Lymphatic‑specific magnetic resonance lymphangiography biomarkers for grading lymphedema in animal models.

Magnetic resonance lymphangiography (MRL) using gadolinium-based contrast agents is valuable for evaluating lymphedema but is limited by its inability to selectively visualize lymphatic structures, hindering accurate diagnosis and severity grading. To overcome this, we demonstrated imaging biomarkers for lymphedema grading using iron oxide-based T1 MRI contrast agent–enhanced MRL (INV-MRL) in animal models. Lymphedema was induced in the hindlimbs of rats by lymph node dissection and radiation, and was successfully established in 29 of 40 animals. Of these, 26 underwent both INV-MRL and near-infrared fluorescence indocyanine green lymphography (NIRF-ICGL) in the same experimental week, and 3 were allocated to excretion studies. Imaging processes were performed under identical injection conditions in the same experimental week. INV-MRL specifically visualized lymphatic vessels, offering superior imaging of deeper lymphatic structures and collateral pathways compared with the predominantly superficial visualization of NIRF-ICGL. Both modalities showed strong concordance in severity grading (Kappa = 0.823), and the Threshold Area Ratio, introduced as a quantitative imaging biomarker, correlated significantly with disease severity. Furthermore, the contrast agent demonstrated rapid excretion within 24 h without notable tissue retention. Collectively, these findings suggest that INV-MRL provides lymphatic-specific qualitative and quantitative biomarkers for lymphedema grading, with the potential to improve diagnostic precision in clinical practice.