Which imaging modality is most sensitive for detecting early ischemia after stroke onset?

• A. Non-contrast CT
• B. CT perfusion
• C. Diffusion-weighted MRI
• D. CT angiography

Answer: (C)

Diffusion-weighted imaging is the most sensitive for detecting early ischemia because it directly shows cytotoxic edema that results from energy failure in brain cells. When a vessel is occluded and blood flow drops, cells lose their ability to regulate ions, water begins to accumulate inside cells, and diffusion of water becomes restricted. This restriction shows up as bright signal on diffusion-weighted images (with a corresponding low signal on ADC maps) within minutes of stroke onset, sometimes even before any visible changes appear on other imaging.

Non-contrast CT often looks normal in the very early hours because density changes take longer to develop. CT perfusion reveals areas of reduced blood flow or altered perfusion, which is helpful for selecting therapies but does not detect the earliest tissue injury as reliably as diffusion-weighted MRI. CT angiography shows the anatomy of vessels and occlusions but does not indicate tissue viability as sensitively as diffusion-weighted imaging.

So, for detecting early ischemia, diffusion-weighted MRI best reveals the initial tissue injury that defines the acute stroke.