Simultaneously constraining SPECT activity estimates with primary and secondary energy window projection data

Single photon emission computed tomography (SPECT) projection data acquired in primary (photopeak) and secondary energy windows provide independent constants on the source activity distribution. A novel image reconstruction method is developed which uses these constraints simultaneously and solves a single matrix equation to estimate the source activity. This simultaneous constraint reconstruction method (I) is evaluated in a Monte Carlo experiment and compared with activity estimation using only primary window projection data with (III) and without (II) modeling the detection of scattered photons in the system matrix, and with estimation using dual energy window scatter subtraction (IV). The detection of photons emitted by Tc-99m within a water-filled cylinder is simulated at different count levels. The energy windows are 91-125 and 126-154 keV, and the images are reconstructed using noise-dependent generalized matrix inverses. Activity estimation using the simultaneous constraint method (I) yields no appreciable improvement in quantitative accuracy or resolution over method III. Methods I and III, which require Monte Carlo modeling of photon scatter, are only slightly superior to activity estimation using dual energy window scatter subtraction (IV)