Diffusion tensor imaging (DTI), a form of magnetic resonance imaging, has become popular in neuroscience. By analyzing the direction of water diffusion in the brain, it can reveal the organization of bundles of nerve fibers, or axons, and how they connect—providing insight on conditions such as autism.

But conventional DTI has its limits. For example, when fibers cross, DTI can’t accurately analyze the signal: the different directions of water flow effectively cancel each other out. Given that an estimated 60 to 90 percent of voxels (cubic-millimeter sections of brain tissue) contain more than one fiber bundle, this isn’t a minor problem. In addition, conventional DTI can’t interpret water flow that lacks directionality, such as that within the brain’s abundant glial cells or the freely diffusing water that results from inflammation—so misses part of the story. …