© MARK MILLER/SCIENCE SOURCEHuman brain tissue is a matter of gray and white. Gray matter tissue derives its moniker from the hue it acquires because of its concentration of neuronal cell bodies and surrounding blood vessels. About half of your brain’s volume is white matter—slender axonal fibers, insulated by fatty myelin, that are bundled together much like large cable cords and have a whitish color. These winding tracts connect distinct brain regions near and far.
White matter is now the object of close scrutiny, with multimillion-dollar efforts underway in the U.S. and Europe that aim to map and characterize the structural and functional links—the connectome—of the healthy human brain. Starring in these projects is diffusion magnetic resonance imaging (dMRI), a specialized type of MRI in use since the mid-1980s that is revealing white-matter structure in unprecedented detail.
Diffusion MRI takes advantage of the fact that water in brain tissue more readily diffuses parallel to axons than across them. In contrast, in the brain’s gray matter and extracellular space, water moves more freely in many directions. Accounting for these differences in diffusion has allowed researchers to indirectly infer the shape of white matter in the brain, and to begin to study ...
