• Recognize the risk factors, signs and symptoms associated with peripheral arterial disease
• Describe the arterial anatomy of the lower extremity
• Describe the technical aspects of imaging the arteries of the lower extremity
• Discuss the imaging and Doppler signal characteristics obtained during arterial duplex imaging

Peripheral arterial disease is estimated to affect approximately 8 – 12 million individuals in the United States.1,2 Originally, the purpose of the noninvasive arterial evaluation was to offer objectivity in the diagnosis of lower extremity arterial disease. It was intended to complement but not to replace a careful history and physical examination of the patient. Currently, after decades of evolution, the noninvasive arterial evaluation may be tailored to a patient’s specific needs, depending on the clinical presentation and the pathologic findings being evaluated.

Arterial duplex imaging provides direct anatomic and physiologic information, but it does not provide information regarding overall limb hemodynamics. Duplex imaging distinguishes between a stenosis and an occlusion, determines the length of the disease segment and patency of the distal vessels, evaluates the results of intervention (angioplasty, stent placement), diagnoses aneurysms and pseudoaneurysms, and monitors a patient's postoperative course with continuing bypass graft surveillance.

The following risk factors are associated with peripheral occlusive arterial disease.3

Non-modifiable Risk Factors

• Age (risk increases with increasing age)
• Sex (Males greater than females)

Modifiable or Controllable Risk Factors

• Hypertension
• Diabetes mellitus
• Hyperlipidemia
• Smoking
• Documented atherosclerosis in the coronary or carotid system

Symptoms

• Claudication: Muscular discomfort of the calf, thigh, hip, or buttock with ambulation. Patients describe a cramping, aching, or pain in the muscles of their legs that is relieved by stopping the walking/exercise, and standing or sitting for 2 to 5 minutes.
• Rest pain: Critical ischemia of the distal limb when the patient is at rest. Patients usually complain of pain in their toes when they are lying down. The pain often awakens a patient that is sleeping, and the patient may find relief by sitting with the affected limb in the dependent position.

Physical Signs

• Decreased peripheral pulses (Femoral, Popliteal, Dorsalis pedis, Posterior tibial)
• Bruits
• Elevation pallor and dependent rubor
• Ischemic ulcers, gangrene
  

The descending aorta is the continuation of the aorta beyond the aortic arch. The descending aorta is divided into a thoracic section and an abdominal section. The thoracic section of the aorta terminates at the aortic opening in the diaphragm. The abdominal aorta begins at the level of the 12th thoracic vertebra as it passes through the aortic hiatus of the diaphragm. The abdominal aorta terminates in the bifurcation of the right and left common iliac arteries (approximately at the level of the fourth lumbar vertebra). Each of the common iliac arteries bifurcates into an internal iliac artery (hypogastric artery) that supplies the pelvis and an external iliac artery that continues distally to supply the lower extremity. The external iliac artery terminates at the inguinal ligament where it becomes the common femoral artery.

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The arteries of the lower extremity.

Leg

The common femoral artery originates beneath the inguinal ligament and terminates by dividing into the superficial femoral and profunda femoris arteries. The profunda femoris artery is posterior and lateral to the superficial femoral artery. The profunda femoris (deep femoral) artery begins at the common femoral bifurcation and terminates in the lower third of the thigh. The profunda femoris artery supplies the muscles of the thigh and the hip joint. The superficial femoral artery travels the length of the thigh, travels through Hunter’s canal, and terminates at the opening of the adductor magnus muscle. The proximal superficial femoral artery is superficial and dives deep in the distal portion of the thigh. The popliteal artery begins at the opening of the adductor magnus muscle and travels behind the knee in the popliteal fossa. Major branches off the popliteal artery are the sural and genicular arteries. The popliteal artery terminates distally into the anterior tibial artery and the tibial-peroneal trunk.

The anterior tibial arteries take off at the popliteal and travel down the lateral calf in the anterior compartment to the level of the ankle. The dorsalis pedis artery is a continuation of the anterior tibial artery on the top of the foot. The arterial branches of the anterior tibial artery join branches of the posterior tibial artery to form the plantar arch. Arising off the plantar arch are the metatarsal arteries that divide into the digital branch arteries.

