Sensitivity of CT Angiography (CTA) for Detecting Vasospasm
CTA demonstrates a sensitivity of 80% and specificity of 93% for detecting cerebral vasospasm, making it the preferred non-invasive imaging modality for this indication. 1
Performance Characteristics by Vessel Location
Proximal (Central) Vessels
CTA achieves highest accuracy in large proximal intracranial vessels with sensitivity ranging from 82-91% and specificity of 90-97% 1, 2, 3
Detection rates are particularly strong for:
- Middle cerebral artery (MCA) M1 segments
- Suprasellar internal carotid artery (ICA)
- Basilar artery
- Anterior cerebral artery (ACA) A1 segments 4
CTA shows excellent performance for severe vasospasm (>50% luminal reduction) with 100% accuracy in proximal locations 4
For absent vasospasm, CTA demonstrates 96% accuracy in proximal vessels 4
Distal (Peripheral) Vessels
- Accuracy diminishes significantly in distal vascular territories 1
- CTA correlation with conventional angiography decreases for smaller, more distal arteries 1
- Detection of M2 segments and A2 segments shows lower correlation (0.152-0.446) compared to proximal vessels (0.88-1.00) 4
Severity Grading Performance
CTA demonstrates variable accuracy depending on vasospasm severity:
- Severe spasm (>50% reduction): 100% accuracy 4
- No spasm: 92% agreement with DSA 4
- Moderate spasm (30-50% reduction): 64% agreement 4
- Mild spasm (<30% reduction): 57% agreement 4
This pattern indicates CTA is most reliable for ruling in severe vasospasm or ruling out vasospasm entirely, but less dependable for intermediate grades 4
Comparison with Other Modalities
CTA vs. Digital Subtraction Angiography (DSA)
- DSA remains the gold standard with 100% sensitivity 1
- CTA approaches DSA accuracy for proximal vessels but cannot replace it for definitive diagnosis of distal vasospasm 1
- CTA has twice the spatial resolution of MRA but only half that of DSA 1
CTA vs. Transcranial Doppler (TCD)
- TCD shows lower sensitivity (38%) and specificity (91%) compared to CTA 3
- TCD achieves sensitivities of 70-90% for MCA vasospasm specifically, but performance drops in posterior circulation 1
- CTA demonstrates superior positive likelihood ratio (27.3) compared to TCD for ruling in vasospasm 3
CTA vs. MRA
- MRA is not recommended for vasospasm detection due to limitations from background hemorrhage and hemodynamic flow alterations 1
- No relevant literature supports MRA use in suspected cerebral vasospasm 1
Clinical Application and Limitations
Optimal Use Cases
- CTA combined with CT perfusion achieves 93% accuracy for vasospasm diagnosis 5
- CTA is particularly useful when TCD readings become elevated and neurological examination is limited 1
- CTA has 90% diagnostic accuracy and only 5% false-positive rate when symptoms develop 1
Important Caveats
- Heavy atheromatous calcifications can produce blooming artifacts that may lead to false-positive results for stenosis 1
- Metal artifacts from coils or clips can obscure adjacent arterial segments, and a negative CTA should not prevent further investigation in these areas 2
- CTA provides static images and cannot assess flow rates, direction, or collateral circulation as effectively as DSA 1
Treatment Decision-Making
- CTA is helpful for determining need for balloon angioplasty with significant predictive value 2
- CT perfusion-derived mean transit time (MTT) ≥6.4 seconds combined with qualitative CTA assessment represents the most accurate combination (93%) for vasospasm diagnosis 5
- Regional cerebral blood flow ≤39.3 mL×100g⁻¹×min⁻¹ on CT perfusion is the most accurate indicator (94.8%) for selecting patients requiring endovascular therapy 5
Guideline Recommendations
The 2023 AHA/ASA guidelines give CTA a Class 2a recommendation (Level B-NR evidence) for detecting vasospasm and predicting delayed cerebral ischemia in patients with aneurysmal subarachnoid hemorrhage 1
The 2021 ACR Appropriateness Criteria support CTA as providing less invasive evaluation compared to catheter angiography, with the caveat that correlation decreases in smaller distal arteries 1