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Vascular Laboratory: Arterial Duplex Scanning
Duplex ultrasound (DUS) is an integral component of diagnostic testing for the evaluation and management of arterial disease. This technology, which combines the acquisition of blood flow (pulsed Doppler spectral analysis) and anatomic (B-mode and color Doppler imaging) information, was developed under the guidance of D. Eugene Strandness, Jr., at the University of Washington in the 1970s. The initial clinical application of arterial duplex scanning assessed the extracranial carotid artery bifurcation for the presence and extent of atherosclerotic plaque and developed velocity criteria to estimate internal carotid artery (ICA) stenosis on the basis of correlations with angiographic measurements. Commercial duplex scanners became available by the 1980s, and the clinical use of DUS rapidly expanded into peripheral arterial, visceral arterial, and peripheral venous applications. The development of real-time, color-encoded Doppler imaging was an important technologic advance that simplified patient testing, enhanced diagnostic accuracy, and led to additional clinical applications in the areas of screening for arterial disease, intraoperative assessment, and surveillance after arterial intervention.
Modern DUS systems provide high-resolution B-mode ultrasound imaging of tissue and vessel anatomy, including three-dimensional vessel reconstruction and evaluation of atherosclerotic plaque morphology. Detailed assessment of blood flow characteristics can be made in real time by one of several techniques, color Doppler imaging, power Doppler imaging, B-flow imaging, or pulsed Doppler spectral analysis.
Test interpretation is based on both imaging and Doppler findings with classification ranging from normal to clinically relevant disease categories. Duplex testing is noninvasive and cost-effective and thus suitable for serial examination; because it not only permits the identification of disease, but also reveals its natural history, including progression, regression, and response to intervention. In many patients, duplex testing can establish a definitive diagnosis and can allow interventions, such as carotid endarterectomy or peripheral artery angioplasty, to be based solely on the B-mode imaging and velocity spectral changes recorded from diseased arterial segments. In the upper and lower limbs, duplex testing should be performed in conjunction with indirect physiologic testing (measurement of systolic blood pressure, pulse volume plethysmography) to assess arterial hemodynamics (see Ch. 21 , Vascular Laboratory: Arterial Physiologic Assessment). When peripheral arterial disease is identified, DUS can be used to map the site or sites of occlusive or aneurysmal lesions, analogous to contrast-enhanced arteriography. Arterial duplex test interpretation combined with the patient’s clinical history and physical examination is often sufficient to counsel the patient on the advisability of intervention and whether it can be treated by an endovascular or conventional open surgical procedure.
The reliability of arterial duplex testing depends on several factors, including the expertise of the examiner (vascular technologist, physician) and the knowledge and experience of the interpreting physician. Testing performed and interpreted in an accredited vascular laboratory has sufficient diagnostic accuracy for clinicians to rely on the final interpretation provided and often avoid performing more invasive, expensive diagnostic testing, such as computed tomography, magnetic resonance imaging, or catheter-based contrast-enhanced angiography, to confirm disease severity (see Ch. 18 , Noninvasive Vascular Laboratory Quality Assurance and Accreditation).
Instrumentation and Basic Concepts
DUS systems use transducers fabricated from piezoelectric crystals to convert electrical activity to mechanical energy (ultrasound) and vice versa, thereby allowing the same device to transmit and receive ultrasound signals to and from the patient to produce images of tissue anatomy as well as to characterize blood flow. Transducers consist of multiple elements that enable focusing of the ultrasound beam, steering of the beam, and resolution sufficient for detailed tissue imaging at depths of less than 1 cm to more than 20 cm. To perform detailed arterial mapping, DUS instrumentation for carotid and peripheral testing should be equipped with linear array transducers with frequencies ranging from 5 to 12 MHz. For visceral artery or abdominal imaging and transcranial Doppler (TCD) examination, lower frequency transducers are needed because of the higher tissue attenuation; typically, 2.5- or 3.5-MHz curved linear or phased array transducers are appropriate. Newer generation transducers have an ultrawide bandwidth that enables harmonic imaging, with its increased resolution and freedom from artifacts, and dynamic frequency tuning for improving image quality at greater tissue depths. Moreover, the development of two-dimensional transducer arrays enables the beam to be focused at a specific depth and steered, which facilitates the use of three-dimensional imaging.
