Analysis of optimal CT spectrum for PET attenuation correction

Abstract

One advantage of a combined PET/CT scanner is that a fast CT acquisition provides a high quality attenuation correction map for PET reconstruction. A diagnostic CT system typically operates in a voltage range from 80 kVp to 140 kVp. The choice of optimal spectrum depends on the specific application. In diagnostic imaging, contrast, noise and dose all play important roles. The designated CT scan for PET attenuation correction has different image quality requirements compared to diagnostic CT. For example, at 511 keV, the contrast between different materials reduces significantly and there is typically no interest in distinguishing small contrast variations. On the other hand, it is important that the error on the attenuation map is small, and especially bias in the CT attenuation correction (CTAC) map could introduce significant artifacts in PET images which may affect clinical diagnosis. We are interested in operating the CT scanner at the lowest possible dose while still providing accurate attenuation correction. However, at these very low dose levels, the effects of electronic noise can become significant. In the study presented here, we analyzed the impacts of the X-ray tube voltage, current and filtration by metal filter on the quality of the CTAC by evaluating noise variance and bias at various dose levels.