A 35-year-old woman with acute onset severe headache ('thunderclap') has normal CT head. Lumbar puncture is traumatic on the first attempt. Which MRI sequence most reliably detects subarachnoid haemorrhage in this scenario?

• A T1-weighted with gadolinium
• B DWI (Diffusion-weighted imaging)
• C FLAIR (Fluid-attenuated inversion recovery) ✓
• D T2*-weighted gradient echo (GRE)

Explanation

FLAIR MRI is the most sensitive MRI sequence for detecting subarachnoid haemorrhage when CT is negative (typically after 6–24 hours when the haemoglobin concentration in CSF dilutes below CT detection threshold). On FLAIR, the CSF signal (normally suppressed black) becomes hyperintense due to the protein/blood products. T2* GRE and SWI detect haemosiderin from chronic haemorrhage but are less sensitive for acute SAH in CSF. DWI detects ischaemia, not SAH. FLAIR is also more reliable than the xanthochromia/spectrophotometry interpretation on a traumatic tap.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.