What is the significance of the CT (Computed Tomography) collateral score in evaluating an adult patient with acute ischemic stroke?

CT Collateral Score in Acute Ischemic Stroke

The CT collateral score is a critical prognostic marker that independently predicts functional outcomes, infarct progression, and mortality in acute ischemic stroke patients, with good collaterals (typically score >3 on multiphase CTA) correlating with smaller infarct volumes, better 90-day functional independence, and greater benefit from reperfusion therapy including thrombectomy. 1

What the Collateral Score Measures

The collateral score quantifies the extent and quality of alternative blood flow pathways that bypass an occluded vessel to supply ischemic brain tissue. 2 This assessment can be performed using:

• Multiphase CT angiography (mCTA): Provides time-resolved depiction of collaterals through multiple phases, allowing semiquantitative grading with superior accuracy compared to single-phase techniques 2
• Single-phase CTA: Less accurate and tends to underestimate collateral status compared to mCTA 3
• CT perfusion (CTP): Can quantify collateral flow through parameters like maximum cerebral blood flow in collateral vessels 4
• Digital subtraction angiography (DSA): Considered the gold standard but invasive 2, 5

Prognostic Significance for Clinical Outcomes

Good collateral circulation independently predicts favorable functional outcome at 90 days after thrombectomy, even after adjusting for baseline stroke severity and recanalization success. 1, 6

Specific Outcome Predictions:

• Functional independence: Patients with good collaterals (mCTA score >3) have 84.8% sensitivity for achieving 90-day modified Rankin Scale (mRS) score of 0-2 3
• Hemorrhagic transformation risk: Higher collateral scores significantly correlate with lower risk of symptomatic intracerebral hemorrhage (sICH) 6, 4
• Infarct volume: Better collaterals correlate with smaller infarcts on CT and lower stroke scale scores 1
• Treatment benefit: Patients with good collaterals on baseline mCTA are more likely to benefit from endovascular thrombectomy (EVT) 2

Optimal Scoring Thresholds

A multiphase CTA collateral score of >3 optimally identifies patients with target mismatch on CT perfusion and predicts good clinical outcome. 3 This threshold provides:

• 78.4% sensitivity and 90.9% specificity for identifying target mismatch 3
• 84.8% sensitivity and 69.4% specificity for predicting 90-day mRS score of 0-2 3

The diagnostic efficacy of mCTA collateral score (AUC 0.697-0.902) is comparable to CT perfusion target mismatch assessment. 6, 3

Advantages of Multiphase CTA Over Single-Phase

Multiphase CTA should be the preferred method for collateral assessment because it:

• Predicts patient outcome better than single-phase CTA and CTP 2
• Has good interrater reliability 2
• Covers the whole brain 2
• Is robust against patient motion 2
• Requires no post-processing 2
• Provides useful information about extracranial vessel anatomy and clot length for procedural planning 2

Single-phase CTA significantly underestimates collateral status compared to mCTA (p < 0.01). 3

Impact on Treatment Decisions

For Thrombectomy Eligibility:

Collateral status should be integrated into treatment decision-making, particularly for patients with low ASPECTS scores or extended time windows. 1 The American Heart Association recommends that:

• Good collaterals predict benefit from reperfusion therapy even in patients with ASPECTS <6 1
• Collateral assessment helps distinguish fast versus slow progressors 1
• In posterior circulation strokes, a 6-point CTA collateral score correlates with good outcomes after thrombectomy 1

For Risk Stratification:

• Patients with poor collaterals have higher risk of hemorrhagic transformation and poor functional outcome despite treatment 2
• The collateral score has good application value in risk-benefit evaluation after mechanical thrombectomy 6

Critical Caveats and Pitfalls

Location-Dependent Effects:

Excellent collaterals do not guarantee good outcomes in all cases. 1 A proximal MCA occlusion may occlude lenticulostriate arteries supplying the basal ganglia and internal capsule—structures that cannot be collateralized from the cortex. 2, 1 Therefore, excellent transcortical collaterals might preserve cortex but permanent hemiplegia can still result from infarction of the internal capsule. 2, 1

Recanalization Dependency:

The benefit of collaterals depends on achieving successful recanalization—collaterals alone without reperfusion do not guarantee good outcomes. 1 This emphasizes that collateral assessment should guide but not replace the decision to pursue reperfusion therapy.

Technical Limitations:

• Extracranial stenoses and poor cardiac output can lead to underestimation of collaterals on mCTA 2
• Collaterals can fail over time, though late window patients still show diverse collateral grades 1
• CTP is subject to technical failures in up to 30% of patients and lacks standardized quantification 2

Practical Implementation

Grading System:

The proposed grading system evaluates collateral function based on:

• Extent of collateral filling to the ischemic territory 2
• Time to collateral filling compared to the contralateral hemisphere 2
• Slow collateral flow: filling >2 seconds slower than contralateral side 2
• Rapid collateral flow: filling within 2 seconds of contralateral side 2

Reading Efficiency:

Automated assistance software (e-CTA) can improve collateral scoring accuracy from 58.6% to 67.5% (p = 0.003), reduce reading time from 103.4 to 59.7 seconds (p = 0.001), and increase inter-rater agreement (Krippendorff's alpha from 0.366 to 0.676). 7

Integration with Other Imaging Markers

Collateral assessment should complement, not replace, ASPECTS scoring and clinical evaluation. 8 While ASPECTS quantifies early ischemic changes, collateral scores provide dynamic information about tissue viability and perfusion status. 2 The combination of ASPECTS and collateral scoring provides more comprehensive risk stratification than either alone. 1, 8

DWI-MRI remains the gold standard for ischemic core assessment, but mCTA collateral scoring offers a faster, more practical alternative in the emergency setting. 2