What does diffusion restriction on MRI indicate in brain imaging?

What does diffusion restriction on MRI indicate in brain imaging?

• A. Vasogenic edema
• B. Cytotoxic edema, often due to acute ischemia
• C. Demyelination
• D. Chronic scar

Answer: (B)

Diffusion restriction on MRI reflects limited movement of water molecules in tissue, which happens when cells swell and the intracellular space becomes crowded. In the brain, this cytotoxic edema occurs early in acute ischemia because energy failure halts the Na+/K+ pumps, causing water to shift inside cells. The result is reduced extracellular space and hindered diffusion, so diffusion-weighted images appear bright while the ADC map shows low values. Vasogenic edema, on the other hand, involves fluid accumulating outside cells due to blood-brain barrier breakdown and typically shows increased diffusion rather than restriction. Demyelination and chronic scar are not defined by restricted diffusion; they have other characteristic MRI features and diffusion patterns.

Diffusion restriction on MRI reflects limited movement of water molecules in tissue, which happens when cells swell and the intracellular space becomes crowded. In the brain, this cytotoxic edema occurs early in acute ischemia because energy failure halts the Na+/K+ pumps, causing water to shift inside cells. The result is reduced extracellular space and hindered diffusion, so diffusion-weighted images appear bright while the ADC map shows low values.

Vasogenic edema, on the other hand, involves fluid accumulating outside cells due to blood-brain barrier breakdown and typically shows increased diffusion rather than restriction. Demyelination and chronic scar are not defined by restricted diffusion; they have other characteristic MRI features and diffusion patterns.