Influence of X Ray Computed Tomography (CT) Exposure and Reconstruction Parameters on Positron Emission Tomography (PET) Quantitation

Abstract

Background:The CT of PET CT provides diagnostic information, anatomic localisation and attenuation correction (AC). When only ACis required very lose dose CT is desirable.CT Iterative reconstruction (IR) improves image quality with lower exposures however there is little data on very low dose IR CT for AC of PET. This work aims to assess the impact of CT exposure and reconstruction algorithm on PET voxel values.Method: An anthropomorphic torso phantom was filled with physiologically typical [18]F concentrations in heart, liver and background compartments. A 17mm diameter “tumour” was included in the right lung. PET was acquired followed by 24 CT acquisitions with varying CT exposures (15-50mAs, 80 – 120kVp, pitch 0.671 - 0.828). Each CT was reconstructed twice using filtered back projection (FBP) or IR and these used for AC of PET. Regions of interest (ROIs) were drawn in liver, soft tissue and over “tumour” on each test reconstruction (TR) and compared with the reference reconstruction (RR), (50mAs, 120kVp, pitch 0.828, IR). Comparison of voxel values in each TR compared to the RR was undertaken using a paired t-test and by calculating which and what proportion of voxels in each TR differed by a quantitatively significant difference (QSD) from the corresponding RR voxel.Results: TRs reconstructed using lower dose CTs underestimated mean and maximum ROI activity relative to the RR; greater with IR than FBP. Once CT dose index (CTDI)increased to 1 mGythe differences were less than QSD. On voxel analysis all TRs were significantly different to the RR (p <0.0001). TRs reconstructed at the lowest CT exposure with IR had 6% of voxels that differed by greater than the QSD. Differences were reduced with increasing CTDI and FBP reconstruction. Voxels which exceeded the QSD were spatially localised to regions of high activity, at interfaces between different attenuation and in areas of CT beam hardening.Conclusions: Very low dose CT exposures are feasible for accurate PET AC. Scanner specific validation should be employed prior very low dose CT AC for PET reconstruction.