Background-adaptive dual-energy-window correction for Compton scattering in SPECT

Abstract

Detection of gamma rays which Compton scatter within a patient but are still within the photopeak window of the Anger camera leads to inaccuracies in quantification of radioactivity from nuclear-medicine images. With the dual-energy-window correction method and a single (universal) scatter multiplier, the activity error is relatively large when there is tissue background with a large range of values including zero. We examine here a procedure that adapts the scatter multiplier to the level of background. In a Monte Carlo investigation, we introduce the iterative technique, examine when it converges, and look at the resultant improvement in quantification. Three geometries are checked: a large-sphere 99mTc target within a cylinder containing 1) a uniform or 2) non-uniform background and 3) a 123I brain phantom. Reconstruction of the data is carried out with the iterative maximum-likelihood, expectation-maximization algorithm with attenuation correction. Results show that the multiplier converges to a stable value after only a few iterations for all cases. Typical errors in target activity are: for the off axis sphere in non-uniform background 23.3% (no correction), -13.9% (universal-multiplier correction), and -0.7% (converged-multiplier correction); for the putamen in uniform white-matter background 20.4% (no correction), -10.6% (universal-multiplier correction), and 3.0% (converged-multiplier correction). The iterative background-adaptive method leads to considerable improvement in all cases tested.