Residency · Residency · Diagnostic Radiology

Acute Stroke Imaging: CT Perfusion and MRI

Overview

Time-Critical Imaging

Stroke is a clinical and imaging emergency where "time is brain." The goals of imaging are threefold: exclude hemorrhage, identify large vessel occlusion, and determine salvageable tissue (penumbra). The standard acute stroke protocol consists of a non-contrast CT of the head, CT angiography of the head and neck, and CT perfusion.

Types of Stroke

Ischemic stroke accounts for approximately 85% of all strokes and results from thrombotic or embolic arterial occlusion. Hemorrhagic stroke accounts for the remaining 15% and includes intracerebral hemorrhage and subarachnoid hemorrhage. Imaging must rapidly distinguish these two entities because their treatments are diametrically opposite.

Non-Contrast CT (NCCT)

Acute Ischemic Stroke Findings

NCCT may be completely normal in the first 6-12 hours of an ischemic stroke. Early signs are subtle and include loss of gray-white matter differentiation (the insular ribbon sign and loss of basal ganglia definition), sulcal effacement indicating focal brain swelling, and the hyperdense vessel sign, which represents thrombus within the occluded artery (the dense MCA sign on axial images or the dot sign in M2/M3 segments).

The ASPECTS (Alberta Stroke Program Early CT Score) is a 10-point scoring system for MCA territory ischemia. Ten regions are evaluated, with 1 point subtracted for each region showing early ischemic change. A score of 0 to 10 is possible, where a lower score indicates a larger infarct. An ASPECTS of 5 or less is associated with poor outcome and higher hemorrhagic transformation risk. ASPECTS is used for patient selection in endovascular thrombectomy trials.

Hemorrhagic Transformation

Hemorrhagic transformation is graded from petechial (HI-1 and HI-2) to parenchymal hematoma (PH-1 and PH-2). PH-2, defined as hemorrhage occupying more than 30% of the infarct volume with mass effect, is associated with clinical deterioration. Risk factors include large infarct core, reperfusion therapy, and anticoagulation.

CT Angiography (CTA)

Technique

CTA is a contrast-enhanced CT acquired from the aortic arch to the vertex that evaluates both extracranial and intracranial vasculature.

Key Findings

The primary findings to identify are large vessel occlusions (LVO) involving the ICA, M1 or M2 segments of the MCA, or the basilar artery. The thrombus location and length should be documented. Collateral vessel status is assessed using CTA collateral scoring, which correlates with outcome. Extracranial carotid disease (stenosis or dissection) and tandem lesions (extracranial stenosis combined with intracranial occlusion) should be evaluated.

Collateral Assessment

Good collaterals show retrograde filling of pial arteries distal to the occlusion. Poor collaterals show absent filling in the affected territory and correlate with larger infarct core and worse outcome. Multiphase CTA improves collateral assessment.

CT Perfusion (CTP)

Principles

CT perfusion involves tracking a dynamic contrast bolus through the brain parenchyma to generate parametric maps: CBF (cerebral blood flow), CBV (cerebral blood volume), MTT (mean transit time), and Tmax (time-to-maximum).

Map Interpretation

The ischemic core shows reduced CBF and reduced CBV, representing tissue that is likely infarcted and irreversible. Automated software such as RAPID defines this as a Tmax exceeding 6 seconds with CBF less than 30% of normal. The ischemic penumbra shows reduced CBF but preserved or elevated CBV (from autoregulatory vasodilation), representing salvageable tissue. This corresponds to a Tmax exceeding 6 seconds with CBF greater than 30% of normal. The mismatch ratio is the penumbra volume divided by the core volume. A mismatch ratio of 1.8 or greater with a core volume less than 70 mL was used in the extended window thrombectomy trials (DAWN and DEFUSE 3).

Clinical Application

In the standard window (0-6 hours), CTP helps identify patients who may benefit from thrombectomy. In the extended window (6-24 hours), CTP is essential for patient selection, identifying patients with small core and large penumbra who benefit from late thrombectomy. CTP is not needed if the patient presents within the standard IV tPA window (0-4.5 hours) with a clear clinical deficit and no contraindications.

Limitations and Pitfalls

Motion artifact degrades map quality. There is variability across software platforms (RAPID versus others) in threshold definitions. Low CBV regions may be overestimated in the setting of incomplete infarction. Posterior fossa coverage may be limited depending on scanner configuration.

MRI in Acute Stroke

Diffusion-Weighted Imaging (DWI)

DWI is the most sensitive sequence for acute ischemic stroke, with sensitivity exceeding 95% within minutes of onset. Cytotoxic edema restricts water diffusion, producing a pattern of DWI bright signal with ADC dark signal, which confirms acute restricted diffusion and acute infarction. DWI can detect stroke within 30 minutes of onset, compared to hours for CT.

