What is the effectiveness of CT (Computed Tomography) perfusion in diagnosing brainstem stroke?

CT Perfusion in Brainstem Stroke: Limited Diagnostic Value

CT perfusion has historically been of limited value in the posterior circulation, particularly for brainstem ischemia, due to reduced accuracy for small lesions and infratentorial structures. 1, 2

Evidence from Posterior Circulation Studies

Diagnostic Performance in Posterior Circulation

The most relevant evidence comes from the Dutch Acute Stroke Study (DUST), a prospective multicenter observational study of 88 patients with suspected posterior circulation stroke: 1

• CT perfusion showed 74% sensitivity and 80% negative predictive value for posterior circulation stroke, which is substantially better than non-contrast CT (31% sensitivity, 61% NPV) or CTA alone (33% sensitivity, 62% NPV) 1
• However, this study did not specifically evaluate large vessel occlusions, and the overall sensitivity remains suboptimal for clinical decision-making 1

Specific Limitations for Brainstem Imaging

The fundamental problem is that CT perfusion has limited accuracy for small ischemic lesions and brainstem ischemia specifically. 2 This occurs because:

• The brainstem contains small, densely packed structures where perfusion abnormalities may be difficult to distinguish from artifacts 2
• Limited spatial resolution makes detection of small brainstem infarcts challenging 2
• Beam hardening artifacts from the skull base can degrade image quality in the posterior fossa 2

Clinical Context from BASICS Trial

In the Basilar Artery International Cooperation Study (BASICS), only 27 of 592 patients (4.6%) underwent CTA and CT perfusion evaluation: 1

• Mean transit time was abnormal in 93% of these patients 1
• Cerebral blood volume correlated with risk of death 1
• For patients with pc-ASPECTS <8 on cerebral blood volume maps, all had died at 1 month compared with 6 of 23 patients with pc-ASPECTS ≥8 (RR 3.8,95% CI 1.9 to 7.6) 1

This suggests CT perfusion may have prognostic value when abnormalities are detected, but the very small number of patients studied (4.6%) indicates it was not considered reliable enough for routine use in posterior circulation stroke. 1

Comparison to Alternative Imaging

MRI Superiority for Posterior Circulation

Diffusion-weighted MRI is the most sensitive test for posterior circulation ischemia, though it can initially be normal in 6-10% of cases—twice as often as in anterior circulation stroke. 1, 3

• DWI achieves 88-100% sensitivity and 95-100% specificity for acute infarction overall 3
• DWI can detect small cortical and subcortical lesions, including those in the brainstem or cerebellum, areas often poorly visualized with CT 3

When CT Perfusion May Still Be Considered

The 2007 AHA/ASA guidelines note that dynamic perfusion CT has potential to provide absolute measures of cerebral blood flow, mean transit time, and cerebral blood volume, but is "currently limited to 2 to 4 brain slices and provides incomplete visualization of all pertinent vascular territories." 1 This limitation is particularly problematic for brainstem evaluation where precise localization across multiple levels is critical.

Practical Clinical Algorithm

For suspected brainstem stroke, the imaging approach should be: 1, 3

1. Non-contrast CT first to exclude hemorrhage (sensitivity 71% for basilar artery occlusion via hyperdense basilar sign) 1
2. CTA to identify large vessel occlusion (primary method for identifying posterior circulation occlusions) 1
3. MRI with DWI as the definitive test when available and not contraindicated, particularly for small brainstem lesions 1, 3
4. CT perfusion may be considered if MRI is unavailable and there is diagnostic uncertainty, but recognize its substantial limitations for brainstem evaluation 1, 2

Critical Caveats

• Do not rely on CT perfusion alone to exclude brainstem stroke—negative CT perfusion does not rule out brainstem ischemia given the 74% sensitivity 1
• CT perfusion should not delay treatment decisions in patients eligible for acute therapies 1
• The 2013 AHA/ASA guidelines emphasize that "the performance of these additional imaging sequences should not unduly delay treatment with intravenous rtPA in the appropriate patients" 1