Transmission imaging with a moving point source: influence of crystal thickness and collimator type

Nonuniform patient attenuation maps can be acquired using an axially moving point source of a high energy isotope that emits a fanbeam of photons. We modeled the Beacon attenuation correction tool attached to multiheaded SPECT cameras which uses this approach. We investigated the scatter order of the photons from the attenuation imaging source reaching the detector, and the scatter contributions from the different detector components were evaluated for different energy windows. Additionally the scatter of the Beacon source photons in the transmission window and the degrading downscatter on an emission photopeak window in simultaneous emission and transmission scanning were investigated. We performed multiple types of simulations including different crystal thicknesses and different collimators for protocol optimization purposes. The main conclusion of this work is that a thick crystal detector coupled to a LEHR collimator is the best solution for acquiring attenuation maps in low energy applications. For medium energy studies attenuation maps have to be rescaled to account for the low sensitivity near the center of the patient. Fully Monte Carlo simulating the projector for medium energy studies on low energy collimators in order to replace the MEGP collimators by their LEHR analogons is another possibility that is currently being investigated. This last approach is however penalized by a high computational burden but can result in an improved image quality after reconstruction.