Changes in local energy spectra with SPECT rotation for two Anger cameras

We investigated the shift of local energy spectra with SPECT rotation for the GE 400 AT and the Picker Prism 3006 tomographs. A Co-57 flood source was taped to the parallel-beam collimator of the GE 400 AT; a Tc-99m line source was placed at the focus of the fan-beam collimator of one head of the Picker Prism. The count-based method, which employs a narrow window (about 4 keV) on the maximum slope of the photopeak, was used with both systems. Non-linear, polynomial spectral fitting was applied to x-y-E data acquisitions with the GE camera. The fitting yielded either shifts or shifts and width changes. Results show: 1) the shifts are pseudo-sinusoidal with angle and similar for different spatial locations, 2) the average of their absolute value is 0.71 keV and 0.13 keV for the GE and Picker cameras, respectively, 3) width changes for the GE camera are small and appear random, 4) the calculated shifts from the count-based method for the central part of the GE camera are correlated with those from the spectral fitting method. They are 12% smaller. The conclusion is that energy shifts with angle may be present with many rotating cameras although they may be smaller with newer cameras. It might be necessary to account for them in schemes designed for high-accuracy compensation of Compton-scattered gamma rays although they possibly could be ignored for newer cameras