CT Based Attenuation Correction for PET Brain Imaging

For research and clinical PET brain studies performed on PET/CT systems, the CT image is often of little benefit beyond attenuation correction. The goal of this work is both to investigate the quantitative accuracy of CT-based attenuation correction (CTAC) for PET brain studies and to determine the lowest-dose protocol that performs adequately. Measurements were performed on a GE Discovery ST PET/CT scanner using the GE kVp-dependent CTAC algorithm. The effects of pitch on CT images were investigated by acquiring CT images of a Defrise disk phantom. The effects of pitch and X-ray tube voltage and current were investigated using a human skull phantom encased in plastic filled with F-18 radioactivity. This phantom was imaged and CT-based attenuation correction factors (CTACF) were created for several permutations of pitch (0.562:1, 0.938:1, 1.375:1, 1.75:1), voltage (80, 100, 120, 140 kVp) and current (10, 100mA). Emission images were acquired and reconstructed using the 3-D reprojection algorithm with the various CTACFs. The measured mean activity concentration is independent of pitch, kVp, and mA and accurate on an absolute scale to ~5%. Anatomically sized regional differences in the brain region indicate that tube voltages less than 100 kVp may not perform adequately (10% of values have a discrepancy greater than 5%). Results indicate that the higher pitch, lower current, and tube voltages down to 100 kVp perform equivalently to higher-dose configurations.