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PURPOSE: To enhance the spatial accuracy of fluorine 18 ((18)F) misonidazole (MISO) PET imaging of hypoxia by using dynamic contrast-enhanced (DCE) MR images as a basis for modifying PET images and by using electron paramagnetic resonance (EPR) partial oxygen pressure (pO(2)) as the reference standard. MATERIALS AND METHODS: Mice (n = 10) with leg-borne MCa4 mammary carcinomas underwent EPR imaging, T2-weighted and DCE MRI, and (18)F-MISO PET/CT. Images were registered to the same space for analysis. The thresholds of hypoxia for PET and EPR images were tumor-to-muscle ratios greater than or equal to 2.2 mm Hg and less than or equal to 14 mm Hg, respectively. The Dice similarity coefficient (DSC) and Hausdorff distance (d(H)) were used to quantify the three-dimensional overlap of hypoxia between pO(2) EPR and (18)F-MISO PET images. A training subset (n = 6) was used to calculate optimal DCE MRI weighting coefficients to relate EPR to the PET signal; the group average weights were then applied to all tumors (from six training mice and four test mice). The DSC and d(H) were calculated before and after DCE MRI–corrected PET images were obtained to quantify the improvement in overlap with EPR pO(2) images for measuring tumor hypoxia. RESULTS: The means and standard deviations of the DSC and d(H) between hypoxic regions in original PET and EPR images were 0.35 mm ± 0.23 and 5.70 mm ± 1.7, respectively, for images of all 10 mice. After implementing a preliminary DCE MRI correction to PET data, the DSC increased to 0.86 mm ± 0.18 and the d(H) decreased to 2.29 mm ± 0.70, showing significant improvement (P < .001) for images of all 10 mice. Specifically, for images of the four independent test mice, the DSC improved with correction from 0.19 ± 0.28 to 0.80 ± 0.29 (P = .02), and the d(H) improved from 6.40 mm ± 2.5 to 1.95 mm ± 0.63 (P = .01). CONCLUSION: Using EPR information as a reference standard, DCE MRI information can be used to correct (18)F-MISO PET information to more accurately reflect areas of hypoxia. Keywords: Animal Studies, Molecular Imaging, Molecular Imaging-Cancer, PET/CT, MR-Dynamic Contrast Enhanced, MR-Imaging, PET/MR, Breast, Oncology, Tumor Mircoenvironment, Electron Paramagnetic Resonance Supplemental material is available for this article. © RSNA, 2021