Integrated Lymphography Using Fluorescence Imaging and Magnetic Resonance Imaging in Intact Mice

We assessed lymph drainage in living mice by an integrated imaging method using fluorescence imaging (FLI) and magnetic resonance imaging (MRI). Mice were subcutaneously injected with quantum dots and gadofluorine 8 into the right rear footpad. They were fixed on a transparent flat plate and underwent FLI and MRI successively. Small markers were attached to the mouse surface for spatial coregistration, and image fusion of FLIs and MRIs was performed. Two-dimensional fluorescence reflectance imaging was used for FLI. FLI and MRI provided generally consistent results and demonstrated lymphatic flow to the popliteal, sacral, and iliac lymph nodes in most mice and to the renal, inguinal, and lumbar-aortic lymph nodes in some mice. On the fusion images, the locations of the lymph nodes in the mouse trunk were in good agreement between FLI and MRI, indicating successful spatial registration even for the deep structures. The popliteal node tended to be visualized a little farther caudally in FLI than in MRI, presumably because the overlying tissues were thicker in the cranial portion. Integrated FLI/MRI lymphography with image fusion appears to be a useful tool for analysis of the murine lymphatic system. HE LYMPHATIC SYSTEM plays a crucial role as a pathway of metastatic spread of malignant tumors and is also an essential component of the immune system. Although mice are the laboratory animals used most commonly in animal experiments, the lymphatic drainage pattern in mice still remains to be investigated. Because the murine lymph nodes are very small, it is difficult to identify them by the naked eye, which may be an obstacle to the evaluation of lymphatic drainage in mice. There are many imaging methods for in vivo evaluation of the lymphatic system, including x-ray lymphangiography, ultrasonography, computed tomography, magnetic resonance imaging (MRI), lymphoscintigraphy, positron emission tomography, optical imaging, and photoacoustic imaging, 1–3 and analysis of small animals can be supplemented by noninvasive in vivo imaging technologies. In vivo fluorescence imaging (FLI) allows assessment of the lymphatic pathway in both animals and humans.