A duplex B-mode, or brightness mode, ultrasound image is displayed as gray-scale pixels reflecting the amplitude and position of returning ultrasound echoes. By processing up to 200 or more separate ultrasound beam signals retrieved from the transducer array, a scan converter organizes both horizontal and vertical pixels to yield a two-dimensional view of the tissue being scanned. Optimal arterial anatomic imaging is achieved when the transducer scan lines (beam) are directed perpendicular to the vessel wall. A 90-degree imaging angle is best used for measuring vessel diameter, identifying intima-media thickening, and assessing atherosclerotic plaque composition. Transmit power and receiver gain should be adjusted to produce a gray-scale image with the best tissue signal-to-noise ratio so that subtle differences can be perceived by the human eye. The examiner can modify image appearance by adjustment of the instrument’s time gain compensation, which is designed to correct for the effects of increasing attenuation with depth. When duplex arterial imaging is performed, the left side of the image should be oriented toward the patient’s head.
There are two types of Doppler ultrasound displays. In one form, a color-flow Doppler image shows the flow velocity distribution over a wide area displayed as a color-encoded map superimposed on the gray-scale B-mode tissue image. The second type, often referred to as spectral Doppler, shows the time-varying flow velocity distribution at a selected sample volume. Spectral Doppler provides quantitative information on the peak velocity within the sample volume, whereas color-flow Doppler provides semiquantitative information on the distribution of velocities over an entire region.
To obtain reliable information from spectral Doppler, it is best to use scan line angles (i.e., Doppler angles) of 60 degrees or less relative to the transducer insonation beam and arterial wall ( Fig. 22.1 ). Assignment of the Doppler angle is controlled by the examiner. Because calculation of blood flow velocity is determined by the Doppler equation, which is proportional to the cosine of the Doppler angle, recording velocity spectra at large Doppler angles results in reduced Doppler frequency shift and thereby increases flow velocity error as a result of uncertainty in knowing the true Doppler angle. For example, an error in Doppler angle assignment such as 5 degrees higher than the recommended 60 degrees (i.e., at 65 degrees) will result in a 15% measurement error in flow velocity, whereas if a 55-degree angle was assigned, an 8% error results. The velocity measurement error caused by incorrect or imprecise assignment of the Doppler angle by the examiner is a common duplex testing inaccuracy that can result in overestimation or underestimation of the severity of the stenosis when disease classification is based on peak systolic velocity (PSV) or end-diastolic velocity (EDV) criteria. When pulsed Doppler flow signals are recorded, the instrument sample volume should be sized to encompass less than a third of the flow lumen and should be positioned in the center stream of flow.
Blood Flow Imaging Techniques
Blood flow detection can be performed with one of three imaging techniques: color Doppler, power Doppler, and B-flow.
Color Doppler Imaging
Color Doppler imaging refers to pixel encoding of blood flow based on a color bar that depicts both flow direction (toward and away from the transducer) and mean velocity (MV). The examiner adjusts the velocity scale, color priority, and saturation of the color bar as well as instrument color gain to show the appearance of normal, laminar arterial flow as homogeneous regions varying in color-coded pixels during the pulse cycle. To set the color gain correctly, the examiner should increase the gain until a noise speckle appears within the flow region and then reduce it slightly. This technique will optimize the display of weak or lower velocity blood flow signals, such as those adjacent to the artery wall. Excessive color gain causes color-coded flow pixels to bleed into or beyond the artery wall, thus making the flow lumen appear larger than it is. Interpretation of real-time color Doppler flow is based on the color bar settings (peak MV, baseline, wall filter, and color assignment of flow toward or away from the transducer). In tortuous vessels, blood flow is color-coded according to its direction relative to the transducer scan lines. When blood flow velocity exceeds the mean peak velocity threshold of the color bar, color aliasing occurs, because the sampling rate as defined by the pulse repetition frequency is no longer sufficient (the Nyquist limit). With aliasing, blood flow is erroneously encoded as the “wraparound” color shown in the color bar, and the color image display will show flow in the opposite direction. Increasing the pulse repetition frequency and increasing the Doppler angle are two techniques that can be used to reduce the color-flow “aliasing” artifact.
