Aggressive breast tumors possess heterogeneity that impacts successful diagnosis and treatment. Mapping this complexity with imaging biomarkers of different biologic specificity supports patient-specific management. We compare biomarkers from diffusion-weighted MRI (intravoxel incoherent motion (IVIM)) and 18F-fluorodeoxyglucose (FDG) PET (dynamic pharmacokinetic modeling) in 10 breast cancer patients in a simultaneous PET/MR system. Voxelwise correlations were performed to study intralesion relationships between biomarkers. Intralesion correlations were observed, such as between PET plasma transfer rate K1 and tissue diffusivity Dt, that also showed potential diagnostic value in tumor classification. This feasibility study establishes a workflow that enables more detailed investigation in larger cohorts.