Carl-Fredrik Westin

Imagine a water park with many pools connected by narrow passages. Visitors, water molecules, splash around in each pool, but occasionally move to the next one. In a brain tumor, these "passages" are protein channels in cell membranes. Their abundance determines how quickly water moves between cells.

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.