Continuing Education Activity

Peripheral arterial disease(PAD) affects approximately 12% of the adult population, with equal prevalence among men and women. PAD which is more common in the lower limbs, comprises atherosclerosis affecting the abdominal aorta, iliac and lower extremity arteries. The main risk factors are diabetes, advanced age, and cigarette smoking. Duplex imaging of peripheral arteries is routinely performed to identify the atherosclerotic burden of these arteries. Duplex testing results are mostly interpreted in conjunction with limb-pressure measurements. The combination of Duplex results and limb pressure measurements optimally categorizes arterial hemodynamics and functional impairment. This activity reviews the assessment, protocol, and interpretation of Duplex imaging of peripheral arteries and highlights the interprofessional team’s role in evaluating and treating patients with PAD.

Objectives:

• Summarize the technique of Duplex imaging of peripheral arteries.
• Review the protocols of Duplex imaging of peripheral arteries.
• Explain the interpretation of Duplex imaging of peripheral arteries.
• Outline the clinical significance of Duplex imaging of peripheral arteries in peripheral arterial disease by an interprofessional team.

Introduction

Peripheral arterial disease (PAD) affects 3 to 5% adult population older than 40 years old.[1] PAD is associated with increased mortality, occurring concomitantly with coronary artery disease and cerebrovascular accidents.[2] PAD might present symptomatically with claudication, rest pain, and or gangrene.[3][4] 

Non-invasive measures are utilized for both diagnosis and post-intervention follow-up purposes.[5] Accordingly, individuals with a suggestive history and physical examination of PAD would be initially evaluated with non-invasive measures, including Duplex ultrasonography. Duplex scanning is an ultrasonic procedure, without contrast exposure, portable and the generated images are reproducible. However, the bowel gas would obscure the aorto-iliac territory in obese patients, so the results should be interpreted cautiously. Arterial Duplex sonography displays two-dimensional structures and movements with time and Doppler ultrasonic signals. The generated signals provide documentation with spectrum analysis and color flow velocity mapping.

Duplex testing results are primarily interpreted in conjunction with limb-pressure measurements. The combination of Duplex results and limb pressure measurements optimally categorizes arterial hemodynamics and functional impairment.[6]

Diagnosis of PAD relies on non-invasive arterial tests, which can provide physiological and morphological information. Physiological tests include segmental pressures, ankle and toe-brachial index, pulse volume recordings, exercise tests, segmental volume plethysmography, transcutaneous oxygen assessments, and photoplethysmography. Morphological tests like CT Angiography, MR Angiography, and catheter-based angiography are ideal for evaluating preprocedural anatomic configuration in individuals with already established vascular disease. Peripheral arterial duplex evaluation provides a unique spectrum of details, including the anatomical location of the lesion, accompanied by further information about flow velocity and volume.[7] 

Duplex ultrasonography combines conventional B-mode imaging with pulsed doppler techniques for spectral analysis.[8] Duplex ultrasonography is a non-invasive and cost-effective procedure used to screen, diagnose, and monitor patients with PAD. It accurately determines the location and degree of stenosis in arteries and helps differentiate stenosis from an occlusion.[9] It helps provide comprehensive anatomic and hemodynamic data of arteries examined and their outflow, which is crucial to planning revascularization.[10] It is also used to follow up with patients with interventions like angioplasty, stent replacement, and bypass grafts.[11]

The accurate and precise interpretation of velocity spectra measures by duplex scanning is essential to successfully applying the Duplex technique.[6]

Anatomy and Physiology

The abdominal aorta divides into the common iliac arteries at the L4-L5 level. The common iliac arteries are situated anterior to the iliac veins. After the internal iliac artery (L5-S1 level), the common iliac artery continues as the external iliac artery. After giving off the inferior epigastric artery medially and deep iliac circumflex artery, the external iliac artery becomes the common femoral artery. The landmark of the external iliac to the common femoral junction is the inguinal ligament. After a short course, the common femoral artery divides into the superficial and deep profunda femoris artery.

