The effect of radionuclide scatter in emission-transmission CT

In order to assess the relative effect of scatter in comparison to attenuation and collimator blur, we imaged concentric cylinder, cold-lesion, and thorax-like phantoms with the emission-transmission computed tomography (ETCT) system, a novel device capable of both X-ray CT and SPECT. For these experiments, the ETCT system was outfitted with a single channel of low-noise electronics which permitted acquisition of SPECT data with 3 keV FWHM energy resolution for good scatter rejection. Reference images were reconstructed with object-specific attenuation correction and accurate collimator response compensation. These images were then compared to images which were reconstructed without attenuation correction, without collimator response, or with data that had been blurred to simulate 13 keV FWHM energy resolution. The results show that although scatter represents a measurable error, its effect is small compared to the effect of attenuation and collimator blur. As part of this study, we have also compared scatter rejection to the well-known dual energy window scatter correction method and to a technique which incorporates an estimate of the scatter distribution directly in the back-projector of the ML-EM algorithm. It is shown that scatter correction may be preferable to scatter rejection due to noise considerations.