A 58-year-old man from Delhi presents to the emergency department with acute onset right-sided weakness and speech difficulty that began 2 hours ago. Blood pressure is 165/95 mmHg, heart rate 88/min, and blood glucose 145 mg/dL. Non-contrast CT head shows no hyperdensity or hypodensity. The patient is being considered for thrombolytic therapy. Which of the following MRI sequences would be MOST sensitive for detecting acute ischemic changes in the first 30 minutes of symptom onset?

Explanation

## Acute Ischemic Stroke Detection on MRI **High-Yield:** Diffusion-weighted imaging (DWI) is the GOLD STANDARD for detecting acute cerebral ischemia within the hyperacute window (first 30 minutes to 6 hours post-onset) [cite:Harrison 21e Ch 445]. ### Mechanism of DWI Sensitivity 1. **Cytotoxic edema** develops within minutes of ischemia onset 2. Water molecules shift from extracellular to intracellular compartments 3. Restricted water diffusion appears as **hyperintensity (bright signal)** on DWI 4. Corresponding **hypointensity on ADC (apparent diffusion coefficient) maps** confirms restricted diffusion (not T2 shine-through) ### Timing of MRI Sequences in Acute Stroke | Sequence | Timing | Sensitivity | Clinical Use | |----------|--------|-------------|---------------| | **DWI** | 0–6 hours | >95% | Acute infarct detection | | **FLAIR** | >6 hours | Better for subacute | T2 hyperintensity + CSF suppression | | **T2-weighted** | >12 hours | Delayed edema | Subacute/chronic phase | | **SWI** | Any time | Microhemorrhages | Detect prior bleeds, contraindicate thrombolysis | **Key Point:** DWI can detect ischemia **before conventional CT or T2-weighted MRI** becomes abnormal. This is critical for thrombolytic eligibility assessment. **Clinical Pearl:** A negative DWI in the first 6 hours essentially excludes acute ischemic stroke; a positive DWI confirms infarction even if CT and conventional MRI appear normal. ### Why DWI Wins in the Hyperacute Phase - Non-contrast CT: Detects only hemorrhage (hyperdensity) or late hypodensity (>24 hours) - FLAIR: Becomes positive only after 6–8 hours (subacute phase) - T2-weighted: Insensitive in first 6–12 hours - DWI: Positive within **minutes** of ischemia onset **Mnemonic:** **DWIT** = **D**iffusion **W**eighted **I**maging **T**iming (earliest positive in acute stroke)