Makindo Medical Notes

Related Subjects: |Acute Stroke Assessment (ROSIER&NIHSS) |Atrial Fibrillation |Atrial Myxoma |Causes of Stroke |Ischaemic Stroke |Cancer and Stroke |Cardioembolic stroke |CT Basics for Stroke |Endocarditis and Stroke |Haemorrhagic Stroke |Stroke Thrombolysis |Hyperacute Stroke Care

Introduction

• Stroke medicine developed as a specialized field with the advent of CT scanning, allowing rapid distinction between ischaemic and haemorrhagic strokes. A non-contrast CT (NCCT) is quick, accessible, and can be used in most patients, including those who are claustrophobic, have metal implants, or are in critical condition.
• CT imaging forms the cornerstone of acute stroke care, providing rapid insights into the type of stroke and guiding early management decisions. It is a vital skill for clinicians to interpret CT scans effectively, correlating findings with clinical presentations.
• Before the widespread use of CT in the 1980s, differentiating between stroke types was often only possible through post-mortem examination. Early diagnostic methods like pineal calcification markers were unreliable, making accurate stroke diagnosis challenging.

History of CT Imaging

• The first CT scanner was developed in 1972 by Sir Godfrey Hounsfield, funded by EMI's profits from The Beatles' records. Hounsfield's idea involved taking X-rays from multiple angles and using a computer to create detailed images in slices.
• The first clinical CT scan was performed in 1971 at Atkinson Morley Hospital in London, successfully diagnosing a cerebral cyst.
• By the late 1980s, CT became more widely available, initially in teaching hospitals, making accurate stroke diagnosis and treatment possible.

Technical Aspects of CT

• A CT scanner consists of an X-ray tube and detectors that rotate around the patient, capturing images from various angles to create detailed cross-sectional images.
• Helical (spiral) CT scans introduced in the 1990s allow continuous imaging as the patient moves through the scanner, providing 3D images of brain structures.
• Image brightness and contrast are adjusted using Hounsfield Units (HU), which measure tissue density. For example, air has a value of -1000 HU, bone ranges from +400 to +3000 HU, and acute haemorrhage appears at +70 HU.

Radiation Exposure

• CT scans involve ionizing radiation, and in the UK, the use of CT is regulated under IRMER guidelines, with scans ordered only when clinically necessary.
• A typical brain CT scan involves about 2 mSv of radiation, equivalent to approximately 100 chest X-rays. It is crucial to balance the need for accurate diagnosis with the risks of radiation exposure.
• For comparison, natural background radiation exposure is around 3.1 mSv per year.

CT Imaging for Stroke: Indications

• CT is essential for assessing patients with suspected stroke who are candidates for thrombolysis or have severe symptoms like decreased consciousness, severe headache, or anticoagulant use.
• Early CT scans can be normal in some ischaemic strokes, especially if performed within the first 6 hours of onset. Clinical judgment remains crucial in these cases.

CT Interpretation Tips

Always review each CT slice from top to bottom, carefully looking for subtle signs of ischaemia or haemorrhage. Even in time-sensitive situations like thrombolysis, it is important to take a moment to carefully assess for subtle signs like subdural blood or a hyperdense artery sign.

Early Signs of Ischaemic Stroke on CT

• Hyperdense MCA sign: Indicates a clot in the middle cerebral artery. May appear before other tissue changes.
• Loss of grey-white differentiation: A subtle sign of early ischaemia, particularly in the cortex.
• Insular ribbon sign: Loss of distinction between the insular cortex and white matter, indicating early infarction.
• Effacement of sulci: Suggests increased oedema due to ischaemia.

ASPECTS Score

The Alberta Stroke Program Early CT Score (ASPECTS) assesses early ischaemic changes in the MCA territory on CT. A normal scan scores 10, with lower scores indicating more extensive infarction. A score below 7 predicts a worse outcome and increased risk of symptomatic haemorrhage.

CTA and CT Perfusion

• CT Angiography (CTA): Visualizes blood vessels from the aortic arch to the circle of Willis, helping to identify clots, occlusions, or dissections. It is useful in guiding endovascular therapies.
• CT Perfusion (CTP): Measures blood flow dynamics, identifying the core infarct and the penumbra. It helps assess the volume of brain tissue at risk and can guide decisions on reperfusion therapies.

Limitations of CT in Stroke Imaging

• CT may miss subtle early ischaemic changes or brainstem strokes due to artefacts from dense bone structures.
• False negatives are more likely if the scan is performed within the first few hours of stroke onset. False positives can occur in older patients with asymptomatic small vessel changes.

Later Signs on CT (6-24 hours)

• Hypodense regions become more apparent as cytotoxic and vasogenic oedema develop.
• Haemorrhagic transformation may occur, especially in larger infarcts or with anticoagulant use.

References

• Use of the Alberta Stroke Program Early CT Score (ASPECTS) for Assessing CT Scans in Patients with Acute Stroke
• Barber PA, Demchuk AM, Zhang J, Buchan AM. Validity and reliability of a quantitative computed tomography score in predicting outcome of hyperacute stroke before thrombolytic therapy. ASPECTS Study Group.
• ASPECTS Score Training and Resources