How do you interpret Computed Tomography (CT) perfusion scan to predict cerebral vasospasm?

From the Guidelines

CT perfusion scans should be interpreted with a focus on cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) to predict cerebral vasospasm, with a sensitivity of 74% and specificity of 93% as demonstrated in a meta-analysis 1. To accurately interpret CT perfusion scans, it is essential to understand the key metrics and their implications.

• Cerebral blood flow (CBF) is a critical parameter, with reductions of more than 30-40% compared to the contralateral side being highly concerning for vasospasm.
• Cerebral blood volume (CBV) is typically preserved or slightly decreased in vasospasm.
• Mean transit time (MTT) is the most sensitive indicator, with prolongation exceeding 6.4 seconds suggesting a significant risk of vasospasm. When evaluating the scan, compare the affected region to both the contralateral hemisphere and baseline values if available.
• Areas showing perfusion mismatch (decreased CBF with normal or near-normal CBV) represent tissue at risk but potentially salvageable. The American Heart Association/American Stroke Association guidelines recommend using CT perfusion to detect vasospasm and predict delayed cerebral ischemia (DCI) in patients with aneurysmal subarachnoid hemorrhage (aSAH) 1. Early detection of vasospasm allows for prompt intervention with hypertensive therapy, calcium channel blockers like nimodipine, or endovascular treatments before permanent ischemic damage occurs.
• The use of CT perfusion to guide treatment decisions in the setting of neurologic symptoms of delayed cerebral ischemia did not improve outcomes when compared with treating all patients without imaging guidance 1. However, the ability to detect vasospasm and predict DCI makes CT perfusion a valuable tool in the management of patients with aSAH.

From the Research

Interpreting Computed Tomography (CT) Perfusion Scan

To predict cerebral vasospasm, the following parameters can be used to interpret CT perfusion scans:

• Mean Transit Time (MTT): MTT is a significant predictor of vasospasm, with abnormal values ranging from 123 to 221% of control values 2. An MTT value of 5.5s has been shown to have 94% specificity and 100% sensitivity for predicting the risk of developing vasospasm 3.
• Cerebral Blood Flow (CBF): Lower mean early CBF values are correlated with the occurrence of delayed cerebral ischemia (DCI) and vasospasm 3.
• Cerebral Blood Volume (CBV): CBV values can be used in combination with MTT and CBF to predict vasospasm, although it is not as sensitive as MTT 2, 4.
• Time to Peak (TTP): TTP can be used to calculate the cerebral circulation time (CCT), which is a significant predictor of vasospasm 5.

Predicting Vasospasm

The following factors can be used to predict vasospasm:

• MTT values: An increase in MTT <1 hour after subarachnoid hemorrhage (SAH) independently predicts mortality within 48 hours of SAH 6.
• CCT values: Prolonged CCT values are significantly correlated with vasospasm, with a cut-off value of 5.62s having a sensitivity of 84.2% and specificity of 82.4% for detecting vasospasm 5.
• Combination of parameters: A combination of CTA and PCT parameters, including MTT, CBF, and CBV, can accurately predict vasospasm and identify patients at high risk of developing DCI and vasospasm 4, 3.

Clinical Implications

The ability to predict vasospasm using CT perfusion scans has significant clinical implications:

• Early identification of patients at high risk of developing vasospasm and DCI can prompt more robust preventative measures and treatment 3.
• CT perfusion scans can be used to monitor patients with SAH and detect vasospasm early, allowing for timely intervention and improving patient outcomes 2, 4.