RESUMO
BACKGROUND: The CT of PET CT provides diagnostic information, anatomic localisation and attenuation correction (AC). When only AC is 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 assesses 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 17-mm-diameter right lung "tumour" filled with [18]F was included (surrounding lung contained no 18[F]). PET was acquired followed by 24 CT acquisitions with varying CT exposures (15-50 mAs, 80-120 kVp, pitch 0.671 or 0.828). Each CT was reconstructed twice using filtered back projection (FBP) or IR and these used for AC of PET. The reference PET reconstruction (RR) used CT acquired at 50 mAs, 120 kVp, pitch 0.828, IR, all others were test PET reconstructions (TR). Regions of interest (ROIs) were drawn in the liver, soft tissue and over "tumour" on each TR and compared with the RR. Voxel values in each TR were compared to the RR 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 RR. 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 mGy, 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 QSD. Differences were reduced with increasing CTDI and FBP reconstruction. Voxels which exceeded the QSD were spatially localised to regions of high activity, interfaces between different attenuation and areas of CT beam hardening. CONCLUSIONS: Very low dose CT exposures are feasible for accurate PET AC. Scanner- and reconstruction-specific validation should be employed prior very low dose CT AC for PET.
RESUMO
Cerebral perfusion single photon emission computer tomography (SPECT) can be used to identify epileptogenic foci. A (99m)Tc ethyl cysteinate dimer SPECT of the brain showed clinically evident differences in uptake between the CT attenuation corrected image and the Chang attenuation corrected image. The upper right hemisphere of the brain showed apparent diffuse hyperperfusion in the CT attenuation corrected image while the Chang attenuation corrected image, after reconstruction that appears to average projections, showed symmetrical cerebral perfusion. On review of archived patient data, this artefact was also observed in multiple previous cerebral SPECT studies undertaken on the same camera. Phantom investigation was used to identify the cause of the artefact as a difference in relative head sensitivity. The investigation also characterised the extent and nature of this artefact for CT attenuation corrected images, Chang attenuation corrected images and non-attenuation corrected images.