The superficial femoral artery is the main arterial supply to the thigh. It then courses through the Hunter's (adductor) canal and becomes the popliteal artery as it emerges. The popliteal artery then bifurcates into the anterior tibial artery and tibioperoneal trunk at the proximal calf. The anterior tibial artery supplies the anterior compartment of the leg and terminates at the ankle as the dorsalis pedis artery. The tibioperoneal trunk bifurcates into the posterior tibial artery and common peroneal artery. The posterior tibial artery supplies the posterior calf and courses posterior to the medial malleolus, and forms the plantar arch of the foot. On the other hand, the peroneal artery supplies the musculature of the lateral calf and courses posterior to the fibula.[9]

The ultrasound examination consists of B mode/color doppler imaging and spectral Doppler waveform evaluation with velocity measurements in the appropriate arterial segments. In the lower extremity, the common femoral artery, proximal superficial femoral artery(SFA), mid-SFA, distal SFA/popliteal artery above the knee, and popliteal artery below the knee should be evaluated in detail. If clinically appropriate, imaging of the iliac vessels, profunda femoris artery, tibioperoneal trunk, anterior tibial artery, posterior tibial artery, and dorsalis pedis artery should be performed.

B mode imaging which is performed initially, shows a 2-dimensional image of the arterial lumen and its wall, thereby allowing a rough assessment of the plaque characteristics if present.[12] Grayscale settings should be adjusted for optimal visualization of the intraluminal lesions. Color doppler should be used to improve the visualization of arterial lesions by detecting visual narrowing and differences in color and to help place the sample volume for spectral doppler assessment.[13] 

Color doppler parameters are optimized to differentiate normal and abnormal flow profiles. The pulse repetition frequency (PRF) determines the extent of the color [6]and the filling of the arterial lumen. It is adjusted so that laminar flow in a normal vessel appears as an area of homogenous color. With stenosis, the color flow pattern changes and is identified as either aliasing or desaturation(whitening) of color display at the site of luminal narrowing.[11] Velocity measurements are taken from angle-corrected longitudinal spectral doppler images. Images should be acquired so that the angle created between the blood flow direction and the ultrasound beam direction should be 60 degrees or less. Velocity measurements obtained from images using larger angles were found to be less accurate.

The principle behind doppler sonography is that the signal frequencies that are reflected from moving objects as the red blood cells change in proportion to the velocity of the target. A major advance in vascular sonography has been the introduction of duplex scanners, where spectral analysis delineates the full spectrum of frequencies (or blood flow velocities) found in the arterial waveform during a single cardiac cycle.[14] Multiple physiologic factors at the site, proximal and distal to the examined site, contribute to the doppler waveform morphology. Metabolic demands of the tissue, changes in resistance and pressure, wave reflection, and propagation all contribute to the waveform patterns.[15]

Several characteristic features assist in differentiating arterial and venous structures in Duplex sonography. Rounded structures with visible, discrete, partially compressible walls are highly suggestive of arteries. Moreover, arteries are typically smaller than the accompanying veins.[16]

A typical peripheral arterial waveform will show a sharply defined narrow tracing, which indicates that all the red blood cells are moving at a similar speed at any time of the cardiac cycle. The configuration of a typical waveform is typically triphasic, with the three components corresponding to the different phases of arterial flow.[11] It consists of a rapid antegrade flow that peaks during systole, transient reversal of flow occurring during early diastole, and slow antegrade flow occurring during late diastole.[14] The high-velocity forward flow causes the rapid antegrade flow component during systole. The early diastolic reverse flow component is caused by distal resistance at the level of small caliber vessels and capillaries. The small amount of forwarding flow causes the late diastolic component in this phase due to the elastic recoil of vessel walls.[11]

Real-time ultrasonography generates images and evaluates the blood velocity with the reflected sound waves (echoes). The two major ultrasound modes commonly used in vascular imaging are the B (brightness) and the Doppler mode (continuous or pulsed wave). Other less frequently used modes are power Doppler, three-dimensional, and contrast-enhanced ultrasound). The B-mode provides a greyscale two-dimensional image, collecting the spectrum of bright dots. The brightness of dots is discriminated according to the different amplitudes of the echo signals. The strength of the echoes or reflected sound waves would identify the quality of the images generated by B-mode. Echo strength would be affected by the process of tissue transfer. The reflected sound waves would be either attenuated or scattered as the echo transfers through the tissue.

