17 Ultrasound Assessment of Lower Extremity Arteries

The purpose of noninvasive testing for lower extremity arterial disease is to provide objective information that can be combined with the clinical history and physical examination to serve as the basis for decisions regarding further evaluation and treatment. One of the most critical decisions relates to whether a patient requires therapeutic intervention and should undergo additional imaging studies. Catheter contrast arteriography has generally been regarded as the definitive examination for lower extremity arterial disease, but this approach is invasive, expensive, and poorly suited for screening or long-term follow-up testing. In addition, arteriography provides anatomic rather than physiologic information, and it is subject to significant variability at the time of interpretation.^1,² Magnetic resonance angiography (MRA) and computed tomographic angiography (CTA) can also provide an accurate anatomic assessment of lower extremity arterial disease without some of the risks associated with catheter arteriography.³^–⁵ There is evidence that the application of these less-invasive approaches to arterial imaging has decreased the utilization of diagnostic catheter arteriography.⁶ The most valid physiologic method for detecting hemodynamically significant lesions is direct, intra-arterial pressure measurement, but this method is impractical in many clinical situations.

As discussed in Chapter 14, the nonimaging or indirect physiologic tests for lower extremity arterial disease, such as measurement of ankle systolic blood pressure and segmental limb pressures, provide valuable physiologic information, but they give relatively little anatomic detail.⁷ Duplex scanning extends the capabilities of indirect testing by obtaining anatomic and physiologic information directly from sites of arterial disease. The initial application of duplex scanning concentrated on the clinically important problem of extracranial carotid artery disease. The focal nature of carotid atherosclerosis and the relatively superficial location of the carotid bifurcation contributed to the success of these early studies.⁸ Ongoing clinical experience and advances in technology, particularly the availability of lower-frequency duplex transducers, have made it possible to obtain image and flow information from the deeply located vessels in the abdomen and lower extremities. This chapter reviews the current status of duplex scanning for the initial evaluation of lower extremity arterial disease. The more specialized applications of intraoperative assessment and follow-up after arterial interventions are covered in Chapter 18.

Instrumentation

A standard duplex ultrasound system with high-resolution B-mode imaging, pulsed Doppler spectral waveform analysis, and color flow Doppler imaging is adequate for scanning of the lower extremity arteries. A variety of transducers is often needed for a complete lower extremity arterial duplex examination. Low-frequency (2 MHz or 3 MHz) transducers are best for evaluating the aorta and iliac arteries, whereas a higher-frequency (5 MHz or 7.5 MHz) transducer is adequate in most patients for the infrainguinal vessels. In general, the highest-frequency transducer that provides adequate depth penetration should be used. The color flow image helps to identify vessels and the flow abnormalities caused by arterial lesions (Figures 17-1 and 17-2). The ability to visualize flow throughout a vessel improves the precision of pulsed Doppler sample volume placement for obtaining spectral waveforms. Thus, color flow imaging reduces examination time and improves overall accuracy. Power Doppler is an alternative method for displaying flow information that is particularly sensitive to low flow rates. The power Doppler display is also less dependent on the direction of flow and the angle of the ultrasound beam than color Doppler, and it tends to produce a more “arteriogram-like” vessel image.

FIGURE 17-1 Duplex scan of a severe superficial femoral artery stenosis. Color flow image shows a localized, high-velocity jet. Spectral waveforms obtained from the site of stenosis indicate peak velocities over 500 cm/sec.

FIGURE 17-2 Color flow image of the posterior tibial and peroneal arteries and veins. The posterior tibial vessels are located more superficially (toward the top of the image). The single arteries and paired veins are identified by their flow direction (color).

Duplex instruments are equipped with presets or combinations of ultrasound parameters for gray-scale and Doppler imaging that can be selected by the examiner for a particular application. These presets can be helpful, especially during the learning process, but these parameters may not be adequate for all patient examinations. A complete understanding of the ultrasound parameters that are under the examiner’s control (i.e., color gain, color velocity scale, wall filter) is essential for optimizing arterial duplex scans.

