Diffusion tensor imaging is actually a specialized form of magnetic resonance imaging (MRI). While MRI traces the hydrogen atoms in water molecules, DTI builds a picture out of the direction in which those molecules are moving. Neuron fibers are long and skinny, and molecules tend to diffuse along them. Researchers can highlight spots where water molecules--and sets of neuron fibers--are headed in the same direction.
Diffusion tensor images like this one (rendered differently than the previous image) can reveal how a brain tumor has affected neuron connections or can help guide brain surgery. It can also reveal subtle abnormalities associated with stroke, multiple sclerosis, schizophrenia, and even dyslexia.