How do you identify subarachnoid hemorrhage (SAH) on a non-contrast computed tomography (CT) scan?

Identifying Subarachnoid Hemorrhage on Non-Contrast CT

Non-contrast CT head is the cornerstone imaging modality for diagnosing SAH, with blood appearing as hyperdense (bright white) material in the subarachnoid spaces, particularly in the basal cisterns, sylvian fissures, and sulci. 1

CT Appearance of SAH

Classic Imaging Findings

• Acute blood appears hyperdense (white) on non-contrast CT, filling the normally dark CSF spaces around the brain 2
• Look for hyperdensity in these key locations 1:
  • Basal cisterns (suprasellar, interpeduncular, ambient cisterns)
  • Sylvian fissures (most common location)
  • Interhemispheric fissure
  • Cerebral sulci over the convexities
  • Fourth ventricle and posterior fossa cisterns

Distribution Patterns

• Diffuse SAH suggests aneurysmal rupture and requires vascular imaging with CTA or catheter angiography 1
• Perimesencephalic pattern (blood centered around midbrain/pons) has better prognosis and CTA may be sufficient to exclude aneurysm 1
• Convexity SAH (isolated to brain surface) suggests non-aneurysmal causes like amyloid angiopathy or reversible vasoconstriction syndrome 1

Diagnostic Performance

Time-Dependent Sensitivity

CT sensitivity is highest immediately after hemorrhage and declines over time 1:

• 98-100% sensitivity in first 12 hours 1
• 93% sensitivity at 24 hours 1
• 57-85% sensitivity at 6 days 1

Recent data suggests 99.5% sensitivity when CT is performed within 6 hours of symptom onset using modern third-generation or newer scanners 3. However, this high sensitivity requires interpretation by an experienced neuroradiologist 2.

When CT is Negative

If CT is negative but clinical suspicion remains high, lumbar puncture must be performed 1, 2. This is particularly critical when:

• CT performed >6 hours after symptom onset 2
• CT not interpreted by experienced neuroradiologist 2
• Clinical presentation strongly suggests SAH (thunderclap headache, sudden severe headache) 2

The only exception: A third-generation or higher CT performed within 6 hours of headache onset and interpreted as normal by a neuroradiologist does not require lumbar puncture 2. However, given that missing SAH has >40% mortality, many centers maintain a lower threshold for LP 2.

Associated Findings to Identify

Beyond the blood itself, look for:

• Hydrocephalus (enlarged ventricles from CSF obstruction) 1
• Intraventricular hemorrhage (blood in ventricles) 1
• Intracerebral hematoma (associated parenchymal bleeding) 1
• Mass effect or midline shift 1

Common Pitfalls

• Small hemorrhages may be missed, particularly in posterior fossa or if scan performed days after ictus 1
• Anemia can reduce blood density, making SAH harder to detect 1
• Severe anemia or coagulopathy may cause blood to appear less hyperdense 1
• Never use contrast for initial SAH detection - the goal is to see blood, not vessels 2

Next Steps After Positive CT

Once SAH is confirmed on CT, immediately obtain CTA head with contrast to identify the bleeding source 1, 2:

• CTA has >95% sensitivity for aneurysms ≥3mm 1, 2
• Catheter angiography remains gold standard with >98% sensitivity and specificity for all aneurysm sizes 1
• 70% of non-traumatic SAH cases have an identifiable aneurysm 2