Arterial stenosis is recognized by color Doppler imaging as a reduction in the color-encoded flow lumen, imaging of a high-velocity flow region with color bar aliasing, and development of a mosaic flow pattern in the lumen signifying turbulent flow. At the site of a high-grade (>75% diameter reduction) stenosis, real-time color Doppler flow will appear as a whitened, color-desaturated “flow jet” with mosaic color flow extending for several vessel diameters downstream corresponding to post-stenotic turbulence ( Fig. 22.2 ). A tissue bruit may appear as low-velocity flow signals outside the artery lumen and is caused by vibration of the arterial wall. The presence of persistence of color, color bar aliasing, and changes in flow lumen diameter on color Doppler imaging is indicative of abnormal flow patterns produced by stenosis. The examiner should then carefully interrogate this diseased arterial segment with pulsed Doppler spectral analysis to measure changes in flow velocity, which are then used to estimate the severity of the stenosis.
Power Doppler Imaging
Power Doppler imaging is a technique in which the display of blood flow is based on the amplitude of the backscattered Doppler signal; it increases the sensitivity of flow detection three to five times with respect to color Doppler imaging. This imaging mode is termed “color angio” and is used by the technologist for imaging of small-diameter vessels, detection of slow flow, assessment of residual lumen diameter at a stenosis, and detection of “trickle” flow associated with high-grade stenosis. Flow direction is not evident with the power Doppler imaging option, and the flow signal is less dependent on the Doppler angle.
B-Flow Imaging
B-flow imaging shows blood flow in gray scale; that is, flowing blood and the surrounding structures are depicted in shades of gray. The imaging technique is a visual depiction of flow hemodynamics and should not be confused with color Doppler imaging, because no velocity information is provided. B-flow imaging relies on the amplification of weak echoes from moving red blood cells and is most useful during arterial imaging to show boundary layer flow adjacent to the vessel wall and traversing atherosclerotic plaque. B-flow imaging can demonstrate the complex flow patterns seen at bypass graft anastomoses and arteriovenous fistulae and within dialysis access conduits where color Doppler artifacts can obscure flow patterns.
Pulsed Doppler Spectral Analysis
Pulsed Doppler velocity spectra recorded from a normal artery have a narrow range of velocities throughout the pulse cycle, which indicates that red blood cells are moving at a similar speed and direction in a nondisturbed, or laminar, flow pattern. If the “sample volume” of the pulsed Doppler is too large relative to the diameter of the artery or positioned adjacent to the arterial wall, low-velocity flow signals will be displayed as “broadening” or increased width of the velocity spectra.
Spectral broadening in the pulsed Doppler signal can also indicate “disturbed” flow or flow turbulence when it is recorded center-stream at bifurcations, regions of abrupt diameter change, and sites of stenosis. The “normal” appearance of arterial duplex flow varies with the artery being studied (peripheral, carotid, renal, or mesenteric) but should demonstrate rapid flow acceleration in systole, narrow spectral width, and varied diastolic flow corresponding to the vascular resistance of the arterial bed.
The velocity spectrum of a normal peripheral (aorta, iliac, extremity, external carotid) artery is triphasic or multiphasic (see Fig. 22.1 ) and consists of high outflow resistance with a systolic flow component, early diastolic flow reversal, and late diastolic forward flow. Low-resistance arterial flow, such as in the internal carotid, vertebral, renal, celiac, splenic, and hepatic arteries, is characterized by continuous flow throughout the pulse cycle with only a single (systolic) phasic flow component producing a monophasic pulsed Doppler spectral waveform. Changes in flow resistance of the microcirculation are primarily reflected as an increase or decrease in diastolic flow velocity.
Measurements
The pulsed Doppler spectral parameters of acceleration time, pulsatility index (PI), resistive index (RI), and maximum spectral velocity measured at peak systole (PSV) and end-diastole (EDV) constitute the primary criteria used for test interpretation. The PSV measurement is reproducible and thus the most common velocity spectral parameter used for the interpretation of normal arterial flow and critical limb ischemia and for the grading of arterial stenosis. The EDV measurement is used in conjunction with PSV for evaluating high-grade stenosis (>70% diameter reduction; see Table 22.1 ).
TABLE 22.1
Velocity Spectra Waveform Parameters Used for Interpretation of Duplex Tests
| Testing Area | PSV | EDV | RI | AT | PI | Mean Flow Velocity |
|---|---|---|---|---|---|---|
| Carotid duplex | X | X | X | X | ||
| Transcranial Doppler | X | X | X | X | ||
| Peripheral duplex | X | X | X | X | ||
| Duplex draft surveillance | X | X | X | X | X | |
| Renal duplex | X | X | X | |||
| Mesenteric duplex | X | X | X | X | X |
AT , systolic acceleration time; EDV , end-diastolic velocity; PI , pulsatility index; PSV , peak systolic velocity; RI , resistive index.
