What Do V1–V4 and Vertebral Artery CTA/MRA Terms Mean?

This page explains terminology commonly found on a Computed Tomography Angiography (CTA) or Magnetic Resonance Angiography (MRA) report. An imaging report describes pictures of your anatomy, not your final diagnosis. Only a healthcare provider can combine these written findings with your symptoms, medical history, and physical examination to determine what the scan means for your overall health.

Important Safety Information: A problem with the vertebral or basilar arteries can cause a stroke. If you experience sudden, new, or worsening symptoms such as trouble walking, double vision, vision loss, one-sided weakness or numbness, slurred speech, difficulty swallowing, fainting, or sudden severe neck pain or headache, seek emergency medical care immediately. Do not drive yourself. While dizziness or imbalance can have many nonvascular causes, do not delay urgent care while trying to interpret a scan report.

When reading a CTA or MRA report, terms like V1 to V4, dominance, hypoplasia, and distal reconstitution are used to map out your blood vessels and describe how they look. Because these words can sound alarming in isolation, doctors look at the whole picture—including how severe any narrowing is, signs of blood vessel injury, and whether your brain is getting the blood it needs.

The V1 to V4 Segments: Mapping the Artery

The vertebral artery is a crucial vessel that supplies blood to the back of your brain. To describe exactly where a variation, narrowing, or blockage is located, doctors divide the artery into four segments:

• V1 (Extraosseous segment): Extraosseous means outside the bone. This segment begins where the artery branches off a larger vessel in your chest or lower neck and travels upward before entering the spine ^[1].
• V2 (Foraminal segment): Foraminal means inside the openings. The artery travels up through small bony tunnels in the vertebrae of your neck. Radiologists sometimes use slightly different definitions for exactly where V2 ends and V3 begins ^[2].
• V3 (Extraspinal or Atlantic segment): The artery exits the bony tunnels of the neck, winding around the uppermost vertebra (C1) and preparing to enter the skull ^[1].
• V4 (Intradural segment): Intradural means inside the dura mater, which is the protective covering of the brain and spinal cord. In this final segment inside the skull, the vertebral artery joins the artery from the opposite side to form the basilar artery, supplying the brainstem and back of the brain ^[1].

Size Variations: Dominance and Hypoplasia

It is entirely normal for your left and right vertebral arteries to be different sizes, but understanding why they are different is critical.

• Dominance: If your report states that one artery is dominant, it simply means that this vessel is naturally wider than the one on the other side ^[3]. Being dominant is a common anatomic variation, but its importance changes if the dominant artery becomes diseased or if the other side is unusually small ^[4].
• Hypoplasia: A hypoplastic vertebral artery is one that is unusually narrow. While often a congenital (developmental) trait you were born with, a small-appearing artery on a scan can also be caused by acquired disease, such as plaque, a tear, or a vessel spasm ^[5]. Different imaging centers use different cutoffs (such as 2.0 mm or 2.7 mm) to define hypoplasia ^[6]. The word alone cannot establish whether the small size is harmless or clinically important.

Distal Reconstitution: Seeing the Vessel Downstream

If a section of the vertebral artery is blocked (occlusion) or severely narrowed, the report might mention distal reconstitution. This means that the contrast dye or flow signal disappears at the blockage but reappears further downstream ^[7].

• What it means: Blood is reaching the downstream vessel, often through smaller detour vessels (collateral flow) or by flowing backward from another artery ^[8].
• What it does not tell you: Reconstitution does not mean the original blockage has cleared, nor does it guarantee that your brain tissue is receiving an adequate or safe amount of blood ^[7]. Your clinician must review the scan to determine if this compensatory flow is sufficient.

How Doctors Read the Whole Report

A radiologist’s job is to describe what the scan looks like. To determine if a finding requires treatment, doctors analyze several factors together rather than focusing on one isolated word:

• Stenosis Severity: Doctors assess how severely an artery is narrowed (stenosis). Some reporting systems categorize this as moderate (50–69%) or severe (70–99%), while complete occlusion is a separate category ^[9]. However, measurement methods vary depending on the facility and the type of scan, so these percentages should not be used to self-diagnose ^[10].
• Signs of Dissection and Thrombus: A narrowed appearance might be from plaque, a blood clot (thrombus) ^[11], or a dissection—a tear in the inner lining of the blood vessel wall ^[12]. Clues for a dissection include an intimal flap (a visible tissue tear), a double lumen (two distinct channels of blood), or an intramural hematoma (blood trapped inside the vessel wall) ^[12].
• The Basilar Artery and Collaterals: Because the left and right vertebral arteries join to form the basilar artery, doctors carefully check this central vessel for blockages ^[13]. They also look at other connections in the brain to assess the anatomy of your collateral circulation ^[14].
• Knowing the Limits of the Scan: No scan is perfect. For example, some noncontrast, flow-dependent MRA techniques (like Time-of-Flight MRA) can make a narrowed artery look completely blocked if the blood flow is just very slow or turbulent ^[15][6]. CTA can be disrupted by artifacts from metal, bone, or patient movement ^[16].
• Dynamic Neck Movements: Rarely, turning the head can physically compress the vertebral artery ^[17]. While dizziness with head movement is common and usually caused by inner ear or other nonvascular issues, dynamic imaging is only considered if your doctor specifically suspects this rare compression ^[18]. Do not repeatedly provoke your symptoms by turning your neck to test them.
• Further Testing: If noninvasive scans are unclear, doctors may use high-resolution vessel-wall MRI to look closer for a dissection ^[19]. In some cases, a traditional catheter angiogram is recommended. Because catheter angiography is an invasive procedure with risks (such as bleeding or a small risk of stroke), it is only used when the diagnostic benefits outweigh those risks ^[20].