Comparison of Neuroimaging Methods
Today, invasive digital subtraction angiography (DSA) is practically not used in routine practice. It has been replaced by modern tomographic and ultrasound techniques:
- Contrast-enhanced MRA (CEMRA): the gold standard for non-invasive diagnosis. According to meta-analyses, for severe stenosis (70–99%), it has the highest sensitivity (0.94) and specificity (0.93), significantly reducing blood flow artifacts.
- CT angiography (CTA): performed on multislice scanners (128 slices and up), it is comparable in accuracy to CEMRA. The method is not prone to flow artifacts, but has a limitation: prominent calcification can mask the true degree of stenosis.
- Time-of-flight MRA (TOF MRA): has high specificity in the absence of pathology. However, turbulence and decreased blood flow velocity can mimic narrowing, causing the physician to overestimate the severity of stenosis.
- Doppler Ultrasound: excellently demonstrates plaque localization and extent, but may underestimate narrowing if a fresh floating thrombus is present in the lumen.
Radiological Signs of Atherosclerotic Plaque
The most critical stage of diagnosis is identifying high-risk plaques prone to rupture and distal embolization. Different methods highlight different morphological features:
- Ultrasound: reveals plaque structure. Hyperechoic areas with acoustic shadowing indicate calcification, while hypoechoic foci indicate a lipid core and risk of stroke.
- CT Angiography: allows clear visualization of juxtaluminal hypodensity. This sign indicates a large lipid plaque or floating thrombus—a condition directly associated with microembolism.
- MRI without contrast: narrowing of the blood flow signal void zone (flow void) indicates stenosis. If the flow void is completely absent and a hyperintense signal is detected within the lumen, this indicates occlusion.
For advanced risk assessment, isotopic imaging (FDG-PET) is used to mark intraplaque inflammation, which directly participates in thrombus formation.
Extracranial Artery Dissection
Dissection is a major non-atheromatous cause of neck vessel narrowing. The internal carotid artery most commonly dissects at the skull base or bulb, and the vertebral artery in the bony canal (a risk factor in children is the presence of the arcuate foramen of the C1 vertebra).
The pathogenesis of the radiological pattern is based on the formation of a false lumen with intramural hemorrhage, which compresses the true vessel lumen.
What it looks like on imaging:
- Non-contrast CT: an enlarged vessel with a crescentic hyperintensity.
- MRI (T1 fatsat sequence): a hyperintense intramural hematoma surrounding the narrowed true lumen. Important: in vertebral arteries, this sign is often masked by the signal from the surrounding venous plexus.
- CTA and MRA: reveal smooth tapered lumen narrowing, a detached intimal flap, or pseudoaneurysmal dilation.
Fibromuscular Dysplasia
Fibromuscular dysplasia (FMD) is another cause of non-atheromatous carotid stenosis, affecting predominantly middle-aged women. The pathology is frequently bilateral (up to 60% of cases).
Most commonly (75% of cases), the middle third of the internal carotid artery is affected, approximately 2 cm distal to the bulb. On angiograms, this appears as a characteristic "corrugation" or "string of beads" sign—regular alternation of vascular narrowing and dilation. Local tubular stenoses are less common, but their degree usually does not exceed 40%. Intracranial vessels are rarely involved, but when they are, the condition may be associated with intracranial aneurysms.
Briefly: answers to topical questions
What complications are characteristic of fibromuscular dysplasia when intracranial vessels are involved?
Intracranial involvement in fibromuscular dysplasia is rare and may be associated with intracranial aneurysms.
Main lesion sites:
- cervical segment of the internal carotid artery — 75% of cases;
- vertebral arteries — 12% of cases.
A characteristic angiographic sign of fibromuscular dysplasia is the "string of beads" sign (string of beads), i.e., alternating stenoses and dilations of the vessel lumen.
To memorize the cross-sectional MRI pattern of arterial dissection, the "fried egg" association is used: the narrowed true vessel lumen is the "yolk", and the surrounding hyperintense hemorrhage in the wall is the "white".
Class and exam questions
How to distinguish complete occlusion from residual, "trickle flow" on imaging?
Ultrasound and time-of-flight MRA handle this poorly (the "flow void" effect occurs). It is most reliable to use CT angiography, analyzing the arterial and slightly delayed contrast phases.
What is the main advantage of time-of-flight MR angiography (TOF MRA) over CEMRA?
Despite the tendency to overestimate stenosis due to flow turbulence, TOF MRA allows for a more accurate assessment of plaque morphology and extent.
Why might MRI yield a false-negative result when vertebral artery dissection is suspected?
When using T1-weighted fat-suppressed images, the hyperintense signal from the intramural hematoma may be masked by a similar signal from the robust venous plexus surrounding the vertebral artery.
What else to study on this topic
- Clinical Significance of Juxtaluminal Hypodensity on CTA
- Differential Diagnosis of Dissection and Fibromuscular Dysplasia
- Takayasu Arteritis: Large Vessel Vasculitis of the Aortic Arch
- The Role of Isotopic Imaging in Assessing Intraplaque Inflammation
- Ischemic Microangiopathy in Hypertension and Diabetes
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