Duplex Ultrasound Technique

Similar to other arterial applications of duplex scanning, the lower extremity assessment relies on high quality B-mode imaging to identify the artery of interest and facilitate precise placement of the pulsed Doppler sample volume for spectral waveform analysis.⁹ Both color flow and power Doppler imaging provide important flow information to guide spectral Doppler interrogation. These imaging modalities are also valuable for recognizing anatomic variations and for identifying arterial disease by showing plaque or calcification. However, it should be emphasized that color flow Doppler and power Doppler imaging are not replacements for spectral waveform analysis, the primary method for classifying the severity of arterial disease.¹⁰

When examining an arterial segment, it is essential that the ultrasound probe be sequentially displaced in small intervals along the artery in order to evaluate blood flow patterns in an overlapping pattern. This is necessary because the flow disturbances produced by arterial lesions are propagated along the vessel for a relatively short distance. Experimental work has shown that the high-velocity jets and turbulence associated with arterial stenoses are damped out over a distance of only a few vessel diameters.¹¹ Consequently, failure to identify localized flow abnormalities could lead to underestimation of disease severity. Because local flow disturbances are usually apparent with color flow imaging (see Figure 17-1), pulsed Doppler flow samples may be obtained at more widely spaced intervals when color flow Doppler is used. Nonetheless, it is advisable to assess the flow characteristics with spectral waveform analysis at frequent intervals, especially in patients with diffuse arterial disease. Lengths of occluded arterial segments can be measured with a combination of B-mode, color flow, and power Doppler imaging by visualizing the point of occlusion proximally and the distal site where flow reconstitutes through collateral vessels. Because flow velocities distal to an occluded segment may be low, it is important to adjust the Doppler imaging parameters of the instrument to detect low flow rates.

For ultrasound examination of the aorta and iliac arteries, patients should be fasting for about 12 hours to reduce interference by bowel gas. Satisfactory aortoiliac Doppler signals can be obtained from approximately 90% of individuals that are prepared in this way. It is usually convenient to examine patients early in the morning after an overnight fast. The patient is initially positioned supine with the hips rotated externally. A left lateral decubitus position may also be advantageous for the abdominal portion of the examination. An electric blanket placed over the patient prevents vasoconstriction caused by low room temperatures.

For a complete lower extremity arterial evaluation, scanning begins with the upper portion of the abdominal aorta. An anterior midline approach to the aorta is used, with the transducer placed just below the xyphoid process. Both ultrasound images and Doppler signals are best obtained in the longitudinal plane of the aorta, but transverse views are useful to define anatomic relationships, assess branch vessels, and determine the cross-sectional lumen (Figure 17-3). If specifically indicated, the mesenteric and renal vessels can be examined at this time, although these do not need to be examined routinely when evaluating the lower extremity arteries. The aorta is followed distally to its bifurcation, which is visualized by placing the transducer at the level of the umbilicus and using an oblique approach (Figure 17-4). The iliac arteries are then examined separately to the level of the groin with the transducer placed at the level of the iliac crest to evaluate the middle to distal common iliac and proximal external iliac arteries (Figure 17-5). This may require applying considerable pressure with the transducer to displace overlying bowel loops. The origin of the internal iliac artery is used as a landmark to separate the common iliac from the external iliac artery.

FIGURE 17-3 Longitudinal B-mode image of the proximal abdominal aorta. The origins of the celiac and superior mesenteric arteries are well visualized. Spectral waveforms obtained just proximal to the origin of the celiac artery show a normal aortic flow pattern.

FIGURE 17-4 Color flow image of a normal aortic bifurcation obtained from an oblique approach at the level of the umbilicus. The changes in color are the result of different flow directions with respect to the transducer. R-CIA, right common iliac artery; L-CIA, left common iliac artery.

FIGURE 17-5 Color flow image of a normal right common iliac artery bifurcation obtained at the level of the iliac crest. The color change in the common iliac segment is related to different flow directions with respect to the transducer. A portion of the common iliac vein is visualized deep to the common iliac artery.