The tibial-peroneal trunk takes off after the anterior tibial artery, and bifurcates into the posterior tibial artery and the peroneal artery. The posterior tibial artery travels down the medial calf in the posterior compartment and terminates between the ankle and the heel into the medial and lateral plantar arteries. The peroneal artery is located deep within the calf and travels near the medial aspect of the fibula. The peroneal artery terminates in the distal third of the calf and its branches communicate with branches of the posterior and anterior tibial arteries.

The examination is explained and a history (risk factors, signs and symptoms) is obtained from the patient. The presence/absence of peripheral pulses and bruits should be documented. Visualization of the proximal arteries is improved if patients do not take anything by mouth the morning of the examination (this should be explained to the patient when they are making the appointment).

Suggested instrument set-ups for arterial duplex imaging are as follows: (1) use a low frequency (2-3.5 MHz) curved array transducer for the proximal segment of the examination and a high frequency (5-10 MHz) linear array transducer for the evaluation of the leg, (2) image orientation: head to the left and feet to the right side of the monitor, (3) color assignment: although color is based on the direction of blood flow (toward or away) in relation to the transducer, red is usually assigned to arterial blood flow, (4) the color scale (PRF) should be adjusted throughout the examination to evaluate the changing velocity patterns, (5) the color gain should be adjusted throughout the examination as the signal strength changes, and (6) the color box width affects frame rates so the color display should be adjusted appropriately.

Arterial duplex imaging is performed with the patient lying in the supine position on an examination table. Peripheral arterial imaging begins at the level of the aortic bifurcation. Visualization of the proximal arteries is improved if patients do not take anything by mouth the morning of the examination. It is best to use a low frequency transducer (2.0-3.5 MHz) for the proximal segment of the examination. The aortic bifurcation is best seen with the patient turned to the left side and with the transducer placed just in front of the right iliac crest in a longitudinal plane. The distal aorta can usually visualized with the origin of both common iliac arteries. Doppler signals should be obtained from all three vessels at this location.

Turn the patient into a lateral decubitus position (side being evaluated up) to evaluate the internal and external iliac arteries with the transducer placed between the iliac crest and the umbilicus. Doppler waveforms should obtained from the internal and external iliac arteries, noting direction of blood flow and velocity. If difficulty is encountered in locating the iliac arteries from this approach, the arteries may be located by identifying the femoral arteries at the groin level and following the arteries proximally.

The patient returns to the supine position and a higher frequency linear array transducer (5-10 MHz) should be used for evaluation of the arteries of the lower extremity. The common femoral artery is located at the level of the groin. The artery lies lateral to the common femoral vein. Imaging should be performed in the longitudinal plane, and a Doppler signal should be obtained from this artery. The vessel should be followed distally on the leg to the origin of the superficial femoral and profunda femoris (deep femoral) arteries. Doppler signals should be obtained from the origin of both the superficial femoral artery and the deep femoral artery. The superficial femoral artery is followed distally as it courses down the medial aspect of the thigh. Doppler signals should be obtained along its pathway and at areas of questionable narrowing. The distal portion of the superficial femoral artery may be easier to evaluate from the distal posterior thigh. This artery is followed distally in the limb and becomes the popliteal artery. The popliteal artery should be followed through the popliteal fossa. The popliteal artery lies deep to the vein, and a Doppler spectral waveform should be obtained from this vessel.

Following the distal popliteal artery in a longitudinal plane, the origin of the anterior tibial artery can usually be visualized diving deep on the monitor. The anterior tibial artery can only be followed for a short distance from this approach. The remainder of the vessel can be located distally by placing the transducer on the lateral calf and it can be followed to the level of the ankle. The tibial-peroneal trunk extends into the calf from the popliteal artery. The posterior tibial and peroneal arteries are usually visualized by placing the transducer on the medial calf. The peroneal artery lies deep and runs parallel to the posterior tibial artery. These vessels are located above the malleolus and followed proximally.

At the end of the arterial duplex examination, the ultrasound gel should be removed from the patient with a clean towel, and any excess gel should be removed from the transducer. The transducer should be cleaned using a disinfectant.

