Water inside nerve fibers doesn't move like it does in a glass: it constantly bumps into the walls of axons (the long projections of nerve cells). If you make the pause between MRI radio pulses very short, water doesn't have time to travel far, and its movement reflects the tightness of the surrounding space.

In the new study on living humans, they compared water mobility across fibers at short and long observation times. The difference was especially striking in the corticospinal tract — the main highway through which the brain commands muscles. That's where axons are thickest, and water takes longer to travel from one wall to the other.

Previously, such differences could only be seen in tissue samples under a microscope. Now MRI with oscillating gradients does the same without a single cut. The method promises to become a tool for tracking axon damage in multiple sclerosis and other diseases — long before symptoms appear.