Comparison of calculated attenuation corrected FDG brain scans with CT

Without attenuation correction (AC) PET images exhibit strong artifacts and do not allow quantitative imaging. Therefore, current PET-CT systems perform a low-dose CT-AC scan. Although of low dose, this still increases the total radiation dose to the patient which is a special concern when scanning children. In addition, patient motion between or during the CT-AC scan and the PET scan can introduce motion AC artifacts. For brain scans a calculated AC is possible which alleviates both problems.Aim:To compare the calculated AC corrected brain scans, using the Siemens Neuro-AC tool, with the gold-standard i.e. CT-AC.Methods:For Alzheimer suspected patients, 10 FDG brain scans were included. (Age: 55-77, M: 6, F: 4) Subjects where positioned using a CT scout scan after which a CT-AC was performed (80 kV, 40 mAs). Scans of 5 min duration where made, 30 min after the injection of 200 MBq FDG. During the uptake period, patients remained in a darkened and quiet room. The scans were reconstructed using the CT-AC scan and the Neuro-AC tool. The reconstructions resulted in different dimensions of the data. Therefore, the scans where first co-registered and then both absolute and relative error images where calculated (Vinci). To prevent extra-cranial voxels to influence the analysis, the different images where masked using the CT-AC corrected FDG scan. The current analysis is based on a visual interpretation of the data, displayed in transaxial, coronal and sagittal sections.Results:Relative errors were largest in areas with low uptake which is a direct result of the division by low values. This is particularly evident for the skull. The smallest relative errors were observed for the cortical regions particularly the more cranial slices. More caudal structures, particularly at the level of the nasal cavities were more susceptible for errors.Conclusion:Calculated AC using the Neuro-AC tool may give large relative errors, although these are usually in low uptake areas. For a visual interpretation of FDG brain scans, particularly of cortical areas, its use seems appropriate. The same holds for the quantitative assessment (i.e. using SUV) of follow-up scans of patients. Generally, the use of SUV ratios is advised since this will reduce potential systematic errors. However, more research is required if this approach is to be used in a research settin