The RI is calculated by subtracting EDV from PSV and then dividing by PSV. It is used clinically to assess the renal and cerebral circulations for abnormal peripheral resistance. Normal values are less than 0.7, and levels higher than 0.85 are associated with increased vascular bed resistance and decreased end-organ perfusion.
The PI is calculated by dividing the peak-to-peak velocity spectral shift by the average (mean) velocity. The PI of normal peripheral arteries is greater than 4.0 (femoral artery, >6; popliteal artery, >8). PI values lower than 4 may reflect proximal inflow or occlusive disease, and change in PI or spectral waveform damping is diagnostic of multilevel occlusive disease. Division of distal artery PI by proximal artery PI calculates the “damping factor;” a normal value is 0.9 or higher, and a value of less than 0.9 is diagnostic of occlusive disease.
The systolic acceleration time during systole can also be used to diagnose occlusive disease proximal to the pulsed Doppler recording site. A normal value is less than 133 ms. As systolic acceleration time increases to longer than 200 ms, the spectral waveform develops a rounded upslope configuration, termed tardus-parvus, because of the prolonged time to PSV. Diagnostic accuracy of the systolic acceleration time is influenced by cardiac conditions (cardiomyopathy, aortic valve disease), but downstream occlusive disease has minimal influence on diagnostic sensitivity.
Artifacts and Errors
Artifacts and errors in ultrasound measurement can limit the effectiveness of the evaluation and create inaccurate results. Various artifacts include mirror image artifacts, shadowing from overlying vessel calcification, inaccuracy due to refraction, and aliasing. Most errors can be attributed to the technologist, because studies using flow models have found that adjustment of Doppler angle, sample volume placement, and Doppler gain were the most significant sources of error in PSV measurement.
Spectral Doppler aliasing is the most common artifact and, similar to color Doppler aliasing, is recognized by a “characteristic” signal wraparound in the spectral display. Adjustment of the velocity scale (i.e., pulse repetition frequency) to above the Nyquist limit or a reduction in the baseline level can shift the spectrum downward and eliminate the artifact. Shadowing from overlying calcification impedes adequate visualization of underlying vessel anatomy with B-mode imaging and interferes with accurate velocity measurement. Mirror image artifacts, created when a tissue structure is reproduced at an incorrect location, occur when a strongly reflecting surface is further reflected by other strongly reflecting surfaces. Refraction can cause misregistration of the image and the Doppler sample volume and occurs when an ultrasound beam passes through mediums with different propagation speeds. Crosstalk, found only in Doppler evaluation, creates a mirror image where identical spectra appear above and below the baseline. It is usually caused by an excessive receiver gain setting or an incident angle near 90 degrees. Ghosting occurs when low-velocity motion from pulsating vessel walls produces small Doppler shifts that can cause color flashing into the surrounding anatomy; it can be fixed with wall filters.
Variability of diagnostic criteria between laboratories stems from methods for defining the percentage of stenosis, different machines, and differences in technique. Factors such as gender and physiologic condition of the patient can also affect the outcomes of DUS evaluations. Studies have found that carotid PSV measurements in women average 10% higher than in men. Congestive heart failure, dysrhythmias, and artificial support measures (ventilators, intraaortic balloon pumps, or pacemakers) can alter cardiac output, which in turn can affect PSV measurements. Regarding technologist error, the largest source is error in accurately aligning the cursor of the sample volume. Even small errors in angle measurement can result in significant errors in velocity measurement and severity of the stenosis. Sample volume assumes that flow is parallel to the walls; however, flow is not usually parallel in tortuous vessels or beyond an asymmetrical stenosis, and these situations can make correct sample volume positioning and true velocity readings difficult. Finally, the most accurate measurement of PSV at a stenosis is within the narrowest portion of the stenosis, and reproducible measurements can be best obtained only if the sample volume is placed at or very near this area.
Duplex Velocity Spectral Classification of Arterial Stenosis
A significant or “critical” arterial stenosis is a lesion that is associated with a resting systolic pressure gradient of more than 15 mm Hg and reduces volume flow. In the peripheral arterial circulation, this correlates with a 50% or greater diameter reduction stenosis or greater than 75% reduction in cross-sectional area. A significant stenosis produces losses in blood energy primarily as a result of losses in inertial energy caused by the development of turbulent flow; such losses are much greater than the friction energy losses predicted by Poiseuille law.
