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 simulated the Beacon attenuation correction tool attached to multiheaded Single Photon Emission Computed Tomography (SPECT) cameras which uses this approach. We investigated the scatter order of the photons reaching the detector, and the scatter contributions from the different detector components were evaluated for different energy windows. In case of simultaneous emission and transmission scanning the spatial signals are electronically windowed to allow differentiation between photons from the attenuation and from the emission source. However, an additional correction needs to be applied for cross-contamination. When applying the Beacon device in hybrid mode [gammacamera-Positron Emission Tomography (PET)] there are no degrees of freedom for crystal and collimator. The inter-window contamination was thus examined in detail to derive possible protocol optimizations for that dedicated setup. For the case of applying Beacon-SPECT, we performed multiple types of simulations including different crystal thicknesses and different collimators to evaluate the inter-window contamination. The main conclusion of this work is that a thick crystal detector coupled to a Low Energy High Resolution (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 system matrix for medium energy studies on low energy collimators in order to replace the Medium Energy General Purpose (MEGP) collimators by the LEHR variants appeared to be a more valuable approach. This last method is penalized by a computational burden but results in an improved image quality after reconstruction.