Peripheral arterial disease (PAD) affects millions of people and is significantly underdiagnosed. Lower extremity arterial duplex is the primary non-invasive diagnostic tool. This is a technically demanding exam — here's how to approach it systematically.

Indications

Claudication (calf, thigh, or buttock pain with walking that resolves with rest)
Rest pain (severe PAD)
Non-healing wounds or ulcers on the foot or lower leg
Abnormal ABI or pulse volume recordings on segmental pressures
Pre-procedure planning for revascularization
Post-intervention surveillance (stent, bypass graft)
Pulsatile mass in the popliteal fossa (popliteal artery aneurysm)

Equipment and Setup

Probe: Start with a 5–9 MHz linear transducer for the thigh and calf. Use a 2–5 MHz curvilinear probe for the aortoiliac segment (deep vessels in larger patients).

Patient position: Supine for most of the exam. Semi-prone or lateral decubitus for popliteal access.

Color Doppler settings: Optimize for arterial flow — higher PRF than venous studies, lower wall filter.

Anatomy

Follow the arterial tree from proximal to distal:

Aortoiliac: Common iliac → external iliac → common femoral
Thigh: Common femoral → profunda femoris (deep femoral) + superficial femoral artery (SFA) → popliteal
Infrageniculate: Popliteal → anterior tibial + tibioperoneal trunk → posterior tibial + peroneal

The SFA is the most commonly diseased segment and runs through Hunter's canal (adductor canal) in the mid-thigh — this is where it dives deep and is the most technically challenging segment to image.

Scanning Protocol

Step 1: ABI First

Before imaging, obtain ankle-brachial indices bilaterally:

Measure brachial systolic pressure in both arms (use the higher)
Measure systolic pressure at the dorsalis pedis and posterior tibial with a handheld Doppler
ABI = ankle pressure ÷ highest brachial pressure

ABI	Interpretation
> 1.4	Non-compressible (calcified vessels — common in diabetics)
1.0–1.4	Normal
0.9–1.0	Borderline
0.7–0.9	Mild PAD
0.5–0.7	Moderate PAD
< 0.5	Severe PAD / critical limb ischemia

Step 2: Waveform Survey

Before detailed duplex, quickly sample waveforms at key levels bilaterally:

Common femoral artery (CFA)
Mid SFA
Popliteal
Posterior tibial

A normal lower extremity artery at rest is triphasic (three phases: forward flow, brief reversal, forward again). Loss of the reversal phase (biphasic → monophasic) indicates proximal obstruction.

Step 3: Aortoiliac Segment

Low-frequency curvilinear probe, patient supine
Sample the distal aorta, both common iliac and external iliac arteries
Evaluate for stenosis, occlusion, aneurysm
Dampened waveforms at the CFA suggest proximal disease even if you can't directly image the iliac vessels

Step 4: Common Femoral Artery

The CFA is your reference point for the entire study.

Located just inferior to the inguinal ligament
Should be triphasic in a normal patient
Evaluate the CFA bifurcation into SFA and profunda (deep femoral)
Plaque and stenosis at the CFA are common

Step 5: Superficial Femoral Artery

The SFA is the workhorse of this exam and the most common site of disease.

Follow from CFA bifurcation to the adductor canal (Hunter's canal)
Image in transverse to identify the vessel, switch to sagittal for Doppler
The adductor canal segment is deep — use enough gain, possibly switch to lower frequency
Key pitfall: Don't confuse the profunda with the SFA. The SFA continues straight down the thigh; the profunda dives posteriorly.

Doppler criteria for SFA stenosis:

Stenosis	PSV Ratio	Waveform
< 50%	< 2:1	Preserved triphasic
50–75%	2:1 to 4:1	Biphasic distal
> 75%	> 4:1	Monophasic distal
Occlusion	No flow	Collateral waveforms distally

PSV ratio = PSV at stenosis ÷ PSV in the normal segment just proximal to it.

Step 6: Popliteal Artery

Patient prone, or supine with knee slightly flexed and foot elevated
Access from the popliteal fossa
Evaluate for stenosis, occlusion, and popliteal aneurysm (normal diameter < 10–12 mm)
The popliteal is the most common site for peripheral aneurysm — always measure in transverse

Step 7: Infrageniculate Vessels

Posterior tibial: behind the medial malleolus → follow proximally up the calf
Anterior tibial: anterior compartment, lateral to the tibia → becomes dorsalis pedis at the ankle
Peroneal: deepest, between tibia and fibula — most difficult to image
In critical limb ischemia, you may only find flow in one vessel. Document which ones are patent.

Waveform Interpretation

Triphasic: Normal peripheral artery at rest — high resistance with forward, reverse, and forward components.

Biphasic: Loss of reversal phase. May be normal in young people or after exercise. Suggests mild proximal disease or vasodilation.

Monophasic/dampened: Significant proximal obstruction. Low velocity, prolonged systolic upstroke.

Tardus-parvus: Slow upstroke, rounded peak, no reversal. Classic for hemodynamically significant stenosis or occlusion upstream.

Bypass Graft Surveillance

Post-bypass patients need regular surveillance. The graft can be native vein or synthetic.

What to look for:

Vein graft stenosis (most common at anastomoses or valve sites)
PSV > 300 cm/s at any graft segment = significant stenosis
PSV < 45 cm/s throughout = at risk for thrombosis
Kinking, perigraft fluid collection

Reporting Template

Document for each segment:

Vessel patency (patent / occluded / not visualized)
Plaque description (calcified / soft / mixed)
Peak systolic velocity
PSV ratio at any stenosis
Waveform character (triphasic / biphasic / monophasic)

Conclusion language:

"Triphasic waveforms throughout the right lower extremity arterial system. No hemodynamically significant stenosis identified."
"Hemodynamically significant stenosis of the left mid SFA. PSV 380 cm/s, PSV ratio 3.8:1. Distal waveforms monophasic."
"Occlusion of the right SFA from mid-thigh to the adductor canal. Reconstitution via collaterals at the popliteal level with monophasic waveform."

Common Pitfalls

Missing Hunter's canal: The SFA dives deep here and is easy to lose. Follow continuously in longitudinal view, adjusting depth and angle as it descends.

Confusing profunda for SFA: They're side by side at the bifurcation. The profunda goes deep and lateral; the SFA continues straight.

Calling a calcified vessel normal: If the ABI is > 1.4, the vessels are calcified and incompressible. The ABI is falsely elevated — this isn't normal. Refer to toe pressures.

Missing popliteal aneurysm: Always measure the popliteal in transverse. An aneurysm here can thrombose or embolize without warning.

SonoBuddy's Calculators include the ABI calculator with interpretation. Protocols section covers lower extremity arterial step-by-step.

Lower Extremity Arterial Duplex: PAD Protocol Step by Step