Overview and Technical Considerations of PET/MRI Systems: System Architecture, MR-based Attenuation Correction, and Modality-specific Artifacts

Positron emission tomography/magnetic resonance imaging (PET/MRI) combines PET and MRI within a single system, enabling simultaneous acquisition of molecular and high–soft-tissue-contrast information. Among various system configurations, the integrated PET/MRI design—where PET detectors are installed inside the MRI gantry—is currently the predominant commercially available whole-body PET/MRI platform in Japan. Compared with PET/CT, in which PET and CT are acquired sequentially and may suffer from misregistration due to patient motion or physiological organ movement, integrated PET/MRI offers improved spatial alignment between PET and MR images through true simultaneous acquisition. Despite this advantage, PET/MRI has modality-specific technical challenges that can affect quantitative accuracy and image interpretation. First, attenuation correction must be derived from MR data (MR-based attenuation correction, MRAC) because MRI does not directly provide photon attenuation information. The most widely used whole-body MRAC approach is Dixon-based 4-class segmentation; however, the absence of bone information can lead to underestimation of standardized uptake value, particularly in and around osseous structures and bone lesions. Bone-inclusive MRAC strategies, such as atlas-based methods and zero-echo-time imaging for the head, as well as model-based approaches that incorporate bone masks, have been introduced to mitigate this limitation. Second, truncation artifacts may occur due to the field-of-view (FOV) mismatch between PET and MRI, especially when parts of the body (e.g., arms or back) extend beyond the MR FOV, leading to erroneous attenuation and scatter correction. Third, the static magnetic field can influence positron range and may generate characteristic artifacts, particularly near high-activity sources adjacent to air. This review summarizes the fundamental architecture of integrated PET/MRI systems, outlines major MRAC methodologies and their pitfalls, and organizes representative artifacts and practical considerations relevant to clinical PET/MRI implementation and quantitative interpretation.