Which imaging modality demonstrates restricted diffusion in acute ischemia?

• A. Diffusion-weighted MRI shows restricted diffusion.
• B. SPECT shows increased uptake.
• C. Ultrasound shows decreased flow.
• D. CT perfusion shows reduced blood flow.

Answer: (A)

Diffusion-weighted MRI reveals restricted diffusion when water movement is limited by cellular swelling that occurs early in ischemia. In acute stroke, energy failure causes the Na+/K+ pump to fail, leading to influx of water into neurons and glia (cytotoxic edema). This reduces the extracellular space and the freedom of water molecules to diffuse, so the diffusion signal becomes bright on DWI and the apparent diffusion coefficient (ADC) falls. This combination—bright on diffusion-weighted imaging with low ADC values—is a hallmark of true diffusion restriction and can appear within minutes to hours, often before changes are visible on other sequences. Other imaging modalities assess perfusion or blood flow rather than diffusion: perfusion studies show reduced blood supply, ultrasound detects flow, and functional imaging like SPECT tracks metabolic activity. So diffusion-weighted MRI uniquely demonstrates the restricted diffusion characteristic of acute ischemia.