To provide good-quality images, both the angle of insonation and the frequency of the sound waves should be considered. Accordingly, high-frequency transducers at 90 degrees of insonation would provide the optimal images for shallow structures. Doppler mode combines the B-mode characteristics with the doppler to identify vascular structures. The sound frequency change caused by the red blood cell movement is reflected in the generated velocity, which has been enhanced in the color mode. The colors do not necessarily represent the arterial or venous flow. However, the flow toward or away from the transducer is displayed in red and blue, respectively. 

The normal arterial velocity waveform includes three discrete phases( triphasic). Normal multiphasic or triphasic waveform with a narrow spectral width (range of velocities) throughout the pulse cycle implies that red blood cells' movements are in identical speed and direction with a laminar flow pattern.[17][18] That includes a sharp systolic peak, early diastolic down peak, and late diastolic forward flow. Once the peripheral resistance is developed, the late diastolic wave decreases or fades. Doppler velocity waveforms differentiate the single and multi-level arterial occlusive disease. The biphasic arterial Doppler velocity waveform consists of a rapid upstroke followed by a relatively rapid downstroke. The flow is above the baseline during all the phases, and each wave is separated from the next with a brief period of antegrade flow during diastole. The monophasic Doppler waveform is distinguished by non-sharp, rounded peaks, absence of flow reversal, and faint up and downstroke. The monophasic waveforms are predicted in multi-level obstructive arterial disease.[19][20]

The generated velocity waveforms correlate with the probe's location relative to the stenotic lesion. The proximal stenotic lesion results in more rounded systolic peaks, an absence of the reversed flow component, and a prolonged late diastolic phase. The lack of reversed flow component is not limited to the decreased peripheral vascular resistance and should be interpreted cautiously. It might be related to either compensatory post-stenotic vasodilation or even a normal finding in older patients.

Duplex signals pitch correlates with the velocity of blood changes. Therefore, high-pitched and thumping sounds are mainly heard in the stenosis and proximal to the stenotic lesion. As peripheral arteries stenoses or develop any flow disturbance, the intravascular flow no longer remains laminar and becomes turbulent. Instead of a well-defined tracing, the doppler waveform will show spectral broadening.[11] 

As the peripheral arterial disease progresses, normal triphasic flow changes to biphasic flow due to the loss of elastic recoil from the hardening of arteries in PAD.[21] As the disease worsens or is at the site of severe stenosis or occlusion, transient flow reversal during early diastole will no longer be present, resulting in a monophasic waveform. According to Bernoulli's principle, as the diameter of an artery decreases, for instance, in the case of arterial stenosis, the flow velocity through the stenosis should increase. However, when the stenosis severity reaches a critical threshold of greater than 95%, the velocity decreases.

The Doppler waveform also helps in identifying the location of arterial obstruction. Delayed systolic upstroke indicates a flow-limiting lesion proximal to the recording site—ischemia in the tissue bed distal to stenosis results in vasodilation and reduced resistance. In addition, the pressure drop across the stenosis leads to a decrease in distal pressure. This reduced resistance and pressure drop leads to increased diastolic flow all through the cardiac cycle distal to the stenosis. On the other hand, there is increased resistance proximal to high-grade stenosis or occlusion. Any antegrade diastolic flow or reflected wave, if present normally, may be absent or reduced. The severity of compromise in arterial caliber is reflected in a continual rise of peak systolic velocity (PSV) and end-diastolic velocity (EDV) to a threshold value suggesting a pre-occlusive lesion.[15]

Optimal velocity measurement requires precise and valid Doppler shift records. The angle of the probe, relative to the flow direction or the angle of insonation, and the probe position relative to the stenotic lesion, should be considered. The angle of insonation should be kept between 30 and 70 degrees. However, it has been shown that the ideal angle of insonation is 60 degrees. Moreover, the ultrasound probe should be positioned at or distal to the suspicious lesion. The characteristics of the color flow imaging would guide the location of the lesion.