By measuring changes in velocity proximal to and across an arterial stenosis, duplex testing can noninvasively estimate its hemodynamic significance and predict reductions in diameter within specified ranges (e.g., 0%–49% diameter reduction, ≥50%, 50%–79%, ≥80%). The relationship between the increase in flow velocity and the reduction in diameter by stenosis is nonlinear, especially with stenoses greater than 50% diameter reduction ( Fig. 22.3 ). The DUS characteristics of greater than 50% diameter reduction arterial stenosis include elevated PSV in comparison to adjacent normal segment, a color Doppler mosaic flow pattern, and pulsed Doppler spectral broadening of highly disturbed flow (i.e., post-stenotic turbulence with simultaneous forward and retrograde velocity spectra during systole).
The ratio of PSV (Vr) across a stenosis is a useful parameter for grading the severity of a stenosis; a Vr value higher than 2 indicates a greater than 50% diameter reduction, and a value higher than 4 correlates with greater than 70% diameter reduction. Typically, a pressure-reducing (peak systolic pressure >20 to 30 mm Hg) and flow-reducing arterial stenosis is associated with a Vr above 3.5, a PSV higher than 250 to 300 cm/s, and an elevation in EDV of 40 cm/s or more. Downstream of a “significant” pressure-reducing arterial stenosis, the spectral waveform should appear damped and monophasic with prolongation of the acceleration time and a decrease in PSV to below normal levels. As stenosis severity increases beyond greater than 90% diameter reduction, volume flow through the stenosis tends toward zero, which can produce PSV at the stenosis in a minimally elevated range (100–200 cm/s) and low-velocity (<10 cm/s) trickle flow downstream. The atherosclerotic plaque associated with greater than 50% stenosis is typically irregular and may be calcified, which produces an acoustic shadow on the image and makes measurements of residual artery diameter or reduction in cross-sectional area too inaccurate on transverse imaging to classify the severity of an arterial stenosis. Correlation studies between duplex testing and angiographic measurements have found that PSV and Vr are the best predictors of the severity of a stenosis when it is expressed as percentage diameter reduction.
Validation studies comparing duplex interpretation of stenosis severity with angiographic measurements have reported different threshold PSVs for lesions with greater than 50% diameter reduction. PSV measurement variation is in the ±15% range, similar to other biologic measurements. This variation is related to the type of ultrasound system used and differences in Doppler angle assessment and sample volume positioning by the examiner. This limitation of duplex scanning can be minimized by reporting stenosis (i.e., diameter reduction) within a specified range (e.g., 0%–49%, 50%–75%, and 76%–99%). In each clinical application, it is recommended that the vascular laboratory conduct ongoing quality assurance studies to confirm the diagnostic accuracy of stenosis interpretation in their laboratory in comparison to angiographic reports.
Patient Testing
Arterial duplex scanning can be performed as a portable bedside or vascular laboratory examination. Scanning should be conducted on a height-adjustable table or stretcher with the patient in a supine position. The bed and room environment should provide a comfortable, quiet atmosphere for patient examination, a warm room temperature (75–77°F) to avoid vasoconstriction of the extremities, and sufficient space to permit bilateral body access for ultrasound scanning. The typical examination time ranges from 30 to 60 minutes. Patients should refrain from tobacco use for at least 1 hour before the examination, and if abdominal scanning or visceral artery testing is planned, the patient should have fasted for 4 hours, and the examination should be performed in the morning to minimize accumulation of intestinal gas. Assessment of visceral artery flow before and after a test meal may be required for the evaluation of patients with symptoms of mesenteric ischemia.
Peripheral Arteries
Vascular specialists should be familiar with the spectrum of testing available as well as the information provided in conjunction with other hemodynamic tests. The duplex testing performed in the vascular laboratory is an extension of clinical assessment and is used to verify the presence and extent of disease. Numerous studies have analyzed the role of DUS, computed tomography, or MRA for preoperative assessment in efforts to limit the need for digital subtraction arteriography. In contemporary vascular practices, imaging with contrast-based modalities is generally limited to interventional procedures. This is because less invasive alternative imaging techniques can usually determine whether interventional treatment is possible without the performance of diagnostic arteriography. In patients with peripheral arterial disease, duplex imaging can determine disease location and length of lesions and help guide interventions.
Indications
DUS evaluation is beneficial for the following: patients with suspected symptomatic, chronic peripheral arterial disease ; patients with suspected acute limb ischemia; patients with pedal infection without a palpable pulse; patients with atheroembolic or thromboembolic disease states; and patients who require surveillance after revascularization procedures.
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