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Longitudinal view of the common femoral artery (power imaging) and the bifurcation of the superficial femoral artery and the deep femoral artery.

The accurate interpretation of arterial duplex imaging depends upon the quality and the completeness of the evaluation. Often the patient’s body habitus will affect the quality of the image and the sonographer’s ability to search the entire arterial system of the lower extremity. The sonographer must be prepared to change transducers if necessary to complete the examination, and have a complete understanding of the equipment controls to optimize the color Doppler image. Careful assessment of the Doppler spectral waveform characteristics is key to the accuracy of the arterial examination of the lower extremity.

The gray scale image and color Doppler display are helpful in recognizing anatomic variations and locating plaque and calcification, but are not accurate in determining the amount of arterial narrowing. The percent narrowing of an artery is determined from the Doppler spectral waveform information. A small Doppler sample volume is used for arterial imaging and the Doppler spectral waveforms are obtained by maintaining a 60-degree angle to the vessel walls. If a 60-degree angle cannot be maintained, documentation of the angle used during the examination is important, especially in following the patient over time. It is best not to use an angle greater than 60 degrees because of the inherent error associated with using larger angles. The Doppler is swept through the color display looking for focal increases in velocity or blood flow disturbances. Representative Doppler signals are recorded from standard sites along the peripheral arteries. Additionally, if an area of narrowing is noted, Doppler signals proximal to the area, at the narrowing, and distal to the narrowing will provide the complete documentation needed for an accurate interpretation.

The duplex imaging criteria for the normal arterial evaluation of the lower extremity is a triphasic Doppler signal from the abdominal aorta to the tibial arteries at the ankle. The characteristic normal arterial waveform has a high velocity forward flow component during systole (ventricular contraction), followed by a brief reversal of flow in early diastole (because of peripheral resistance), and a final low velocity forward flow phase in late diastole (elastic recoil of the vessel wall). Peak systolic velocity gradually decreases from the proximal to the distal arteries. The peak velocity in the abdominal aorta is approximately 100 cm/sec and the velocity gradually decreases to 70 cm/sec in the popliteal artery.

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A Doppler spectral waveform from a normal right femoral artery. The Doppler signal was obtained from a longitudinal view and with the Doppler sample volume placed in middle of the lumen. The characteristic triphasic Doppler signal shows a fast upstroke to peak systole (1), reversal of blood flow during early diastole (2), and a forward flow component during late diastole (3).

Criteria have been developed for duplex imaging to detect abnormal arterial segments. A triphasic waveform with an increase in peak velocity of 30% to 100% relative to the adjacent proximal segment indicates disease, defined as a stenosis with a narrowing of less than 50% of the diameter of the artery. A 50% - 99% diameter reduction stenosis produces a monophasic waveform with extensive spectral broadening (due to turbulence) and a peak systolic velocity of more than 100% relative to the adjacent proximal segment, and reduced systolic velocity is present distal to the stenosis. The three major changes in the spectral Doppler arterial waveform that occur because of a significant stenosis are: an increase in peak systolic velocities (greater than 100%), marked spectral broadening because of turbulence, and the loss of the reversal of blood flow during diastole. The color Doppler display also provides information that identifies the presence of a significant arterial narrowing. Color aliasing, color persistence (continuous signal), and color bruit (tissue vibration caused by severe blood flow disturbance) indicate the presence of a blood flow abnormality.

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A longitudinal view of the distal superficial femoral artery in a patient with symptoms of claudication. There is an area of increased velocity demonstrated by color Doppler aliasing and an increase in peak systolic velocity to approximately 550 cm/sec. The Doppler signal from the arterial segment proximal and distal to this area had an abnormal waveform shape and the peak systolic velocity was approximately 60 cm/sec. These duplex imaging findings suggest an arterial stenosis of the distal superficial femoral artery.

No color Doppler or Doppler spectral waveform will be obtained in areas of occlusion. Additionally, damped proximal arterial Doppler spectral waveforms are obtained which demonstrate a low velocity with little or no diastolic blood flow. The color Doppler display may reveal collaterals near the occluded segment. Power Doppler imaging or B-flow imaging improves visualization of areas of tight stenosis, especially in vessels running parallel to the skin line.

