Impact of X-ray Scatter When Using CT-based Attenuation Correction in PET: A Monte Carlo Investigation

Current dual-modality PET/CT systems offer significant advantages over stand-alone PET including decreased overall scanning time and increased accuracy in lesion localization and detectability. However, the contamination of 3-D cone-beam CT data with scattered radiation during CT-based attenuation correction (CTAC) is known to generate artifacts in the attenuation map and thus the resulting PET images. The aim of this work is to quantitatively measure the impact of X-ray scatter in CT images on the accuracy of CTAC on future designs of volumetric PET/CT systems. Our recently developed MCNP4C-based Monte Carlo X-ray CT simulator capable of modeling both fan- and cone-beam CT scanners and the Eidolon dedicated 3D PET Monte Carlo simulator were used to generate realigned PET/CT data sets. The impact of X-ray scatter was investigated through simulation of a uniform cylindrical water phantom for both a commercial multi-slice and prototype flat panel detector-based cone-beam CT scanners. The analysis of attenuation correction factors (ACFs) for the simulated cylindrical water phantom showed that the contamination of CT data with scattered radiation in the absence of scatter removal underestimates the true ACFs, namely by 7.3% and 28.2% in the centre for both geometries, respectively. It was concluded that without appropriate X-ray scatter compensation, the visual artifacts and quantitative errors in flat panel detector-based geometry are substantial and propagate cupping artifacts to PET images during CTAC.