The degree of stenosis is identified according to the velocity ratio. To calculate the velocity ratio, the ratio of the peak systolic velocity at the site of the stenotic lesion to the uninvolved section of the vessel is evaluated. The ratio of 1.5 to 2 refers to a 30 to 49% stenosis, while the ratios of 2 to 4 and above 4 identify 50 to 75% and more than 75% stenosis, respectively. The accuracy of the limb Duplex arterial sonography depends on the scanned segment and involved levels. The relative accuracy of Duplex scanning to CT-angiography in grading peripheral arterial disease is greater than 80%. According to the other classification, the peripheral arterial lesion is considered significant when it causes above 50% stenosis in the presence of an atherosclerotic plaque. The following Duplex features are predictable:

1. PSV of greater than 200 cm/s
2. PSV ratio of greater than 2.4 (PSV in the stenotic site to the proximal non-stenotic area).

However, PSV of greater than 300 cm/s and a ratio of above 3.5 were classified as severe stenotic lesions.[22]

To interpret the PAD severity, the following indexes are provided by duplex-acquired velocity spectra, including 1. acceleration time, 2. pulsatility index (PI), and maximum spectra velocity measured at PSV.[6]

Indications

The main indications of peripheral arterial duplex imaging include:

1. Detection of stenosis or occlusion of peripheral arteries in symptomatic patients with suspicious evidence related to PAD, including exertional leg pain, diagnosis of claudication, absent distal pulses, dependent rubor, and nonhealing ulcer.[23]
2. Monitoring areas of previous open vascular procedures, including sites of previous bypass surgery, and providing the appropriate prediction of graft thrombosis.[6]
3. Monitoring sites of previous percutaneous and endovascular interventions like angioplasty, thrombolysis/thrombectomy, and stent placements.
4. Follow-up for progression of the previously documented disease, such as stenosis in an artery that had undergone intervention, atherosclerosis, or other occlusive disorders.
5. Evaluation of vascular and perivascular pathologies like aneurysms, pseudoaneurysms, arteriovenous fistula, and vascular malformations
6. Mapping arteries before surgical intervention and providing anatomic and physiologic data to plan the optimal surgical and endovascular intervention.[10]
7. Evaluation of arterial integrity in trauma.[13]
8. Providing a grading scale for vascular trauma
9. Initial evaluation of the nonatherosclerotic diseases that affect the peripheral arteries, including popliteal entrapment syndrome, external iliac artery endofibrosis, lower-extremity inflammatory vasculitides, and dissection.[24] 
10. A diagnostic and therapeutic measure of the peripheral arterial embolization in the upper extremity

Equipment

Modern ultrasound instrumentation provides an assessment of blood flow with one of several techniques as follows:

• Color Doppler imaging
• Pulsed Doppler
• Spectral analysis
• Power Doppler imaging
• B-flow imaging
• High-resolution B-mode imaging of artery anatomy[6]

Peripheral arterial ultrasound should be performed using a real-time scanner and transducer with pulsed and color doppler capability. A linear array transducer is preferred over a curved array transducer if it allows adequate penetration.[13] 

A 5 MHZ linear array transducer is ideal for examining an average-sized adult. Linear 3.5 MHz probes for iliac and 5 MHz for CFA interventions, including PTA, might be preferred. Higher frequency probes (7.5 to 10 MHZ) are used for thin or small individuals and identifying superficial vessels. However, some centers mention utilizing a 3 to 12 MHz linear probe occasionally in very thin patients.[25] Lower frequency probes (3 MHZ) are used for larger patients to identify deep vessels.[26]

