What parts of the arteries are typically included in an MR (Magnetic Resonance) angiogram of the brain report?

Arterial Components in Brain MR Angiography Reports

A comprehensive brain MR angiography report should systematically evaluate the entire cerebral circulation from the aortic arch through the first divisions of major intracranial arteries, including both extracranial and intracranial segments. 1

Extracranial Cerebrovascular Arteries

The extracranial evaluation encompasses the proximal arterial supply to the brain:

• Aortic arch and its branches - The brachiocephalic, subclavian, and common carotid arteries should be assessed when gadolinium-enhanced MRA is performed 1
• Common carotid bifurcation (CCB) - Critical junction point where the common carotid divides into internal and external carotid arteries 2
• Extracranial internal carotid arteries - From the bifurcation through the neck 1, 3
• Vertebral arteries (VA) - Bilateral assessment from their origin through the cervical segments 2, 4

Important caveat: Gadolinium-enhanced MRA allows imaging of the entire circulation from the aortic arch to the first division of major intracranial arteries in a single acquisition, providing superior anatomical coverage compared to non-contrast techniques 1

Intracranial Arterial Segments

Anterior Circulation

The anterior circulation supplies the majority of the cerebral hemispheres:

• Intracranial internal carotid arteries (ICA) - Including the petrous segment (C5), carotid siphon, and terminal portions 2, 5
• Middle cerebral arteries (MCA) - Bilateral M1 segments and major divisions 2, 5
• Anterior cerebral arteries (ACA) - Including A1 segments and A2 segments bilaterally 6, 5
• Anterior communicating artery (ACoA) - Connecting the two anterior cerebral arteries 6, 4

Critical pitfall: Horizontal segments of the carotid artery, particularly the intracranial segment, are less well visualized on time-of-flight MRA due to dephasing effects in the horizontal plane, which can lead to overestimation of stenosis severity 1

Posterior Circulation

The posterior circulation supplies the brainstem, cerebellum, and occipital lobes:

• Basilar artery (BA) - Formed by the confluence of vertebral arteries 2, 5
• Posterior cerebral arteries (PCA) - Bilateral P1 and P2 segments 2, 5
• Posterior communicating arteries (PcomA) - Connecting the internal carotid to the posterior cerebral arteries 4, 5

Technical consideration: Non-visualization of the basilar artery on time-of-flight MRA may represent either true anatomical absence/severe hypoplasia OR a technical limitation; contrast-enhanced MRA should be performed for definitive assessment 7

Circle of Willis Components

The circle of Willis represents the critical collateral network:

• Complete anatomical assessment of all communicating arteries is essential for understanding collateral flow dynamics 4
• Anatomic variants should be documented, including A1 segment aplasia (5.6% incidence), three A2 segments (3.0%), unpaired A2 segment (2.0%), and fenestrations (1.2%) 6
• Posterior cerebral artery origin - Whether from the basilar artery or internal carotid artery (fetal origin) should be specified 4

Special Vascular Patterns

Moyamoya Disease Assessment

When moyamoya disease is suspected, specific findings should be documented:

• Stenosis or occlusion of the terminal portion of intracranial internal carotid artery 1
• Decreased outer diameter of the terminal ICA and horizontal portion of MCA bilaterally 1
• Abnormal vascular networks (moyamoya vessels) in the basal ganglia or periventricular white matter - at least 2 visible flow voids unilateral or bilateral are needed 1, 8

Diagnostic requirement: MRI and MRA using a scanner with static magnetic field strength of ≥1.5 T is necessary for adequate assessment 1

Technical Artifacts and Limitations

Understanding MRA limitations is essential for accurate interpretation:

• Turbulence effects - MRA may overestimate stenosis severity due to dephasing effects of turbulence 1
• Slow flow - Can mimic occlusion or severe stenosis 1
• Movement artifacts - Patient motion degrades image quality 1
• Phase dispersion - Particularly problematic in the C5 segment, carotid siphon, and MCA 2

Best practice: 3D reconstructions should be reviewed in multiple planes (anteroposterior, lateral, and horizontal rotation views) to identify eccentric or highly focal lesions not apparent on standard views 1