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-[¹⁸F]fluoro-D-glucose (¹⁸F-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.