Some studies mentioned that duplex ultrasound instrumentation for peripheral arterial testing requires the application of curved and linear array transducers. A 3.5- to 7-MHzimaging frequencies to provide both appropriate penetration and comprehensive high-resolution images of the abdomen and lower extremity arteries are recommended.[6]

There are two types of Doppler ultrasound displays: 1. a color flow Doppler image and 2. a spectral Doppler image. While the former demonstrates mean flow velocity distribution as a color-encoded map integrated over the gray-scale B-mode tissue image, the latter shows the time-varying flow velocity distribution within a specific sample volume.[6][17] Therefore, further information, including  1. peak velocity during the pulse cycle and 2. the spectral content, and 3. range of velocities, are expected to be provided by spectral Doppler imaging.[17][6]

Personnel

Ultrasound technicians perform most of the peripheral ultrasound examination. Any trained medical personnel is eligible to perform the examination. Competent physicians can provide a point-of-care ultrasound and help in triage care.[9]

Technique

To perform the lower extremity duplex scanning, the patient should be examined supine with slight abduction and external rotation of the hip. The duplex scanning initiates at the inguinal crease. The transducer should be placed transversally over the common femoral artery. A typical feature of superficial and deep femoral arteries, and femoral vein just distal to the inguinal crease, on the transverse duplex, is a mickey mouse face. However, the longitudinal scanning reveals an inverted-Y feature for CFA, SFA, and DFA. The scanning would be continued by moving the transducer distally. The popliteal artery examination starts from the popliteal fossa while flexing the knee and continues proximally to the level of the Hunter canal. The alternative left lateral and prone positions are recommended to evaluate the popliteal fossa arteries, including popliteal, posterior tibialis, and peroneal.

The popliteal artery is located centrally in the popliteal fossa between the medial and lateral heads of the gastrocnemius muscles. The examination of the posterior tibialis starts from the distal end, posterior to the medial malleolus. To scan the peroneal artery, the lateral aspect of the posterior calf parallel to the fibular bone should be followed. To scan the anterior tibialis artery, the arterial course from the distal end at the talar bone neck should be followed proximally. Alternatively, the arterial course could be followed from the proximal arterial end at the tibiofibular plane and continued distally.

Angle-corrected spectral Doppler waveforms are acquired from longitudinal images proximal to, at, and distal to the sites of suspected stenosis.[13] A 90-degree imaging angle is recommended to evaluate the following arterial characteristics; 1. diameter, 2. intima-medial thickening, and 3. atherosclerotic plaque composition.[6]

Doppler beam angle of equal to or less than 60 degrees relative to the transducer insonation beam and the arterial wall of interest should be used to provide optimal pulsed spectral Doppler recordings.[17]

Diagnostic criteria for hemodynamically significant stenosis (50 to 99% stenosis) require that the PSV at the site of stenosis is double or more when compared with a more proximal segment.[21] The ratio of PSVs is superior to the absolute PSV measurements for peripheral arterial stenosis evaluation. Distal to a stenotic lesion, the post-stenotic jet will dissipate into eddy currents with a marked decrease in PSV. The waveform will lose its rapid systolic upstroke and demonstrate a delay to peak systole, termed a tardus-parvus pattern. It is essential to identify this waveform as it indicates either proximal high-grade stenosis or frank occlusion. In the case of occlusion, there will be absent flow within the arterial segment. In the absence of collaterals, the proximal segment to the occlusion shows high-resistance waveforms. A low resistance, the low-velocity tardus-parvus flow pattern, is found in the reconstructed segment if collaterals are present.[11] The estimated lengths of the stenosed or occluded segments should be documented during the evaluation.

While evaluating suspected pseudoaneurysms, the size of the pseudoaneurysm, measurements of the residual lumen, and that of the communicating channel(length and width) should be documented using grayscale and color doppler techniques. Color doppler will show the characteristic 'yin-yang' sign due to the blood swirling within the pseudoaneurysm.

