Objective To observe the drainage pathways of interstitial fluid(ISF)and substance clearance pattern in rat brain with fluorescence tracing imaging and treacer-based MRI.Methods Thirty-three male SD rats were randomly divided into fluorescence tracing group(F group,n=18)and treacer-based MRI group(MRI group,n=15),then further divided into thalamic,hippocampal and caudate nucleus subgroups,respectively.Evans blue was injected to rats in F group,and cardiac perfusion was performed after injection,then brain tissue was harvested,and frozen sections were made to observe the drainage pathways of IFS in different subgroups.MRI was performed on rats in MRI group before and after injection of gadolinium-diethylenetriamine pentaacetic acid(Gd-DTPA)to observe signal intensity in ROI of brain regions in different subgroups,the signal unit ratio was calculated,and the changing trend was explored.Results ISF in thalamus,hippocampus and caudate nucleus had different dominant drainage pathways,and the time of tracer reached to adjacent brain regions and whole brain in F group were different.In MRI group,within 4 h after injection of Gd-DTPA,there were differences in direction and clearance rate among tracer in thalamus,hippocampus and caudate nucleus,mainly manifesting as the tracer in thalamus and hippocampus drained to the ipsilateral cortex and lateral ventricle,while the tracer in the caudate nucleus diffused to the cortex and midbrain,and there were differences of the peak time of tracer signal among adjacent drainage brain regions.Conclusion Fluorescence and MR dual-mode imaging showed that there were differences in the dominant drainage pathways of IFS and clearance rates of small molecule substances among hypothalamus,hippocampus and caudate nucleus of rats.