DWI-FLAIR Mismatch

When DWI is positive but FLAIR is negative, the infarct is likely less than 4.5 hours old. This serves as a tissue-based clock for wake-up strokes or strokes of unknown onset time. The WAKE-UP trial demonstrated that patients with DWI-FLAIR mismatch benefit from IV alteplase even when the time of onset is unknown.

MR Angiography (MRA)

Time-of-flight (TOF) or contrast-enhanced MRA evaluates the vasculature, identifying LVO, intracranial stenosis, and dissection. TOF MRA may overestimate stenosis due to flow gap artifact.

MR Perfusion

MR perfusion follows similar principles to CTP but uses gadolinium bolus or arterial spin labeling (ASL). The DWI-perfusion mismatch compares the diffusion lesion (core) with the perfusion deficit (at-risk tissue). MR perfusion is less commonly used than CTP in the acute setting due to time constraints.

Vascular Territories

Anterior Circulation

The ACA territory supplies the medial frontal and parietal lobes and the anterior corpus callosum. The MCA territory supplies the lateral frontal, parietal, and temporal lobes, the insular cortex, and the basal ganglia via the lenticulostriate branches. The anterior choroidal artery supplies the posterior limb of the internal capsule, medial temporal lobe, and optic tract.

Posterior Circulation

The PCA territory supplies the occipital lobes, medial temporal lobes, and thalamus. Basilar artery branches supply the pons and cerebellum. The PICA territory supplies the lateral medulla and inferior cerebellum. The SCA territory supplies the superior cerebellum and superior cerebellar peduncle.

Watershed (Borderzone) Infarcts

External (cortical) watershed infarcts occur between the ACA-MCA and MCA-PCA territories. Internal watershed infarcts involve the periventricular white matter between deep and superficial MCA perforators. Watershed infarcts are associated with hemodynamic compromise from hypotension or severe carotid stenosis.

<image>A CT perfusion map panel showing acute left MCA territory stroke. Four parametric maps displayed: (1) CBF map showing a region of decreased blood flow in the left MCA territory. (2) CBV map showing a small area of decreased CBV (ischemic core) surrounded by a larger area of preserved CBV (penumbra). (3) MTT map showing prolonged mean transit time throughout the left MCA territory. (4) Tmax map showing the Tmax >6 second region delineating the at-risk tissue. The mismatch between the small core and large penumbra is highlighted, indicating a favorable profile for thrombectomy. Labels indicate core, penumbra, and mismatch ratio.</image>

<image>An MRI panel demonstrating acute ischemic stroke with DWI-FLAIR mismatch. Top row: DWI showing a bright (restricted diffusion) lesion in the right MCA territory, with the corresponding ADC map showing dark signal (confirming true restricted diffusion). Bottom row: FLAIR image at the same level showing NO corresponding signal abnormality (FLAIR negative), indicating the infarct is likely less than 4.5 hours old. An arrow indicates the DWI lesion and the normal-appearing FLAIR for comparison.</image>

<image>An illustration of the major intracranial vascular territories shown on axial brain sections at three levels (basal ganglia, corona radiata, and vertex). Color-coded regions show: ACA territory (blue/medial), MCA territory (red/lateral), PCA territory (green/posterior), and watershed zones (hatched areas at the boundaries between territories). The lenticulostriate perforator territory (deep MCA) is shown supplying the basal ganglia. A lateral brain diagram shows the surface territories with labels for each artery.</image>

Clinical Pearls

NCCT may be completely normal in the first 6-12 hours of ischemic stroke; a "normal" CT does not exclude stroke, and DWI is far more sensitive. The dense MCA sign (hyperdense vessel on NCCT) is a direct sign of thrombus and should prompt immediate CTA. An ASPECTS score of 5 or less predicts large infarct core and higher risk of hemorrhagic transformation and is used in thrombectomy patient selection. In the extended window (6-24 hours), CT perfusion with automated software (RAPID) is essential for identifying patients who benefit from thrombectomy based on mismatch between core and penumbra. DWI-FLAIR mismatch can serve as a tissue clock for wake-up strokes: DWI positive with FLAIR negative suggests onset less than 4.5 hours and eligibility for thrombolysis. Posterior fossa coverage on CTP should always be checked, as basilar artery occlusion is a devastating stroke that may be incompletely evaluated on standard CTP coverage.

References

• Powers WJ, et al. "Guidelines for the Early Management of Patients with Acute Ischemic Stroke." Stroke (AHA/ASA), 2019
• Nogueira RG, et al. "Thrombectomy 6 to 24 Hours after Stroke with a Mismatch between Deficit and Infarct (DAWN Trial)." NEJM, 2018
• Albers GW, et al. "Thrombectomy for Stroke at 6 to 16 Hours with Selection by Perfusion Imaging (DEFUSE 3)." NEJM, 2018
• Thomalla G, et al. "MRI-Guided Thrombolysis for Stroke with Unknown Time of Onset (WAKE-UP Trial)." NEJM, 2018
• ACR Appropriateness Criteria: Cerebrovascular Disease, 2019

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