A transmission-dependent method for scatter correction in SPECT.

Abstract

A method of scatter compensation has been developed that incorporates planar transmission measurements in the estimation of photopeak scatter in SPECT. The scatter distribution is first estimated by convolving the planar projections with a monoexponential scatter function. The number of scattered events that subsequently reach the detector as a proportion of total events (i.e., scatter fraction) is then determined for each point in the projections based on narrow-beam transmission values, obtained using an external source. The assumptions of the method were tested using 99mTc and 201Tl point and line sources. The quantitative and qualitative impact of transmission-dependent scatter correction was assessed in realistic phantom experiments simulating blood-pool, lung and myocardial perfusion studies. The method accurately predicts the scatter distribution from 99mTc and 201Tl line sources in a phantom with variable density. Reconstructed counts are artificially enhanced in regions of high tissue density when scattered events are not removed from the projections prior to attenuation correction. Using convolution-subtraction with a constant scatter fraction (k = 0.4), scatter is underestimated in the heart and overestimated in the lungs, whereas transmission-dependent scatter correction enables activity to be quantified with > or = 95% accuracy in heart and lung regions. We conclude that incorporating transmission data enables accurate scatter compensation in objects with nonuniform density.