When is CT perfusion indicated for adults with suspected acute ischemic stroke?

When to Perform CT Perfusion in Acute Ischemic Stroke

CT perfusion is not routinely indicated for patients presenting within 3-4.5 hours who are straightforward IV tPA candidates—non-contrast CT alone is sufficient to exclude hemorrhage and make treatment decisions. 1 However, CT perfusion becomes essential for patient selection when considering endovascular therapy beyond 6 hours or when evaluating patients with uncertain time windows. 1, 2

Within the First 6 Hours: CTP is Optional

• For IV tPA candidates within 4.5 hours, non-contrast CT to exclude hemorrhage is all that is required—additional perfusion imaging should not delay treatment. 1, 2
• For endovascular therapy candidates within 6 hours, vascular imaging (CTA) is strongly recommended to identify large vessel occlusions, but perfusion imaging remains optional since >80% of patients in this window have salvageable tissue. 1, 2
• The role of CT perfusion in making acute treatment decisions within 6 hours has not been established, and it should never delay IV tPA administration. 1

Beyond 6 Hours: CTP is Mandatory

Multimodal imaging with perfusion assessment becomes essential for treatment selection in patients presenting 6-24 hours after symptom onset. 1, 2 This is when CT perfusion transitions from optional to mandatory:

• CT perfusion identifies the ischemic penumbra (salvageable tissue) and distinguishes it from the irreversibly infarcted core, which is critical for selecting patients who will benefit from late-window thrombectomy. 1, 2, 3
• The DAWN and DEFUSE-3 trials established that thrombectomy in the 6-24 hour window produces excellent outcomes (45-49% functional independence vs. 13-17% with medical therapy alone) when patients are selected using perfusion imaging criteria. 2
• Key perfusion thresholds for late-window selection include: core volume <70 mL, mismatch ratio ≥1.8, and absolute mismatch volume >10-15 mL. 2, 4

Specific Clinical Scenarios Requiring CTP

Wake-Up Stroke or Unknown Time of Onset

• Perfusion imaging helps identify patients with salvageable tissue when the precise time of stroke onset cannot be determined. 1
• This allows biologically-based rather than time-based management decisions. 1

Large Vessel Occlusion Beyond 6 Hours

• Mandatory for any patient with ICA or M1 occlusion presenting 6-24 hours after onset who is being considered for mechanical thrombectomy. 2, 4
• A mismatch ratio <1.8 excludes patients from late-window thrombectomy, as the risk-benefit profile becomes unfavorable. 4

Patients Ineligible for IV tPA

• When considering intra-arterial thrombolysis or mechanical thrombectomy as primary therapy, perfusion imaging helps identify at-risk brain regions that may be salvageable. 1

Technical Capabilities and Limitations

Dynamic perfusion CT provides absolute measures of cerebral blood flow, mean transit time, and cerebral blood volume, though it is currently limited to 2-4 brain slices with incomplete visualization of all vascular territories. 1

• Optimal thresholds validated by research: delay time ≥3 seconds defines penumbra, and relative CBF ≤30% within delay time ≥3 seconds defines ischemic core. 5, 6
• CT perfusion demonstrates high sensitivity and specificity for detecting cerebral ischemia and can differentiate thresholds of reversible versus irreversible ischemia. 1, 7
• Disadvantages include iodine contrast exposure, additional radiation, and the need for rapid data acquisition with conventional CT equipment. 1

Critical Pitfalls to Avoid

• Never delay IV tPA administration to obtain perfusion imaging in patients presenting within 4.5 hours—the performance of additional imaging sequences should not unduly delay treatment. 1, 2
• Do not extend DAWN or DEFUSE-3 eligibility criteria beyond the strict imaging thresholds (mismatch ratio ≥1.8, core <70 mL) established by randomized trials, as doing so exposes patients to procedural risks without proven benefit. 4
• Focusing solely on non-contrast CT without vascular imaging may miss large vessel occlusions requiring endovascular therapy, representing a critical missed opportunity for potentially life-saving intervention. 2
• Recognize that perfusion imaging interpretation can be complex, with technical challenges including motion artifact, contrast timing issues, and variable post-processing methods that can affect perfusion parameter calculations. 7, 8
• A mismatch ratio of 1.3 does not meet DEFUSE-3 criteria (which requires ≥1.8) and therefore does not qualify a patient for late-window thrombectomy, even if other imaging features appear favorable. 4

Practical Algorithm for CTP Decision-Making

0-4.5 hours + IV tPA candidate:

• Non-contrast CT only 1, 2
• Add CTA if considering endovascular therapy, but do not add CTP 2

0-6 hours + endovascular candidate:

• Non-contrast CT + CTA mandatory 2
• CTP optional (provides additional information but not required for decision-making) 1, 2

6-24 hours + large vessel occlusion:

• Non-contrast CT + CTA + CTP all mandatory 1, 2, 4
• Must meet strict perfusion criteria (core <70 mL, mismatch ratio ≥1.8) to proceed with thrombectomy 2, 4

Unknown time of onset (wake-up stroke):

• CTP recommended to identify salvageable tissue and guide treatment decisions 1