A 58-year-old man presents to the emergency department 90 minutes after acute onset of right-sided weakness and expressive aphasia. Non-contrast CT head is unremarkable. MRI brain with DWI is performed, which shows a hyperintense lesion in the region marked **A** (cortical gray matter of the left frontal lobe) with corresponding hypointensity on the ADC map. Which of the following best explains the imaging findings in this acute ischemic stroke?

Explanation

## Why option 1 is correct The hyperintensity on DWI with corresponding hypointensity on the ADC map in the cortical gray matter region marked **A** is pathognomonic for restricted diffusion due to cytotoxic edema. This is the hallmark of acute ischemic stroke and is the MOST SENSITIVE MRI sequence for detecting acute cerebral ischemia. DWI detects ischemic changes within MINUTES of symptom onset (in this case, 90 minutes), achieving 90-95% sensitivity in the first 24 hours. The restricted water diffusion reflects the shift of water from the extracellular to intracellular compartment during acute ischemia, before irreversible cell death occurs. This finding directly supports the diagnosis of acute ischemic stroke and guides urgent thrombolytic or thrombectomy intervention (Harrison 21e Ch 426). ## Why each distractor is wrong - **Option 2 (Vasogenic edema)**: Vasogenic edema from blood-brain barrier disruption is a later phenomenon (develops over hours to days) and appears hyperintense on T2-weighted and FLAIR sequences, NOT on DWI. Moreover, vasogenic edema shows INCREASED diffusion (hyperintense on ADC), not restricted diffusion. This pattern is more typical of subacute/chronic infarction or vasculitis, not acute ischemia. - **Option 3 (Chronic infarction)**: Chronic infarcts do show persistent T2-FLAIR hyperintensity but DWI normalizes or pseudonormalizes after 7-10 days as cytotoxic edema resolves. The presence of restricted diffusion (DWI hyperintense + ADC hypointense) is incompatible with chronic infarction, which would show normal or increased ADC values. - **Option 4 (Hemorrhagic transformation)**: Hemorrhagic transformation is a complication that occurs AFTER thrombolysis or spontaneously in the subacute phase (typically 24-48 hours post-stroke). It is better detected on gradient echo or susceptibility-weighted imaging (SWI) and non-contrast CT. Acute hemorrhage does not produce the characteristic DWI hyperintensity + ADC hypointensity pattern of cytotoxic edema. **High-Yield:** DWI hyperintense + ADC hypointense = ACUTE ISCHEMIC STROKE (cytotoxic edema); detectable within MINUTES; normalizes after 7-10 days. NCCT rules out hemorrhage first; MRI detects ischemia faster than CT but takes longer to acquire. [cite: Harrison 21e Ch 426 — Acute Ischemic Stroke]