Diffusion tensor imaging of magnetic resonance imaging, including diffusion tensor tractography, is a unique tool to visualize and segment the white matter pathways in vivo and one can evaluate the segmented trace quantitatively. Three dimensional visualization of the white matter fibers, such as corticospinal (pyramidal) tracts, with relationship to brain lesions (infarcts, vascular malformations and brain tumors) is extremely helpful for stereotactic radiosurgery, preoperative evaluation and intraoperative navigation. Quantitative measurement of the tract is a very sensitive method to detect differences in the tract in neurodegenerative/neurocognitive/psychiatric patients such as amyotrophic lateral sclerosis, schizophrenia and Alzheimer diseases. Importance of this tool will become more significant in clinical and neuroscience fields in the future.