Microstructural changes of the corticospinal tract (CST) in idiopathic normal pressure hydrocephalus (iNPH) before and after CSF shunt surgery were studied using NODDI and WMTI. Pathological increase of orientational coherence and its postoperative normalization were shown, indicating axon stretching and its recovery. To the contrary, decrease of axon density was present in iNPH and remained after the surgery. Simulation using undulating cylinder model demonstrated both NODDI and WMTI can separate the effects from axon density and undulation. These results suggest possibilities of diffusion MRI to distinguish between reversible and irreversible microstructural changes in iNPH, and raise expectation for prediction of treatment outcome.