THE EFFECT OF RADIONUCLIDE SCATTER IN EMISSION-TRANSMISSION CT

In order to assess the relative effect of scatter in comparison to att enuation and collimator blur, we Imaged concentric cylinder, cold-lesi on, and thorax-like phantoms with the emission-transmission computed t omography (ETCT) system, a novel device capable of both x-ray CT and S PECT. For these experiments, the ETCT system was outfitted with a sing le channel of low-noise electronics which permitted acquisition of SPE CT data with 3 keV FWHM energy resolution for good scatter rejection. Reference images were reconstructed with object-specific attenuation c orrection and accurate collimator response compensation. These images were then compared to Images which were reconstructed without attenuat ion correction, without collimator response, or with data that had bee n blurred to simulate 13 keV FWHM energy resolution. The results show that although scatter represents a measurable error, its effect is sma ll compared to the effect of attenuation and collimator blur. As part of this study, we have also compared scatter rejection to the well-kno wn dual energy window scatter correction method and to a technique whi ch incorporates an estimate of the scatter distribution directly in th e back-projector of the ML-EM algorithm. It is shown that scatter corr ection may be preferable to scatter rejection due to noise considerati ons.