Validation of MR-Based Attenuation Correction of a Newly Released Whole-Body Simultaneous PET/MR System

The aim of this study was to validate quantitative performance of a newly released simultaneous positron emission tomography (PET)/magnetic resonance imaging (MRI) scanner, by using MR-based attenuation correction (MRAC), both in phantom study and in patient study. PET/MRI image uniformities of a phantom under different hardware configurations were tested and compared. Thirty patients were examined with 2-deoxy-2-[(18)F]fluoro-D-glucose ((18)F-FDG) PET/computed tomography (CT) and subsequent PET/MRI. PET images from PET/MRI were corrected with MRAC (PET(MR)), CT-based attenuation maps (μ-maps, PET(CT)), and segmented CT μ-maps (PET(CTSeg)) derived from PET/CT. Standardized uptake values (SUVs) were compared among the 3 sets of PET in main organs (bone, liver and lung) and in 52 FDG-avid lesions, including soft-tissue lesions and bone lesions. The result showed that PET imaging uniformities of PET/MRI under different configurations were good (<8.8%). The SUV differences among the 3 sets of PET varied with organs and lesion types. In detail, the mean relative differences of SUV between PET(MR) and PET(CT) were as follows: −18.8%, bone (SUV(mean)); −8.0%, liver (SUV(mean)); −12.2%, lung (SUV(mean)); −18.1%, bone lesions (SUV(mean)); −13.3%, bone lesions (SUV(max)); −8.2%, soft-tissue lesions (SUV(mean)); and −7.3%, soft-tissue lesions (SUV(max)). The mean relative differences between PET(MR) and PET(CTSeg) were as follows: −19.0%, bone (SUV(mean)); −3.5%, liver (SUV(mean)); −3.3%, lung (SUV(mean)); −19.3%, bone lesions (SUV(mean)); −17.5%, bone lesions (SUV(max)); −5.5%, soft-tissue lesions (SUV(mean)); and −4.4%, soft-tissue lesions (SUV(max)). The differences of SUV between PET(MR) and PET(CT) were larger than those between PET(MR) and PET(CTSeg), in both soft tissue and soft-tissue lesions (P < 0.001), but not in bone or bone lesions. In conclusion, MRAC in the newly released PET/MR system is accurate in most tissues, with SUV deviations being generally less than 10%, compared to PET/CT. In bone, however, underestimations can be substantial, which may be partially attributed to segmentation of the MR-based μ-maps.