Quantitative evaluation of atlas-based MR attenuation correction for brain PET imaging using a time-of-flight PET/MRI system: A direct comparison with CT-based attenuation correction

259 Objectives CT-atlas-based bone-anatomy compensation for MR-based attenuation correction (MRAC) in brain PET imaging using PET/MRI is a current standard. It is important to ensure the quantitative accuracy of this technique in the combined PET/MRI system. Methods Whole-body FDG-PET/CT followed by PET/MRI were performed for twelve patients. Using the same PET raw data from PET/MRI, time-of-flight (TOF) iterative reconstruction was performed with the attenuation correction applied using atlas-based MRAC as well as CTAC from PET/CT. CT images were aligned to MR images before CTAC was applied. For quantitative evaluation, PET mean activity concentration values were measured and compared in eight 10 ml volumes-of-interest (VOI). Results PET activity concentration with the atlas-based MRAC was systematically underestimated on average by 0.64±0.29 kBq/ml (3.9±1.5%). In addition, the results were patient-specific (highest: 6.9% vs. lowest: 1.0%) and VOI-specific (highest: 5.6% vs. lowest: 2.6%). Specifically, the highest discrepancy (5.6%) between PET/MRAC and PET/CTAC occurred at the cerebellum (VOI #7). Also, the patient-specific difference images for AC maps and PET reconstructions show that skull differences between MRAC and CTAC resulted in a small but measurable underestimation of activities on PET/MR. Conclusions For the first time, the atlas-based MRAC was evaluated for brain PET imaging in an integrated TOF-PET/MRI system. Overall, the MRAC achieves quantification accuracy similar to CTAC with a small but measurable difference of 5% in values. Further studies are needed to determine if this difference is clinically significant as well as the etiology of the patient specific differences. Additionally, since the skull has the greatest contribution to AC difference, bone-enhancing MR pulse sequences such as ultra-short or zero echo-time (UTE/ZTE) may improve the accuracy of PET quantitation compared to the CT-based bone atlas.