 Characteristic Duplex feature of a femoral pseudoaneurysm is a "to and fro" flow pattern in the stalk of the aneurysm and sac emptying during the systolic and diastolic phases, respectively. Spectral Doppler waveforms obtained in the communicating channel demonstrate a 'to and fro' (bidirectional) flow pattern. The waveform will have a rapid systolic upstroke with exaggerated deceleration and an elongated reverse flow component.[15]

Arteriovenous fistulas (AVF) are less common than pseudoaneurysms.[27] Evaluating AVF should include spectral Doppler waveform from the artery proximal to, at the fistula site, and distal to it. Flow within the fistula and also the draining vein should also be documented. Color Doppler helps identify the level of communication as the flow disturbance in the fistula will create color in the surrounding soft tissues from the pressure changes and transmitted vibrations (color bruit).[13] Blood flow from a high-pressure artery to a low-pressure vein will result in spectral broadening, increased systolic and diastolic velocities, and continuous forward flow throughout the cardiac cycle.[15] 

On spectral doppler, there will be arterialization of flow within the vein.[28] AVT might occur following catheter-based interventions. Duplex features of AV fistula formation include a high-velocity flow jet with increased peak systolic velocity greater than 300 cm/s. Moreover, in femoral artery cannulation circumstances, the external iliac artery velocity spectra will demonstrate the following features; 1. Increased PSVs, 2. altered flow signal proximal to the AVF, and 3. triphasic distal signal. 

While evaluating peripheral aneurysms, their location and widest diameter of the aneurysm should be documented on grayscale images. If present within the aneurysm, their patency, and intraluminal thrombus should be documented using color doppler. Popliteal artery aneurysm is the most common peripheral aneurysm contributing to 80% of all cases. It is often misdiagnosed as Deep vein thrombosis (DVT), peroneal nerve palsy, ruptured Baker's cyst, and other causes of acute limb ischemia(ALI).[29]

Clinical Significance

PAD affects a large percentage of the population worldwide and is associated with significant mortality and morbidity rates. Its most severe form, Critical limb ischemia, presents as ischemic rest pain for greater than two weeks in duration. Lower extremity PAD is a common complication of systemic atherosclerosis in the elderly. This population has a 2 to a 4-fold higher risk of myocardial infarction, stroke, and coronary artery disease. Early identification allows PAD treatment and modification of risk factors to decrease the risks associated with cardiovascular disease.[30] 

Duplex ultrasound is a readily available and non-invasive method of arterial visualization in the lower extremity.[21] When used in conjunction with other non-invasive physiological tests, duplex imaging can effectively screen for PAD. It provides information regarding the location, severity, and frequency of the disease and helps determine the ideal therapeutic approach before more invasive procedures. It is also an effective tool for monitoring disease progression and follow-up after management.[11]

Enhancing Healthcare Team Outcomes

An interprofessional team approach is required due to the complex nature of PAD and its multiorgan involvement. A team approach has been found to significantly improve the patient's treatment outcomes and prevent further disease progression and risk of significant sequelae of the disease. The team can be broken down into three main sections; medical management, wound care, and revascularization, to avoid duplication of care and promote cooperation. Medical management is provided by primary care, cardiologists, endocrinologists, infectious disease team, and dietitians. Wound care is provided by podiatrists, vascular surgeons, nursing staff, orthopedics, and the plastic surgery team. Ultrasound technicians provide the scans themselves, which form the basis for directing clinical decision-making. Nurses can also provide valuable assistance in patient evaluation, counseling, and assisting during revascularization procedures. This interprofessional approach will yield the best patient results. [Level 5]

Revascularization is dealt with by intervention radiologists and cardiologists, and the vascular surgery team. A complete team should include at least one member from each team to treat patients diagnosed with PAD or critical limb ischemia. By bringing together the knowledge of different team members and effective communication, optimal patient-centered care can be provided, and the limb salvage rate can be increased.[31] [Level 3]