Diffusion-Weighted Imaging (DWI): NEET PG Radiology
1. The Physics (Simplified)
DWI measures the random (Brownian) motion of water molecules. In healthy brain tissue, water moves relatively freely. In areas of cytotoxic edema (as seen in acute stroke), cell membranes fail, water becomes trapped intracellularly, and its motion is restricted.
2. The DWI/ADC Paradigm (The “Gold Standard”)
| DWI Signal | ADC Map | Interpretation |
|---|---|---|
| Bright (Hyperintense) | Dark (Hypointense) | True Restricted Diffusion (Acute Infarct). |
| Bright (Hyperintense) | Bright (Hyperintense) | T2 Shine-Through (Not restricted; usually subacute). |
3. High-Yield Clinical Pearls
- Sensitivity: DWI is the most sensitive sequence for detecting early ischemic changes, often showing abnormalities within minutes of symptom onset (before changes are visible on CT).
- Differential Diagnosis for Restricted Diffusion: Besides an acute infarct, look for:
- Acute Abscesses: Typically show restricted diffusion centrally.
- Cellular Tumors: Highly cellular tumors (e.g., Lymphoma, Medulloblastoma) can show restricted diffusion.
- Epidermoid Cysts: Often restrict diffusion (helps distinguish from Arachnoid Cysts, which do not restrict).