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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Radiology

The Diagnostic Journey: Imaging Your Arteries

At a Glance

Carotid artery thrombosis is usually evaluated first with duplex ultrasound and, when blockage is suspected, confirmed with CT or MR angiography. These tests help distinguish a complete occlusion from a near-occlusion; catheter angiography is reserved for unclear cases or planned procedures.

The journey to a carotid diagnosis usually involves several different types of scans. Because no single test is perfect, your medical team often uses a “step-up” approach for stable outpatient evaluation—starting with a simple screening and moving to more detailed imaging to confirm the findings before deciding on a treatment plan [1][2]. (Note: Emergency pathways for acute stroke or suspected acute carotid occlusion differ; rapid brain and CTA/MRA imaging may be performed immediately without waiting for ultrasound).

Step 1: The Screening (Duplex Ultrasound)

For stable outpatient evaluation, a Duplex Ultrasound (DUS) is almost always the first test performed. It uses sound waves to create a picture of your artery and measure the speed of blood flow [3].

  • Why it’s used first: It is non-invasive, does not use radiation or dyes, and is very effective at spotting significant narrowing [4].
  • The Limitation: Ultrasound can be “operator-dependent,” meaning the results rely on the skill of the person performing the test. It can also be difficult to see through heavy calcium deposits, which create “shadows” on the image [2][5].

Step 2: The Confirmation (CTA or MRA)

If an ultrasound shows a serious blockage or complete occlusion, doctors typically order a second, more detailed scan to confirm the result [1]. This is usually a CT Angiogram (CTA) or an MR Angiogram (MRA).

  • CTA: Uses a specialized CT scan and an iodinated contrast dye to map the arteries from your heart all the way into your brain [6]. It is excellent for seeing the “anatomy” of the blockage but does involve radiation [3].
  • MRA: Uses magnetic fields to visualize blood flow. It may be noncontrast or use gadolinium. The radiology team selects the test based on kidney function, prior contrast reactions, implants, and clinical urgency [7]. A prior reaction to iodinated contrast does not automatically make all CTA impossible, and many modern pacemakers are MRI compatible if checked.

The “String Sign” and Pseudo-Occlusion

One of the most critical reasons for a second scan is to distinguish between a true occlusion and a pseudo-occlusion (also called near-occlusion) [8].

  • True Occlusion: The artery is 100% blocked, and no blood is moving through it [9].
  • Pseudo-Occlusion: Near-occlusion is a severe stenosis with markedly reduced distal internal-carotid caliber or flow—sometimes producing an angiographic appearance called a string sign [10].

On a standard ultrasound, these two can look identical because the flow in a pseudo-occlusion is so slow it may not be detected [11]. This distinction is vital. However, revascularization for near-occlusion is highly individualized—especially when distal vessel collapse is present—and is not automatically beneficial compared to medical therapy. Specialist review of the CTA/MRA is required [8][12].

Digital Subtraction Angiography (DSA)

A Digital Subtraction Angiography (DSA) is an invasive procedure where a catheter is threaded through an artery (usually in the groin or wrist) up to the neck [13].

While CTA and MRA have replaced DSA for most patients, it is reserved for situations in which noninvasive imaging is inadequate or as part of a planned intervention [6]. Your doctor may recommend a DSA if:

  • Your ultrasound and CTA results contradict each other [14].
  • The anatomy is too complex to see on standard scans [15].
  • You are already undergoing a procedure, like a stent placement, where the doctor needs real-time “live” images [16].

Be aware that DSA carries small but important risks, including bleeding, contrast reaction or kidney injury, and a small risk of embolic stroke.

Your Imaging Report: A Completeness Checklist

When you receive your imaging report, look for these specific details to discuss with your doctor:

  1. Laterality: Does it clearly state which side is affected (Left vs. Right)? [17]
  2. Degree of Stenosis: Is the narrowing listed as a specific percentage or category (e.g., 70-99%)? Percentages are often reported using the NASCET method [4].
  3. The “String Sign” Check: Does the report explicitly mention if it’s a total occlusion or if there is a “threadlike” flow (near-occlusion)? [8]
  4. Plaque Features: Does it mention “ulceration” (pits in the plaque) or “mobile thrombus” (a loose clot), which can increase stroke risk? [18]
  5. Intracranial & Collateral Check (If clinically relevant): Did the specialized scans look at the vessels inside the brain to check for “tandem” blockages or collateral flow? [19][20]

Common questions in this guide

What imaging test is usually done first for suspected carotid artery blockage?
For stable outpatient evaluation, duplex ultrasound is usually the first test. It uses sound waves to measure blood flow and does not use radiation or contrast dye; emergency stroke evaluations may go directly to rapid brain and CT or MR angiography.
Why might I need a CTA or MRA after a carotid ultrasound?
If ultrasound suggests severe narrowing or complete blockage, a CTA or MRA can provide a more detailed view and confirm the finding. These scans can help distinguish a true occlusion from a near-occlusion, and the choice depends on kidney function, contrast reactions, implants, and urgency.
What is a carotid near-occlusion or string sign?
A near-occlusion is very severe narrowing that leaves unusually little blood flow and a smaller artery farther along the internal carotid; the remaining channel may look like a thin string on an angiogram. It is different from a complete occlusion, and treatment decisions require specialist review.
Can an ultrasound mistake a near-occlusion for a total blockage?
Yes. Very slow flow may be below the ultrasound’s ability to detect it, and calcium can create shadows that obscure the artery; results also depend partly on the operator. CTA or MRA may be used to clarify an uncertain or serious finding.
What should I look for in my carotid imaging report?
Check which side is affected, the reported degree of narrowing and whether it uses the NASCET measurement method, and whether the report describes total occlusion, near-occlusion, or a string sign. It may also mention plaque ulceration, a mobile thrombus, tandem blockages inside the brain, or collateral blood flow.
When is a catheter angiogram or DSA needed?
Digital subtraction angiography is usually reserved for cases in which noninvasive scans are unclear, disagree, or cannot show complex anatomy, or when live images are needed during a planned procedure. Because it uses a catheter and contrast, it has risks such as bleeding, kidney injury, contrast reaction, and a small risk of stroke.
Can I have an MRA if I have a pacemaker or a contrast reaction?
The radiology team chooses CTA or MRA based on kidney function, prior contrast reactions, implants, and how urgently imaging is needed. A previous reaction to iodinated contrast does not automatically rule out every CTA, and many newer pacemakers are MRI-compatible after the device is checked.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my ultrasound report suggest a complete blockage (occlusion) or 'near-occlusion' with a threadlike flow?
  2. 2.If my ultrasound says 'occlusion,' can we confirm this with a CTA or MRA to make sure it isn't a 'pseudo-occlusion'?
  3. 3.What specific velocity criteria did the ultrasound lab use to determine my percentage of narrowing?
  4. 4.Do I have any 'acoustic shadowing' from calcium that might be making my ultrasound results less accurate?
  5. 5.Based on my imaging, is there any evidence of a 'tandem' blockage further up inside my brain?
  6. 6.Is a catheter-based angiogram (DSA) necessary for me, or can we make a treatment plan based on my non-invasive scans?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page explains carotid artery imaging for informational purposes only and does not constitute medical advice. Your clinician and radiology team should interpret your results and recommend the safest next step for you.

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