A 58-year-old man from Delhi presents with acute left-sided weakness and speech difficulty of 2 hours duration. Non-contrast CT head is normal. Regarding the imaging findings in acute ischemic stroke, all of the following statements are true EXCEPT:

Explanation

## Acute Ischemic Stroke — Imaging Timeline and Modalities **Key Point:** Understanding the temporal evolution of imaging findings is critical for stroke diagnosis and management decisions. ### DWI-PWI Mismatch Concept Diffusion-weighted imaging (DWI) represents the gold standard for detecting acute ischemic stroke: - **DWI hyperintensity** appears within **minutes** of symptom onset (typically 30 minutes) - Reflects **restricted water diffusion** in cytotoxic edema - Remains hyperintense for 7–10 days (or longer) - **ADC hypointensity** confirms true restriction (rules out T2 shine-through) **High-Yield:** DWI is more sensitive and specific than conventional CT or FLAIR for acute ischemia in the first 24 hours. ### FLAIR Timing — The Trap **Warning:** FLAIR hyperintensity in acute stroke is **NOT an early finding**. Timeline: - FLAIR becomes hyperintense **after 6–8 hours** (or later) - DWI detects ischemia **hours before** FLAIR - FLAIR is useful for **subacute** (>24 hours) and **chronic** infarcts - In hyperacute phase, FLAIR may appear **normal** while DWI is already positive **Clinical Pearl:** A normal FLAIR with hyperintense DWI in the first few hours is pathognomonic for acute ischemia and rules out artifact. ### CT Perfusion (CTP) Role CTP maps tissue perfusion and identifies the **ischemic penumbra** (tissue at risk but not yet infarcted): - Helps identify candidates for **thrombolysis beyond 4.5 hours** (up to 24 hours in selected cases) - Mismatch between perfusion deficit and DWI infarct core guides intervention - Increasingly used in **extended window** protocols ### Summary Table | Modality | Timing | Finding | Sensitivity | | --- | --- | --- | --- | | **DWI** | Minutes (30 min–7 days) | Hyperintense + ADC hypointense | Highest (>95%) | | **FLAIR** | 6–8 hours onward | Hyperintense | Lower in hyperacute phase | | **Conventional CT** | Any time | May be normal acutely | Low for early ischemia | | **CTP** | Any time | Perfusion deficit, penumbra | Guides extended window therapy | **High-Yield:** Option 3 (FLAIR hyperintensity appearing before DWI) is **FALSE** — DWI is earlier and more sensitive.