Vascular Imaging

Which Imaging Is Best for Suspected FMD or SAM in the Lower Extremities?

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Which Imaging Is Best for Suspected FMD or SAM in the Lower Extremities?

A 45-year-old woman with no history of smoking or diabetes presents with progressive right calf claudication. Ankle-brachial indices are borderline, and her presentation doesn’t fit the typical profile for atherosclerosis. You suspect a less common, noninflammatory vascular cause like fibromuscular dysplasia. The critical question is which imaging study will provide a definitive diagnosis without missing subtle arterial wall abnormalities. This article provides a clinical workflow for this specific scenario, guiding you through the differential diagnosis and the rationale for the recommended imaging. Based on the American College of Radiology (ACR) Appropriateness Criteria, conventional digital subtraction `Arteriography lower extremity` is rated Usually Appropriate for this initial workup.

Who Fits This Clinical Scenario?

This guidance applies to patients, often younger than 60 and without significant atherosclerotic risk factors, who present with signs of lower-extremity arterial insufficiency. The key feature is the suspicion of a noninflammatory, nonatherosclerotic vasculopathy. This includes patients with unexplained claudication, limb ischemia, non-traumatic arterial dissection, or aneurysms found incidentally.

This workflow is specifically for conditions like fibromuscular dysplasia (FMD) or segmental arterial mediolysis (SAM). It is crucial to distinguish this scenario from others that may present similarly but require a different diagnostic approach:

  • Inflammatory Vasculitides: If the patient has systemic symptoms like fever, weight loss, or elevated inflammatory markers (ESR, CRP), the workup should follow the pathway for suspected inflammatory vasculitis, as the imaging goals and potential treatments differ significantly.
  • Popliteal Entrapment Syndrome: If a young, athletic patient experiences claudication primarily with specific physical maneuvers like plantar flexion, the primary suspicion should be popliteal entrapment, which has its own dedicated imaging protocol often involving provocative maneuvers.
  • Connective Tissue Disorders: If the patient has a known diagnosis like Marfan syndrome or Ehlers-Danlos syndrome, the imaging workup is tailored to look for dissection and aneurysm, following a distinct clinical pathway.

What Diagnoses Are You Working Up in This Scenario?

When atherosclerosis is unlikely, the differential for lower-extremity arterial disease shifts to a set of less common but important conditions. The choice of imaging is driven by the need to visualize the specific pathologies associated with these diagnoses.

Fibromuscular Dysplasia (FMD): This is a leading consideration, especially in middle-aged women. FMD is a noninflammatory, nonatherosclerotic disease that causes abnormal cell growth in the artery walls, leading to stenosis, aneurysm, or dissection. The classic angiographic finding is the multifocal “string of beads” appearance, though focal stenoses can also occur.

Segmental Arterial Mediolysis (SAM): A rare and poorly understood condition characterized by the degeneration of the outer muscular layer (media) of the arterial wall. This weakening can lead to spontaneous dissection, hemorrhage, aneurysm formation, or occlusion. SAM often presents more acutely than FMD and can be life-threatening if it leads to arterial rupture.

Adventitial Cystic Disease: This uncommon condition typically affects the popliteal artery in young to middle-aged men. A mucin-filled cyst develops within the adventitial layer of the artery wall, compressing the lumen and causing progressive claudication. The artery itself is often otherwise healthy.

External Iliac Artery Endofibrosis: Primarily seen in high-performance cyclists and other elite endurance athletes, this condition involves fibrosis and thickening of the intimal layer of the external iliac artery, leading to flow limitation during peak exertion.

Why Is Arteriography the Recommended Study for This Presentation?

While multiple modalities are highly rated, conventional digital subtraction arteriography (DSA) offers the highest spatial resolution, making it the gold standard for visualizing the subtle mural and luminal changes characteristic of FMD and SAM. The ACR panel rates `Arteriography lower extremity` as Usually Appropriate.

The primary advantage of arteriography is its ability to clearly delineate the “string of beads” pattern in multifocal FMD or identify subtle dissections and small aneurysms in SAM that might be less conspicuous on cross-sectional imaging. This level of detail is often necessary for a definitive diagnosis and subsequent treatment planning, which may include angioplasty. The procedure carries an adult radiation level of ☢☢ (0.1-1 mSv) but is invasive, requiring arterial access.

Other modalities are also highly effective and rated Usually Appropriate:

  • CTA lower extremity with IV contrast: Offers excellent spatial resolution and rapid acquisition, making it a strong non-invasive alternative. It is highly sensitive for detecting stenoses, aneurysms, and dissections. However, it involves a higher radiation dose (☢☢☢, 1-10 mSv) and iodinated contrast.
  • MRA lower extremity without and with IV contrast: Avoids ionizing radiation and provides excellent soft-tissue contrast, which can be valuable for identifying intramural hematoma or adventitial cystic disease. Its spatial resolution may be slightly lower than CTA or DSA, potentially missing the most subtle FMD findings.

