Diffusion-weighted magnetic resonance imaging is a specialized technique that measures the degree of diffusion of water molecules within extracellular space and between intracellular and extracellular space. Diffusion-weighted imaging signal is high (bright) when diffusion is restricted, as occurs in cytotoxic damage from ischemia, inflammation, trauma, or tumor. This technique, now available on most magnetic resonance imaging units, is especially helpful in detecting early ischemic stroke and multiple sclerosis and in differentiating arachnoid cyst from epidermoid tumor and brain abscess from neoplasm.