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Objectives: To compare the diagnostic accuracy of 2-[ 18 F]fluoro-2-deoxy-D-glucose-enhanced positron emission tomography (2-[ 18 F]FDG-PET) and diffusion-weighted magnetic resonance imaging (DW-MRI) for the detection of bone marrow metastases in children and young adults with solid malignancies., Methods: In this cross-sectional single-center institutional review board-approved study, we investigated twenty-three children and young adults (mean age, 16.8 years ± 5.1 [standard deviation]; age range, 7-25 years; 16 males, 7 females) with 925 bone marrow metastases who underwent 66 simultaneous 2-[ 18 F]FDG-PET and DW-MRI scans including 23 baseline scans and 43 follow-up scans after chemotherapy between May 2015 and July 2020. Four reviewers evaluated all foci of bone marrow metastasis on 2-[ 18 F]FDG-PET and DW-MRI to assess concordance and measured the tumor-to-bone marrow contrast. Results were assessed with a one-sample Wilcoxon test and generalized estimation equation. Bone marrow biopsies and follow-up imaging served as the standard of reference., Results: The reviewers detected 884 (884/925, 95.5%) bone marrow metastases on 2-[ 18 F]FDG-PET and 893 (893/925, 96.5%) bone marrow metastases on DW-MRI. We found different "blind spots" for 2-[ 18 F]FDG-PET and MRI: 2-[ 18 F]FDG-PET missed subcentimeter lesions while DW-MRI missed lesions in small bones. Sensitivity and specificity were 91.0% and 100% for 18 F-FDG-PET, 89.1% and 100.0% for DW-MRI, and 100.0% and 100.0% for combined modalities, respectively. The diagnostic accuracy of combined 2-[ 18 F]FDG-PET/MRI (100.0%) was significantly higher compared to either 2-[ 18 F]FDG-PET (96.9%, p < 0.001) or DW-MRI (96.3%, p < 0.001)., Conclusions: Both 2-[ 18 F]FDG-PET and DW-MRI can miss bone marrow metastases. The combination of both imaging techniques detected significantly more lesions than either technique alone., Key Points: • DW-MRI and 2-[ 18 F]FDG-PET have different strengths and limitations for the detection of bone marrow metastases in children and young adults with solid tumors. • Both modalities can miss bone marrow metastases, although the "blind spot" of each modality is different. • A combined PET/MR imaging approach will achieve maximum sensitivity and specificity for the detection of bone marrow metastases in children with solid tumors., (© 2022. The Author(s), under exclusive licence to European Society of Radiology.)