[Magnetic resonance diffusion tractography in the brain--its application and limitation].

Diffusion tractography of magnetic resonance imaging (MRI), such as diffusion tensor tractography, allows us to visualize white matter tracts in vivo and to study white matter integrity quantitatively. Virtual dissection of the living human brain can be performed in the first time. We developed tracking software, dTV and VOLUME-ONE, in 2001, as a freeware (https://www.ut-radiology.umin.jp/people/masutani/dTV. htm), and we used it to visualize eloquent white matter bundles with relationship to brain tumors, cerebral infarctions and other lesions. We also used it for quantitative measurement of the specific tracts segmented by diffusion tensor tractography (tract-specific analysis) to reveal abnormalities in so-called normal appearing white matter. Three dimensional visualization of the white matter fibers such as corticospinal (pyramidal) tract, optic radiation and arcuate fasciculus with relationship to brain tumors such as gliomas was extremely helpful for preoperative evaluation and intraoperative navigation. We correlated tracking with intraoperative electric fiber stimulation to validate fiber tracking. In patients with small lacunar infarctions near the corticospinal tracts, relationship between the tract and fresh infarction correlated well with final (2 weeks-later) motor function. Quantitative measurement of the tract is a very sensitive tool. We analyzed the corticospinal and corticobulbar tracts in patients with amyotrophic lateral sclerosis (ALS). Changes of the diffusion parameters (fractional anisotropy and ADC) of the tracts were observed not only between normal controls but also between subtypes of ALS (limb- and bulbar-onset). Tract- specific analysis can also apply for the limbic-related tracts such as fornix, cingulum, uncinate fasciculus and etc. We observed differences in some of the fibers in neurocognitive/psychiatric patients such as schizophrenia and Alzheimer disease. Using fiber tracking, we can now develop white matter mapping. We visualized components of the pyramidal tract (fibers from foot, hand, face motor areas separately) and made a probabilistic map. Diffusion tractography is a unique tool to visualize and segment the white matter pathways and one can evaluate the segmented tract quantitatively. Importance of this tool will become more significant in clinical and neuroscience fields in the future.