Duplex imaging has been compared with lower extremity arteriography to define its accuracy.4-8 These studies demonstrate a sensitivity of 77% to 92% and a specificity of 92% to 98% for correctly categorizing a stenosis as greater or less than 50% diameter reduction. These reports evaluated the capabilities of duplex imaging in the proximal vessels, but there is limited information about its sensitivity in the calf arteries.9

All studies reported high negative predictive values (87% to 98%), indicating that significant occlusive arterial disease can be excluded in patients with normal duplex imaging examinations.

Limitations of the lower extremity arterial duplex imaging examination are:

• Nonvisualization of the iliac system because of bowel gas or obesity
• Shadowing because of calcification
• Difficulty imaging the popliteal trifurcation
• Difficulty evaluating lesions distal to tight stenoses because of low velocities in these segments

Aneurysms and Pseudoaneurysms

Arterial duplex imaging has also become valuable for evaluating patients with aneurysms and pseudoaneurysms of the extremity.10-15 Duplex imaging distinguishes between aneurysms, pseudoaneurysms, perigraft fluid collections, and hematomas. Information about the size, location, site of communication, and presence of luminal thrombus is easily obtained in most cases.

The evaluation of arterial aneurysms by duplex imaging is considered an accurate method in determining the size, position, patency, and associated arterial blood flow dynamics. Aneurysms may be located in the distal abdominal aorta, iliac, common femoral, and popliteal arteries. To accurately measure the size (length and width) and shape of an aneurysm the artery should be evaluated in both longitudinal and transverse imaging planes.

Pseudoaneurysms occur as a result of trauma, at a vascular anastomoses, in angioaccess grafts, or at puncture sites (usually following cardiac catheterization). A pseudoaneurysm is a perivascular collection (hematoma) that communicates with an artery or a graft and has the presence of pulsating blood entering the collection. A track (neck) of variable length connects the native vessel to the collection.

A pseudoaneurysm may be unilocular or multilocular, and may partially contain thrombus. Pseudoaneurysms occur in variable sizes and the size of the pseudoaneurysm changes during each cardiac cycle. Although spontaneous thrombosis of pseudoaneurysms have been reported in the literature, fatal spontaneous hemorrhage of pseudoaneurysms have also occurred.

Swirling blood within the collection is often visualized in gray scale or B-flow imaging. The use of color Doppler imaging helps to identify the neck of the pseudoaneurysm. Identification of the neck of the pseudoaneurysm is important when ultrasound guided compression therapy is attempted and color Doppler imaging permits identification of the vessel of origin which is important when planning surgical interventions.

A right common femoral artery (CFA) pseudoaneurysm. In this color Doppler image, the neck of the pseudoaneurysm (arrow) is visualized connecting the CFA to the perivascular collection.		The Doppler spectral waveform obtained from the neck of the common femoral artery pseudoaneurysm. Note the characteristic to-and-fro pattern of the Doppler spectral waveform.

A spectral Doppler waveform obtained from the neck of a pseudoaneurysm displays a to-and-fro (bidirectional) pattern. During systole blood flows from the native artery into the pseudoaneurysm, and during diastole blood flow returns to the native artery. Additionally, the size (length, width, and depth) of the pseudoaneurysm should be measured. Some pseudoaneurysms are very large and it is difficult to capture the entire area in one image for measurement. The sensitivity and specificity in the identification of pseudoaneurysms ranges from 94-100%.

Compression therapy of the pseudoaneurysm may be attempted if the neck of the pseudoaneurysm has been clearly identified during arterial duplex imaging. The neck of the pseudoaneurysm is the area that is compressed with pressure placed on the skin by the ultrasound transducer. The goal is to stop the blood flow into the pseudoaneurysm by occluding the neck. It is extremely important not to occlude blood flow in the native artery so distal arterial blood flow should be monitored during compression therapy.

Ultrasound compression therapy of pseudoaneurysms takes a significant amount of time and upper body strength to maintain the compression for extended periods of time. Compression cycles of 15 to 20 minutes are usually performed with progress being evaluated between cycles. In most cases it will take at least 30 to 60 minutes to achieve a successful thrombosis of the pseudoaneurysm. Most investigators suggest bed rest (6-24 hours) following a successful compression procedure and re-examination of the area the next day. Recurrence rates have been reported for ultrasound compression and a second attempt to compress the pseudoaneurysm may be warranted.

