A 58-year-old man from Delhi presents to the emergency department with acute onset left-sided weakness and speech difficulty that began 2 hours ago. His wife reports he was fine while watching television. On examination, he has left hemiparesis (grade 3/5) and expressive aphasia. Blood pressure is 156/92 mmHg, heart rate 88/min, regular. Non-contrast CT head is normal. What is the next best imaging modality to confirm the diagnosis and guide thrombolytic therapy?

Explanation

## Clinical Context This patient presents with acute ischemic stroke symptoms (left hemiparesis, expressive aphasia) within the thrombolytic window (2 hours from onset). Non-contrast CT has ruled out hemorrhage, which is the critical first step. ## Why DWI-MRI is the Gold Standard **Key Point:** DWI-MRI is the most sensitive imaging modality for acute cerebral ischemia, detecting cytotoxic edema within minutes of symptom onset, whereas conventional CT may appear normal for 6–24 hours. **High-Yield:** DWI shows restricted water diffusion (bright signal on DWI, low ADC) in ischemic tissue, confirming acute infarction and helping: 1. Confirm diagnosis when CT is normal 2. Identify the vascular territory (right middle cerebral artery territory in this case — Broca's area) 3. Exclude stroke mimics (Todd's paralysis, hemiplegic migraine, functional weakness) 4. Guide thrombolytic eligibility and timing ## Imaging Comparison Table | Modality | Sensitivity (Acute) | Timing | Clinical Use | Limitations | | --- | --- | --- | --- | --- | | **Non-contrast CT** | ~10% (first 24 h) | Immediate | Rule out hemorrhage | Insensitive for early ischemia | | **DWI-MRI** | >90% (within minutes) | 5–10 min | Confirm acute ischemia, guide thrombolysis | Contraindicated if metal implants; longer acquisition | | **Perfusion CT** | Good (mismatch detection) | 5–10 min | Assess penumbra; useful in extended window | Less specific than DWI; requires contrast | | **CTA** | Good (vessel occlusion) | 5–10 min | Identify large vessel occlusion (LVO) | Does not confirm infarction; requires contrast | | **DSA** | Gold standard (vessel anatomy) | 30–60 min | Therapeutic intervention (thrombectomy) | Invasive; not for diagnosis alone | ## Clinical Pearl **DWI-MRI + perfusion-weighted imaging (PWI) mismatch** identifies the ischemic penumbra — tissue at risk of infarction. This is crucial for thrombolytic decision-making beyond the standard 4.5-hour window (up to 24 hours in selected cases with mismatch). ## Workflow in Acute Stroke ```mermaid flowchart TD A[Acute stroke symptoms]:::outcome --> B[Non-contrast CT]:::action B --> C{Hemorrhage?}:::decision C -->|Yes| D[Neurosurgery consult]:::urgent C -->|No| E[DWI-MRI]:::action E --> F{Acute ischemia confirmed?}:::decision F -->|Yes| G[Assess vessel occlusion]:::decision G -->|LVO present| H[Thrombectomy candidate]:::action G -->|No LVO| I[IV thrombolysis if eligible]:::action F -->|No| J[Stroke mimic — treat underlying cause]:::outcome ``` **Reasoning:** In acute ischemic stroke within the thrombolytic window, DWI-MRI is the imaging gold standard for confirming acute infarction when CT is normal, identifying the vascular territory, and guiding treatment decisions.