A 58-year-old man from Delhi presents with sudden-onset severe headache and neck stiffness. CT head shows hyperdense material in the subarachnoid space. Regarding the imaging features of subarachnoid hemorrhage (SAH), all of the following are true EXCEPT:

Explanation

## Imaging Features of Subarachnoid Hemorrhage ### CT Findings in Acute SAH **Key Point:** Non-contrast CT (NCCT) is the gold standard for acute SAH detection. Hyperdensity (>40 HU) in the subarachnoid space, particularly in the basal cisterns and sulci, is the hallmark finding and remains visible for 5–7 days. ### Fisher Grading Scale **High-Yield:** Fisher grade predicts vasospasm risk and clinical outcome: - Grade 1: No blood - Grade 2: Diffuse thin layer (<1 mm) - Grade 3: Localized clot or thick layer (≥1 mm) - Grade 4: Intraventricular hemorrhage (IVH) ± SAH Grades 3 and 4 carry higher vasospasm risk. ### MRI in SAH **Clinical Pearl:** MRI is NOT first-line for acute SAH (CT is faster and more sensitive for blood). However, when MRI is performed: - **FLAIR sequences** show hyperintensity in subarachnoid spaces within **hours** of SAH, not minutes - **T2 sequences** may show hypointensity (blood) or hyperintensity (edema/CSF) - MRI is more useful for **subacute/chronic SAH** and detecting vasospasm complications **Warning:** The claim that T2/FLAIR hyperintensity appears "within minutes" is inaccurate. FLAIR changes appear within **hours**, not minutes. Acute blood (oxyhemoglobin) is isointense on T2 and may not show FLAIR hyperintensity immediately. ### CTA for Source Identification **Key Point:** CTA of the head and neck (including intracranial and extracranial vasculature) is the standard first-line imaging to identify the source of SAH: - Detects aneurysms in ~85–90% of cases - Evaluates for arteriovenous malformations (AVMs) - Assesses for arterial dissection - Sensitivity increases with modern multidetector scanners and thin-slice acquisition ### Summary Table | Modality | Acute SAH | Sensitivity | Role | | --- | --- | --- | --- | | NCCT | Hyperdensity in basal cisterns | ~95% | Gold standard, initial screening | | CTA | Aneurysm, AVM, dissection | ~85–90% | Source identification | | MRI/FLAIR | Hyperintensity (hours later) | Lower in acute phase | Subacute/chronic, complications | | DSA | Gold standard for aneurysm | ~100% | Therapeutic, not diagnostic | [cite:Harrison 21e Ch 296] ## Why the Correct Answer Is Correct Option 2 (T2/FLAIR hyperintensity within minutes) is **FALSE**. FLAIR hyperintensity in SAH appears within **hours**, not minutes. Acute blood (oxyhemoglobin) is isointense on T2 and does not produce immediate FLAIR signal change. This is a common misconception in neuroradiology.