Each lower extremity is examined in turn, beginning with the common femoral artery and working distally. After the common femoral and the proximal deep femoral arteries are studied, the superficial femoral artery is followed as it courses down the thigh. At the distal thigh, it is often helpful to turn the patient into the prone position to examine the popliteal artery. However, some examiners prefer to image the popliteal segment with the patient supine and the leg externally rotated and flexed at the knee. As the popliteal artery is scanned in a longitudinal view, the first branch encountered below the knee joint is usually the anterior tibial artery. The tibial and peroneal arteries distal to the tibioperoneal trunk can be difficult to examine completely, but they can usually be imaged with color flow or power Doppler. Identification of these vessels is facilitated by visualization of the adjacent paired veins (see Figure 17-2). These vessels are best evaluated by identifying their origins from the distal popliteal artery and scanning distally or by finding the arteries at the ankle and working proximally. Several large branches can often be seen originating from the distal superficial femoral and popliteal segments. These are readily visualized with color flow or power Doppler imaging and represent the geniculate and sural arteries.

Pulsed Doppler spectral waveforms are recorded from any areas in which increased velocities or other flow disturbances are noted. Recordings should also be made at the following standard locations: (1) the proximal and distal abdominal aorta; (2) the common, internal, and external iliac arteries; (3) the common femoral and proximal deep femoral arteries; (4) the proximal, middle, and distal superficial femoral artery; (5) the popliteal artery; and (6) the tibial/peroneal arteries at their origins and at the level of the ankle. As with other applications of arterial duplex scanning, Doppler angle correction is required for accurate velocity measurements. Although an angle of 60 degrees is usually obtainable, angles below 60 degrees can be utilized to provide clinically useful information.

A complete examination of the aortoiliac system and the arteries in both lower extremities may require 1 to 2 hours, but a single leg can usually be evaluated in less than 1 hour. An example of a vascular laboratory worksheet for lower extremity arterial duplex scanning is shown in Figure 17-6.

FIGURE 17-6 Example of a vascular laboratory worksheet used for lower extremity arterial assessment.

Classification of Disease

Normal Flow Characteristics

Jager and colleagues ¹² determined standard values for arterial diameter and peak systolic flow velocity in the lower extremity arteries of 55 healthy subjects (30 men, 25 women) ranging in age from 20 to 80 years (Table 17-1). Although women had smaller arteries than men, peak systolic flow velocities did not differ significantly between men and women in this study. However, the peak systolic velocities (PSVs) decreased steadily from the iliac to the popliteal arteries. There is no significant difference in velocity measurements among the three tibial/peroneal arteries in normal subjects.

TABLE 17-1 Mean Arterial Diameters and Peak Systolic Flow Velocities ^*

Artery	Diameter ± SD (cm)	Velocity ± SD (cm/sec)
External iliac	0.79 ± 0.13	119.3 ± 21.7
Common femoral	0.82 ± 0.14	114.1 ± 24.9
Superficial femoral (proximal)	0.60 ± 0.12	90.8 ± 13.6
Superficial femoral (distal)	0.54 ± 0.11	93.6 ± 14.1
Popliteal	0.52 ± 0.11	68.8 ± 13.5

SD, standard deviation.

* Measurements by duplex scanning in 55 healthy subjects.

Data from Jager KA, Ricketts HJ, Strandness DE Jr: Duplex scanning for the evaluation of lower limb arterial disease. In Bernstein EF, editor: Noninvasive diagnostic techniques in vascular disease, St. Louis, 1985, Mosby, pp 619–631.

Spectral waveforms taken from normal lower extremity arteries show the characteristic triphasic velocity pattern that is associated with peripheral arterial flow (Figure 17-7). This flow pattern is also apparent on color flow imaging.¹³ The initial high-velocity, forward flow phase that results from cardiac systole is followed by a brief phase of reverse flow in early diastole and a final low-velocity, forward flow phase late in late diastole. The reverse flow component is a consequence of the relatively high peripheral vascular resistance in the normal lower extremity arterial circulation. Reverse flow becomes less prominent when peripheral resistance decreases. This loss of flow reversal occurs in normal lower extremities with the vasodilatation that accompanies exercise, reactive hyperemia, or limb warming. The reverse flow component is also absent distal to severe occlusive lesions. A similar triphasic flow pattern is seen in the peripheral arteries of the upper extremities (see Chapter 15).