detectors focused on the heart, the problem is further complicated by the fact that the plane of the pinhole may not be parallel to either the detector or the flood source. To address this, we derived an expression for the geometrical response to an ideal uniform flood source, then added further terms to approximate the attenuation and scatter behavior of a real flood source. The model was validated with Monte Carlo simulations for a range of angles for both flood and pinhole. Accurate knowledge of the flood source orientation is essential for good uniformity correction; we developed a jig that permits repeatable flood positioning for rapid daily QC. Alternatively we have shown that the angle of the flood can be calculated from observed systematic flood non-uniformity. Experimental measurements show that variations in pinhole penetration as a function of angle can be detected in the residual error of the floods; the magnitude of the effect agrees very well with predictions from a simple model of the knife edge collimator. Uniformity effects attributable to the finite stopping power of CZT were also observed. The methods described in this paper have been implemented in the GE Discovery NM 530c and Discovery NM/CT 570c imaging systems, and portions of the technology are patent pending.