Neuroobrazowanie Udarów Mózgu Akutnych

Neuroimaging of Acute Stroke

• Neuroimaging is crucial in evaluating acute stroke to differentiate hemorrhage from ischemic stroke, assess brain damage, and pinpoint the responsible vascular lesion.
• Multimodal computed tomography (CT) and magnetic resonance imaging (MRI), including perfusion imaging, distinguish between irreversibly infarcted brain tissue and potentially salvageable tissue.
• This allows for patient selection for reperfusion therapy.
• Neuroimaging is essential during the initial 24 hours of stroke.
• Other acute stroke aspects, diagnostic types of stroke, subacute, and long-term assessment are discussed separately.

Approach to Imaging

• Neuroimaging is essential for acute stroke or transient ischemic attacks (TIAs).
• Differentiating ischemia from hemorrhage is an essential role of imaging.
• Excluding stroke mimics, such as tumors, is a key part of the process.
• Assessing large cervical and intracranial arteries is important for imaging.
• Estimating the volume of irreversibly infarcted tissue (infarction core) is crucial for imaging.
• Estimating the extent of salvageable brain tissue (ischemic penumbra) is a key aspect to identify patients who are likely to benefit from reperfusion therapies, such as intravenous thrombolysis and mechanical thrombectomy.

CT or MRI for Initial Imaging?

• CT with CT angiography (CTA) is the standard imaging modality for acute stroke at most centers.
• CT's advantages include widespread availability, rapid scan times, and lower cost, making it suitable for differentiating ischemic from hemorrhagic stroke
• CT's disadvantage is that early signs of ischemic stroke can be subtle and often absent in the first few hours, resulting in suboptimal sensitivity and interrater agreement in detecting early infarct signs.

Advantages of MRI

• Diffusion-weighted imaging (DWI) in MRI is more sensitive for detecting acute ischemic stroke than noncontrast CT and can help exclude some stroke mimics.
• MRI does not expose the patient to radiation, as CT does.
• MRI protocols, including conventional T1-weighted, T2-weighted, fluid-attenuated inversion recovery (FLAIR), and T2*-weighted gradient-recalled echo (GRE) sequences, can reliably detect both acute ischemic and hemorrhagic stroke.
• Susceptibility-weighted imaging (SWI) in MRI is equivalent to noncontrast CT for detecting acute intraparenchymal hemorrhage, and superior in detecting chronic hemorrhage.

Multimodal Imaging

• Multimodal CT (CT with CT angiography (CTA) of the head and neck and perfusion (CTP))can detect acute ischemic stroke better than a single CT.
• Multimodal CT helps to diagnose large vessel occlusion and determine the core and penumbra of acute ischemic stroke.
• Multimodal MRI(MRI of the brain without contrast, high-susceptibility imaging (to exclude hemorrhage), MRA of the head and neck, DWI, and perfusion-weighted imaging (PWI))can identify acute infarction, large vessel occlusion, infarct core, and salvageable penumbral brain tissue.

Time-Based Selection of Imaging

• For patients presenting within 4.5 hours of their last known well time, noncontrast head CT and CTA are the standard imaging modalities to assess for acute stroke and exclude hemorrhages.
• For patients presenting between 4.5 to 24 hours of their last known well time, multimodal CT (including noncontrast CT, CTA, and CTP) or multimodal MRI (if available), which can determine eligibility for mechanical thrombectomy, is considered a suitable alternative.

Imaging Findings of Ischemic Stroke

• Early infarct signs on CT may include loss of grey-white matter differentiation in the basal ganglia, loss of insular ribbon, and cortical hypoattenuation.

ASPECTS method (Alberta Stroke Program Early CT Score)

• The ASPECTS score is a simple and reliable method for clinically assessing the severity of early ischemic stroke on noncontrast CT scans, which proves helpful for treatment decisions.
• The score is calculated from two standard axial, noncontrast CT images, one at the level of the thalamus and basal ganglia, and one immediately rostral to the basal ganglia. (Each image will evaluate 10 regions)

Time Course of Ischemic Changes

• Ischemic changes using DWI (bright signal on DWI and matching low signal on the ADC map) are observed within 3-30 minutes after stroke onset.

Sensitivity of DWI for Acute Ischemic Stroke

• Abnormal DWI is a sensitive and specific indicator of acute ischemic stroke, particularly for those presenting within 6 hours of symptom onset.

Acute Intravascular Thrombus

• CT, and MRI are useful for detecting hyperdense artery/vessel sign on noncontrast CT. It’s important to distinguish from early infarct signs which are time-dependent.
• Susceptibility-weighted MRI is advantageous for early detection of thrombosis.

Vessel Imaging

• CT angiography (CTA) or MR angiography (MRA) are important tools for evaluating the aortic arch and large extracranial and intracranial vessels as this is important in mechanical thrombectomy.
• CTA has shown high sensitivity and specificity (92-100% and 82-100%, respectively) for detecting intracranial vessel stenosis and occlusion compared to conventional angiography.

Collateral Blood Flow

• Evaluation of collateral blood flow, which is important due to the ability of collateral vessels to preserve the brain tissue, when direct blood supply is blocked by thromboembolism.

Perfusion-Core Mismatch

• Mismatch is an imaging marker to help select patients needing mechanical thrombectomy. Assessing the core infarct and ischemic penumbra helps determine if tissue is salvageable.
• It involves imaging with automated perfusion-software to help quantify the core and penumbra of an ischemic stroke.

Clinical-Core Mismatch

• A clinical-core mismatch occurs when the severity of neurological deficits, as assessed by the National Institutes of Health Stroke Scale (NIHSS), is greater than expected from the core infarct damage.

Clinical-ASPECTS Mismatch

• The mismatch between the severity of neurologic deficits (NIHSS) and the core infarct (ASPECTS) indicates that reperfusion treatment may be beneficial if the mismatch is present.

DWI-FLAIR Mismatch

• The finding of a hyperintense lesion on DWI (indicating acute infarction) but no corresponding signal change on FLAIR images (indicating vasogenic edema) suggests a relatively acute stroke, potentially warranting intravenous thrombolysis, especially if the time of stroke onset is unknown or unwitnessed.

Imaging of Hemorrhagic Stroke

• Acute or subacute hemorrhage is a contraindication for reperfusion therapy (thrombolysis and mechanical thrombectomy).
• Management and evaluation of hemorrhagic stroke should be performed separately. Intracerebral, intraventricular, subarachnoid, subdural, and epidural hemorrhages should be diagnosed individually.

Digital Subtraction Angiography

• DSA is a method of visualizing the carotid and vertebral arteries in the neck, and the large and medium-sized arteries in the head through a catheter.

Ultrasound Methods

• Carotid duplex ultrasound (CDUS) and transcranial Doppler (TCD) ultrasound are widely used to evaluate stroke patients.

Other Imaging Considerations

• Specifics for other modalities (e.g., angiography, CT perfusion) are mentioned in relevant sections.