Acute Ischemic Stroke - Noncontrast CT, CTA, and CT Perfusion

Key Points

• Noncontrast CT is the first immediate test to exclude intracranial hemorrhage.
• A normal early CT does not exclude ischemic stroke.
• CTA should be obtained rapidly when large-vessel occlusion is possible.
• Do not delay otherwise eligible thrombolysis for unnecessary advanced imaging.
• CT perfusion helps select patients in extended or unknown-onset windows but is vulnerable to technical and physiologic artifacts.
• MRI is highly sensitive for ischemia but should not slow time-sensitive reperfusion.
• Posterior circulation symptoms warrant vascular imaging even when noncontrast CT is normal.
• Large-core infarction requires current thrombectomy criteria rather than an outdated automatic exclusion.

Algorithm

1. Activate the stroke pathway and establish last-known-well or symptom-discovery time.
2. Obtain immediate noncontrast head CT.
3. If no hemorrhage and the patient is otherwise eligible, proceed with IV thrombolysis without waiting for unnecessary tests.
4. Obtain CTA head and neck rapidly when large-vessel occlusion is possible.
5. Use CT perfusion or MRI selection in extended-window, wake-up, or otherwise complex reperfusion decisions according to local stroke-center protocol.
6. Transfer promptly to a thrombectomy-capable center when an eligible large-vessel occlusion is identified.
7. Repeat imaging for neurologic deterioration or when protocol requires assessment before antithrombotic therapy.
8. Use MRI selectively for small, posterior fossa, uncertain, or stroke-mimic cases when it will not compromise time-sensitive care.

Clinical Synopsis & Reasoning

Brain imaging in suspected acute ischemic stroke has two immediate jobs: exclude intracranial hemorrhage and identify patients who may benefit from reperfusion. Noncontrast head CT is fast and highly sensitive for acute hemorrhage. Early ischemic changes can be subtle, including loss of gray-white differentiation, insular ribbon obscuration, sulcal effacement, or a hyperdense artery. A normal-appearing CT does not exclude early ischemic stroke and should not delay treatment when the clinical diagnosis is compelling.

CT angiography from the aortic arch or neck through the intracranial circulation identifies large-vessel occlusion, tandem lesions, dissection, and relevant vascular anatomy. It should be obtained rapidly in patients who may be thrombectomy candidates, including those with disabling deficits that suggest an anterior or posterior circulation large-vessel occlusion. Modern workflows often perform noncontrast CT and CTA in immediate sequence. Kidney disease should not cause prolonged delay when the potential benefit of thrombectomy is substantial; the risk-benefit balance favors timely vascular imaging in many emergencies.

CT perfusion estimates ischemic core and hypoperfused but potentially salvageable tissue. It is especially useful in selected patients presenting beyond the traditional early window, with unknown onset or wake-up stroke, and in some centers for complex early-window cases. Automated software is helpful but not infallible. Motion, poor bolus, chronic infarct, severe stenosis, low cardiac output, and very early presentation can create misleading core or penumbra estimates. Imaging should be interpreted with neurologic findings and CTA rather than accepted as an isolated software verdict.

Treatment Strategy & Disposition

For patients eligible for IV thrombolysis within a standard treatment window, do not delay treatment to obtain advanced perfusion imaging when noncontrast CT has excluded hemorrhage and no other imaging is required to establish eligibility. CTA can proceed rapidly in parallel when thrombectomy is possible. The 2026 AHA/ASA guideline continues to emphasize fast imaging-to-treatment workflows and expanded use of reperfusion in appropriately selected patients.

MRI with diffusion-weighted imaging is more sensitive than CT for small or early infarction and can identify posterior fossa lesions that are difficult to see on CT. FLAIR and diffusion mismatch can help estimate lesion age in selected wake-up strokes. However, MRI should not replace a rapid CT pathway when scanner availability, monitoring, implant status, or acquisition time would delay reperfusion. For suspected basilar occlusion or posterior circulation stroke, CTA is particularly important because noncontrast CT may initially be unrevealing.

Imaging should also assess hemorrhage risk and infarct extent. ASPECTS provides a standardized estimate of early ischemic change in the middle cerebral artery territory. Large established infarction no longer automatically excludes thrombectomy in every case; newer evidence supports treatment in selected patients with larger core infarcts, so decisions should follow current stroke-center criteria. After reperfusion, repeat imaging is obtained for neurologic deterioration or according to protocol before antithrombotic therapy after thrombolysis.

Epidemiology / Risk Factors

• Stroke remains a major cause of disability and death.
• Large-vessel occlusions account for a minority of ischemic strokes but a disproportionate share of severe disability.
• Expanded thrombectomy indications have increased the number of patients who benefit from advanced vascular and perfusion imaging.
• Time from arrival to imaging and reperfusion is a major quality metric.

Investigations

Typical Workflow

Pharmacology

Prognosis / Complications

• Earlier reperfusion generally produces better functional outcomes.
• Imaging that identifies salvageable tissue can extend treatment opportunities in selected late-presenting patients.
• Large infarct core and poor collaterals are associated with worse prognosis but are no longer absolute exclusions in all patients.
• Failure to recognize posterior circulation occlusion can lead to catastrophic deterioration.

Patient Education / Counseling

• Explain that a normal first CT does not mean a stroke did not occur; it mainly rules out bleeding and major established injury.
• When CTA or perfusion is recommended, emphasize that the purpose is to determine whether a blocked artery can be treated.
• Patients and families should understand that treatment decisions may need to occur before every imaging detail is finalized.
• After discharge, review secondary-prevention imaging results such as carotid disease, dissection, or intracranial stenosis with the stroke team.

Notes

The best stroke-imaging protocol is the one that answers the treatment question quickly. A beautiful perfusion map obtained after a long delay is less valuable than a rapid CT/CTA pathway that gets an eligible patient to reperfusion.

Radiology reports should communicate actionable findings directly to the stroke team, especially LVO, hemorrhage, dissection, critical carotid disease, and unexpected mimics. Time-critical communication is part of interpretation quality.

References

1. AHA/ASA 2026 Guideline for Early Management of Acute Ischemic Stroke — Link
2. American College of Radiology Appropriateness Criteria: Cerebrovascular Diseases and Stroke — Link