Success rates of ultrasound compression of pseudoaneurysms have varied from 70% to 86%. Success varies depending on the size and age of the pseudoaneurysms and if the patient is on anticoagulation therapy. Failure to thrombose a pseudoaneurysm occurs more often in large pseudoaneurysms, if the pseudoaneurysm has been present for a prolonged period of time, and if the patient is or has been anticoagulated. Additionally, some patients cannot tolerate the pain associated with the compression technique. The complications of ultrasound compression therapy of pseudoaneurysms has been occlusion of the native artery, development of deep vein thrombosis, and rupture of the pseudoaneurysm.

As an alternative to ultrasound compression therapy, some investigators have performed ultrasound guided thrombin injection of pseudoaneurysms.16-18 Reports indicate that this technique is highly successful and may be suitable in select patients.

Bypass Graft Surveillance

The patency of bypass grafts can be significantly prolonged if developing graft lesions are corrected before graft thrombosis.19-22 Therefore, all lower extremity bypass grafts should be monitored for technical adequacy, hemodynamic function, and development of postimplantation lesions. Graft abnormalities such as myointimal stenosis, retained valve cusps, arteriovenous fistulas, degenerative aneurysmal formation, and low flow states can be detected with arterial duplex imaging even if it has not been obvious by changes in ankle pressures. A graft surveillance program should identify bypass grafts at risk for thrombosis, provide information on the mechanism of failure, and with appropriate intervention should reduce the incidence of unexpected graft failure.

Surveillance programs by arterial duplex imaging have resulted in assisted primary patency rates of 82% - 92% at 5 years compared to 60% - 70% patency for bypasses followed clinically, and 30% to 50% patency rates after secondary procedures to salvage thrombosed vein grafts.

To accurately evaluate bypass grafts with arterial duplex imaging it is important to know the location and the type of graft prior to beginning the study. The technique is similar to the color Doppler imaging of native arteries. The entire length of the bypass graft should be evaluated with color Doppler and Doppler signals. Additionally, the inflow and outflow vessels should be evaluated as well as close attention to the proximal and distal anastomses.

The timing of bypass graft surveillance will vary with each surgeon. Investigators suggest performing an examination following the operation prior to the patient being discharged from the hospital. This baseline duplex examination permits identification of bypass grafts with residual graft defects. If the baseline postoperative duplex evaluation is considered normal, follow-up examinations are performed 4-6 weeks, 3 months, and 3 to 6 month intervals for the first postoperative year. The second postoperative year, evaluations are performed at 18 and 24 months and than on an annual basis. If graft surveillance detects a stenosis, more frequent evaluations may be performed to follow the patient or the decision to intervene may be indicated.

A change from the triphasic Doppler signal to a monophasic waveform with a decrease in peak systolic velocity to below 45 cm/sec is diagnostic of a lesion placing the graft at risk. Additionally, repair of graft stenosis is recommended with peak systolic velocities of greater than 300 cm/sec or a peak systolic ratio of greater than 3.5.

Investigators have also noted that a wide range of peak velocities may be identified in normal grafts. The peak systolic velocity in a bypass graft is due to the size of the graft and the outflow resistance. A low velocity in a bypass graft may relate to a large graft diameter, poor arterial inflow, and small vessel runoff. A significant decrease in peak systolic velocity within a graft on serial duplex imaging may be a better indicator of pending graft failure.

Other investigators suggest using a velocity ratio to determine graft stenosis. A doubling of the velocity ratio was used to indicate a significant graft stenosis.22 The velocity ratio is calculated by dividing the peak systolic velocity at the site of the flow disturbance by the peak systolic velocity in the adjacent proximal segment. A velocity ratio greater than 2.0 was associated with a sensitivity of 95% and a specificity of 100% for detection of stenosis greater than 50% diameter reduction.

Arterial duplex imaging plays an important role in the follow-up of patients with lower extremity arterial bypass grafts. Identification of arterial lesions during the preocclusive phase is critical for prolonging the patency of the graft.

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