A 58-year-old man from Delhi presents to the emergency department 2.5 hours after acute onset of right-sided weakness and expressive aphasia. Blood pressure is 165/95 mmHg, heart rate 88/min. Non-contrast CT head shows no hyperdensity. MRI brain with DWI sequence is performed. What finding on DWI-weighted imaging would be most specific for acute ischemic stroke in this patient?

Explanation

## Acute Ischemic Stroke: DWI-ADC Correlation **Key Point:** Diffusion-weighted imaging (DWI) is the most sensitive MRI sequence for detecting acute ischemic stroke, with sensitivity >90% within the first few hours of symptom onset—far superior to conventional CT or standard MRI sequences. ### DWI-ADC Relationship in Acute Ischemia In acute ischemic stroke, cytotoxic edema develops due to failure of the Na⁺/K⁺-ATPase pump: 1. **Hyperintense DWI signal** — restricted water diffusion in the ischemic tissue 2. **Low ADC (Apparent Diffusion Coefficient) values** — confirms true restriction, not T2 shine-through 3. **Timing:** Appears within **minutes** of symptom onset; peaks at 3–7 days; may normalize by 2–3 weeks **High-Yield:** The combination of **DWI hyperintensity + low ADC** is pathognomonic for acute ischemic stroke and is present even when conventional CT is normal (as in this case). ### Why DWI Outperforms Other Sequences | Sequence | Acute Stroke (0–24 hrs) | Chronic Stroke | Sensitivity | |---|---|---|---| | **Non-contrast CT** | Normal or subtle hypodensity | Hypodensity | ~20% in first 6 hrs | | **Conventional MRI (T1/T2)** | May be normal in first 6–8 hrs | Hyperintense on T2/FLAIR | ~60% in first 24 hrs | | **DWI + ADC** | Hyperintense DWI + low ADC | May show ADC pseudonormalization | >90% in first hours | | **FLAIR** | May be normal early | Hyperintense | ~70% after 6 hrs | **Clinical Pearl:** In this patient, the normal non-contrast CT rules out hemorrhage and makes thrombolysis/thrombectomy safe; DWI-ADC confirmation of acute ischemia guides urgent reperfusion therapy. ### Interpretation Algorithm ```mermaid flowchart TD A[Acute focal neurological deficit]:::outcome --> B{Time since onset?}:::decision B -->|< 24 hours| C[Perform DWI-ADC MRI]:::action C --> D{DWI hyperintense?}:::decision D -->|Yes| E{ADC low?}:::decision E -->|Yes| F[Acute ischemic stroke confirmed]:::outcome E -->|No| G[T2 shine-through or vasogenic edema]:::outcome D -->|No| H[Acute stroke unlikely; consider other diagnosis]:::outcome B -->|> 24 hours| I[Conventional MRI or CT may suffice]:::action ``` **Mnemonic:** **DWI-ADC = Acute Ischemia** - **D**iffusion-weighted imaging hyperintense - **W**ater restricted (low ADC) - **I**schemia confirmed - **A**cute (minutes to days) - **D**iagnosis made - **C**ytotoxic edema [cite:Harrison 21e Ch 379]