Initial Imaging for Suspected Subarachnoid Hemorrhage
Non-contrast CT scan of the head is the recommended initial imaging test for suspected subarachnoid hemorrhage, with sensitivity of 93-98% overall and approaching 98.7-100% when performed within 6 hours of symptom onset using modern third-generation or higher CT scanners. 1, 2
Why Non-Contrast CT is Superior
- Non-contrast CT is the modality of choice because it is superior to MRI in detecting acute subarachnoid blood, which appears hyperdense (white) on imaging 1
- The American College of Radiology specifically recommends against using contrast in the initial CT scan because the goal is to detect blood, not characterize vessels 1
- Modern 16-slice or better CT scanners have demonstrated sensitivity of 97.8% (95% CI: 93.1-99.4%) for detecting SAH 3
Critical Timing Considerations
Within 6 hours of symptom onset, non-contrast head CT alone is sufficient when performed on high-quality scanners and interpreted by experienced radiologists, with sensitivity approaching 98.7-100% 2
- The sensitivity of CT decreases significantly after 6 hours: 93% at 24 hours and 57-85% by day 6 4
- If CT is performed after 6 hours of symptom onset and is negative, lumbar puncture for xanthochromia evaluation becomes necessary 1, 2
Important Caveats
The 6-hour rule does not apply to patients with atypical presentations, who require full workup regardless of timing 2:
- Primary neck pain without headache
- Syncope as presenting symptom
- New seizure
- New focal neurological deficits
Subsequent Vascular Imaging
Once SAH is confirmed on non-contrast CT, the diagnostic sequence proceeds as follows 1:
- CT angiography (CTA) with contrast is the preferred next step to identify aneurysms or malformations, with sensitivity >95% for aneurysms ≥3mm and 96.5% overall sensitivity 1, 2
- CTA is faster and less invasive than conventional angiography 1
- Digital subtraction angiography (DSA) remains the gold standard with sensitivity and specificity >98%, and should be performed when CTA is negative but SAH pattern suggests aneurysmal source, especially for small aneurysms <3mm 1, 2
Common Pitfalls to Avoid
- Misdiagnosis of SAH occurs in up to 12% of cases, with the most common error being failure to obtain a non-contrast cranial CT 4
- Do not order contrast-enhanced CT as the initial study—this defeats the purpose of detecting blood 1
- Equipment quality matters: the high sensitivity rates only apply to third-generation or higher CT scanners meeting ACR specifications 2
- In patients with non-diagnostic lumbar puncture results, secondary imaging (CTA or MRA) should still be performed, as aneurysms requiring treatment have been identified in this population 5