Makindo Medical Notes

Related Subjects: |Brain Herniation syndromes |Haemorrhagic stroke |Traumatic Head/Brain Injury |Acute Hydrocephalus |Epidural Haematoma |Subdural haematoma |Basic Neuroscience |Basal Ganglia |Apraxia |Cerebral Angiography and Perfusion

CT Angiography (CTA)

CT Angiography (CTA) is performed by administering a single intravenous bolus of contrast through a well-positioned IV line. Using a helical CT scanner with multislice capabilities, the scan can capture the contrast as it travels through the arterial and venous phases in the brain, allowing for detailed imaging of both arterial and venous vessels.

Procedure: The scan is timed to capture images at peak opacification, providing clear visualization of the vascular structures, including the circle of Willis, its branches, and extracranial vessels. The data can be reconstructed into three-dimensional images using post-acquisition software, particularly Maximum Intensity Projection (MIP), which highlights the vascular structure while filtering out other soft tissues.

Clinical Applications: CTA is valuable in:

• Diagnosing conditions like basilar artery stroke and planning endovascular interventions by locating clots in major vessels.
• Evaluating for aneurysms, with sensitivity rates between 94-98% for aneurysms larger than 3 mm, though detection drops to 70% for aneurysms smaller than 3 mm.
• Assessing thrombus presence in acute stroke, guiding intra-arterial thrombolysis or mechanical thrombectomy considerations.
• Identifying arterial dissections, pseudoaneurysm formation, and detecting vasospasm in subarachnoid hemorrhage.

Limitations: Despite its utility, CTA may have limited accuracy compared to Digital Subtraction Angiography (DSA) and requires weighing risks such as contrast-induced nephropathy, especially in patients with renal impairment.

CT Perfusion (CTP)

CT Perfusion (CTP) allows for mapping the brain's blood flow dynamics in real-time using rapid CT scanning following IV contrast injection. It is often performed alongside CTA for a comprehensive assessment of cerebral perfusion in acute stroke.

Procedure: Modern multislice scanners, capable of capturing 16 to 64 slices simultaneously, generate a time-density curve for each pixel, measuring how contrast perfuses brain tissue. This enables calculation of:

• Cerebral Blood Volume (CBV): Blood volume per unit of brain tissue (4-5 ml/100 g).
• Cerebral Blood Flow (CBF): Rate of blood flow, especially to grey matter (50-60 ml/100 g/min).
• Mean Transit Time (MTT): Average time for contrast to travel from arterial inflow to venous outflow.
• Time to Peak (TTP): Time from the start of contrast injection to peak enhancement in a region.

Clinical Applications: CTP helps differentiate between:

• Infarct Core: Areas that are irreversibly damaged with significantly reduced CBF and CBV and prolonged MTT/Tmax. These regions are unlikely to benefit from reperfusion.
• Penumbra: Viable but at-risk tissue with moderately reduced CBF, near-normal or increased CBV, and prolonged MTT/Tmax. Identifying the penumbra can guide decisions regarding reperfusion therapy.

Challenges: CTP accuracy can be affected by factors such as cardiac output, arrhythmias, and local stenosis, leading to potential misinterpretation of flow maps. Seizure activity may also cause hyperperfusion that mimics stroke in contralateral areas.

Current Use: CTP is increasingly utilized in evaluating acute large vessel occlusions and in research, though its role in routine hyperacute stroke protocols remains under investigation. It is more commonly available in advanced stroke centers.

CT in Haemorrhagic Stroke

In non-contrast CT (NCCT), blood appears as a hyperdense (bright) area due to its high absorption of X-rays. The sensitivity of CT for detecting hemorrhagic stroke is about 99% within the first 24 hours, but this decreases as the blood is gradually reabsorbed.

Imaging Findings:

• Acute Hemorrhage: Appears as a well-defined hyperdense region, often unilateral and asymmetric. Look for mass effect, midline shift, or ventricular compression, which can indicate significant pressure changes.
• Subarachnoid Hemorrhage (SAH): Manifests as a hazy hyperdensity along the brain's sulci and cisterns, such as the Sylvian fissure and the interhemispheric fissure. Look for blood pooling in gravity-dependent areas like the prepontine cistern.
• Subdural Hematoma: Can appear crescent-shaped, often along the brain's periphery, and can shift midline structures. Chronic subdural hematomas may appear hypodense over time.
• Intraventricular Hemorrhage: Presence of blood within the ventricles may cause hydrocephalus, identifiable by dilation of the third ventricle or other ventricles.

MRI with gradient-echo sequences can be used when CT fails to show residual hemorrhage, particularly after 1-2 weeks.

Cerebral Digital Subtraction Angiography (DSA)

DSA is the gold standard for detailed visualization of cerebral vasculature, used primarily in specialized centers for conditions such as aneurysms and arteriovenous malformations (AVMs).

Procedure: A catheter is inserted through the femoral artery and navigated through the aorta to reach the cerebral vessels. Iodinated contrast is injected directly into the vessels, and high-resolution X-ray images capture the flow through the cerebral arteries.

Clinical Applications:

• Identification of small aneurysms, AVMs, and detailed assessment of vasculitis.
• Post-hemorrhage assessment to pinpoint the source of bleeding, such as a ruptured aneurysm.
• Planning endovascular procedures like coil embolization or stent placement for aneurysms.

Risks: DSA has a small stroke risk (~1%) due to potential embolization during catheterization. Other risks include vascular injury, bleeding at the insertion site, infection, and contrast nephropathy, especially in patients with pre-existing renal impairment.