A lower-rated alternative, `US duplex Doppler lower extremity`, is rated May be appropriate. While non-invasive and radiation-free, its utility is often limited to screening. It can detect hemodynamically significant stenoses but may fail to visualize the specific morphological features of FMD or SAM, especially in deeper vessels like the iliac arteries. It is not typically sufficient for a definitive diagnosis in this context. For a detailed overview of the scanning technique, see our protocol guide: US Lower Extremity Doppler (DVT).

What’s Next After Arteriography? Downstream Workflow

The results of the initial imaging study will guide the subsequent clinical pathway, from medical management to potential intervention.

If the study is positive for Fibromuscular Dysplasia (FMD): The next step involves assessing the clinical significance of the findings. If a hemodynamically significant stenosis is identified as the cause of claudication, percutaneous transluminal angioplasty (PTA) without stenting is often the preferred treatment. Medical management with antiplatelet therapy and blood pressure control is also crucial. The patient should be screened for FMD in other vascular beds, particularly the renal and cerebrovascular arteries.

If the study is positive for Segmental Arterial Mediolysis (SAM): Management depends on the presentation. For symptomatic dissections or expanding aneurysms, endovascular or open surgical repair may be necessary. Asymptomatic or stable findings may be managed conservatively with close surveillance imaging and strict blood pressure control.

If the study is negative: A negative high-quality arteriogram, CTA, or MRA makes intrinsic arterial disease unlikely. The workup should pivot to consider other causes of leg pain. If claudication is exertional in a young athlete, consider the specific workup for popliteal entrapment syndrome or external iliac artery endofibrosis, which may require dynamic imaging with provocative maneuvers.

If the study is indeterminate: If a non-invasive study like CTA or MRA is suggestive but not definitive, conventional arteriography is the logical next step to provide the highest-resolution images for a final diagnosis.

Pitfalls to Avoid (and When to Get Help)

Navigating the workup for rare vascular diseases requires careful attention to detail to avoid common missteps. One major pitfall is misattributing symptoms in a younger patient to premature atherosclerosis, which can delay the correct diagnosis. Another is ordering a non-contrast study; a CTA or MRA performed without intravenous contrast is inadequate for evaluating the arterial lumen and wall in this scenario. Finally, failing to consider the entire arterial tree can be a mistake, as conditions like FMD are systemic and may be present in other vascular beds. If imaging reveals complex dissections, rapidly expanding aneurysms, or signs of acute limb ischemia, immediate consultation with a vascular surgeon or interventional radiologist is critical.

Related ACR Topics and Tools

This article focuses on a single clinical scenario. For a comprehensive overview of the ACR’s recommendations for all nonatherosclerotic peripheral arterial disease presentations, from popliteal entrapment to vasculitis, please consult our parent guide. You can also use the tools below to explore other scenarios, protocols, and radiation safety considerations.

Frequently Asked Questions

Why is conventional arteriography still recommended when non-invasive CTA and MRA are available and also rated ‘Usually Appropriate’?

Conventional arteriography (DSA) provides the highest spatial resolution of the three modalities. This is critical for visualizing the very subtle findings of some noninflammatory vasculopathies, such as the classic ‘string of beads’ in fibromuscular dysplasia (FMD) or small intimal tears. While CTA and MRA are excellent and often sufficient, DSA remains the problem-solving tool when a definitive diagnosis is required or if endovascular intervention is planned in the same session.

If I suspect FMD in the lower extremities, should I also image the renal and carotid arteries?

Yes. Fibromuscular dysplasia is a systemic disease. A diagnosis of FMD in one vascular bed should prompt a one-time, baseline imaging evaluation of all other potential territories from head to pelvis, most commonly with CTA or MRA. This is to screen for asymptomatic aneurysms or significant stenoses, particularly in the renal and cerebrovascular arteries, which could have major clinical consequences.

Can Duplex ultrasound be used as the first imaging test for suspected FMD?

Duplex ultrasound is rated ‘May be appropriate’ by the ACR for this scenario. While it is a good non-invasive tool for detecting flow-limiting stenoses, it has limitations. It is highly operator-dependent and may not be able to visualize the specific morphological changes of FMD, especially in the iliac arteries. It is often used as an initial screening test, but a negative or equivocal ultrasound in a patient with high clinical suspicion should be followed by CTA, MRA, or arteriography.

What is the difference in presentation between FMD and Segmental Arterial Mediolysis (SAM)?

While both are nonatherosclerotic vasculopathies, their presentations often differ. FMD typically presents with chronic symptoms like claudication or can be found incidentally. SAM, on the other hand, often presents more acutely with symptoms related to spontaneous arterial dissection, intramural hematoma, or aneurysm rupture, which can cause sudden, severe pain or signs of malperfusion.

Does a finding of FMD or SAM always require intervention?

No. Treatment is guided by the clinical presentation and the severity of the imaging findings. Asymptomatic or non-flow-limiting lesions are typically managed medically with antiplatelet agents, blood pressure control, and routine surveillance imaging. Intervention, such as angioplasty or surgery, is reserved for patients with lifestyle-limiting symptoms (like severe claudication), acute limb ischemia, or high-risk features like enlarging aneurysms or complicated dissections.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 29, 2026