Core Curriculum, The: Ultrasound
1st Edition

11
Vascular Ultrasound
Vascular US is based upon the use of the Doppler effect first described in 1842 by the Austrian mathematician Johann Christian Doppler. As applied to diagnostic US, the Doppler effect describes the change in sound frequency that occurs when sound is reflected from a moving object. In medical diagnostics, the US beam is reflected by clumps of moving red blood cells (RBCs) within blood vessels. The resulting change in sound frequency (the Doppler shift) is carefully measured to determine the presence, direction, and velocity of blood flow. Doppler shifts may also be produced by microbubbles induced by ureteral peristalsis producing a ureteral jet that confirms patency of the ureter. Doppler shifts produced by movement of the patient (breathing, heartbeat, and bowel peristalsis) or by movement of the transducer produces Doppler artifacts.
Understanding Doppler Ultrasound
The Doppler shift is the difference in sound frequency between the US beam transmitted into tissue and the echo produced by reflection from the moving RBCs. A Doppler interrogation beam is directed into tissue in a direction controlled by the operator. The Doppler beam intercepts moving blood within a blood vessel at an angle called the Doppler angle. Blood flow that is relatively toward the Doppler beam increases the frequency of the echo returning to the transducer (Fig. 11.1). The movement toward it compresses the Doppler sound wave. A “positive frequency shift” to a higher frequency indicates that blood flow is relatively toward the Doppler beam. A “negative frequency shift” to a lower frequency indicates that blood flow is relatively away from the Doppler beam. Doppler frequency shifts are fortuitously within the range of human hearing, so the sound of moving blood can be heard as well as measured by US.
The amount of frequency shift is proportional to the velocity of the moving RBCs. By using a mathematical formula (the Doppler equation) and the handy computer intrinsic to our US units, we can measure the Doppler shift and calculate blood flow velocity. It is most important to recognize that Doppler can accurately measure blood flow velocity, but is not
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accurate at measuring blood flow volume. Measurement of blood flow volume requires accurate measurement of the cross-sectional area of blood vessels. This measurement is difficult and constantly changing because of the pulsatile nature of blood flow.
Figure 11.1 Doppler Frequency Shift. Flow relatively away from the Doppler beam shifts the Doppler echo to a lower frequency. Flow relatively toward the Doppler beam shifts the Doppler echo to a higher frequency. Ft is the frequency of the transmitted Doppler beam. Fr is the frequency of the Doppler echo returned to the transducer.
The Doppler equation can be written in two ways:
Ft is the frequency of the Doppler interrogation beam. Fr is the frequency of the echo shifted by the Doppler effect. (Fr - Ft) is the Doppler shift. C is the velocity of sound in human tissue, assumed to be constant at 1540 m/sec. V is the velocity of moving blood. The Doppler angle is indicated by the Greek letter theta (θ). The Doppler shift is proportional to the cosine function of the Doppler angle. The Doppler angle must be estimated by the operator and is communicated to the US unit by steering the “wing” of the Doppler angle indicator (Fig. 11.2).
For those mathematically inclined, the Doppler equation immediately demonstrates several important features of Doppler US. For those not mathematically inclined, just memorize the following facts:
Figure 11.2 Spectral Doppler Display. A. Drawing illustrates the Doppler beam, Doppler angle, Doppler sample volume, and the operator-controlled “wing” used to communicate to the US computer the estimated direction of blood flow. B. In this illustration, color Doppler and gray-scale US are used to locate and display the vessel being interrogated in the box at the top of the image. The spectral Doppler sample volume, the direction of the spectral Doppler US beam, and the Doppler angle indicator are shown. The Doppler spectrum is shown at the bottom of the image. See text for explanation of the spectral Doppler display (see Color Figure 11.2B).
Table 11.1: Cosine Values
Doppler Angle (θ) Cosine Value
1
10° 0.98
20° 0.93
30° 0.87
40° 0.77
50° 0.64
60° 0.50
70° 0.34
80° 0.17
90° 0
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  • The Doppler frequency shift is the signal that we must optimize to obtain reliable blood flow velocity information. The Doppler signal is intrinsically weak, as little as 1/10,000 as strong as the gray-scale US signal. RBC reflectors in blood are very limited in number compared to innumerable sound reflectors within soft tissue. So, Doppler technique must always be optimized to produce useful and accurate information. The Doppler frequency shift is proportional to the Doppler transmission frequency. The higher the transmission frequency, the higher the Doppler shift. When blood flow velocity is slow, higher transducer frequency improves its detection. However, higher-frequency US is limited in penetration and lower frequencies must often be used to produce a Doppler signal from deep vessels.
  • The Doppler frequency shift is proportional to the cosine of the Doppler angle. This has important implications as seen by inspection of the cosine values listed in Table 11.1. The maximum Doppler frequency shift will be obtained by directing the Doppler interrogation beam straight down the barrel of the vessel—the Doppler angle is 0 degrees. The cosine of 0 degrees is 1, the maximum cosine value. Unfortunately, most blood vessels course parallel to the skin and a zero Doppler angle is seldom obtainable. The cosine of 90 degrees is zero. No Doppler shift is obtained at precisely 90 degrees of interrogation to direction of blood flow. In practice, a weak Doppler signal is often obtained because the Doppler interrogation beam diverges slightly. However, this weak signal is misleading as to flow direction and is inaccurate in determining velocity.
  • Acute Doppler angles (<60 degrees) must be created to obtain accurate Doppler information. Note from Table 11.1 that the cosine values change slowly at acute angles (10 degrees, 20 degrees) and change rapidly at larger angles (70 degrees, 80 degrees). The operator routinely assumes that blood flow is parallel to the walls of visualized blood vessels and aligns the Doppler angle “wing” to align with the wall of the vessel. However, blood vessels, especially diseased arteries, are commonly tortuous and the exact orientation of blood flow and the Doppler angle must be estimated. Erroneous estimates of the Doppler angle cause smaller errors in blood flow velocity calculations at acute angles than at angles close to 90 degrees. The Doppler shift diminishes to 50% of maximum at 60 degrees, and falls rapidly at larger angles, reducing the quality of Doppler information. A 60 degrees, or smaller, Doppler angle is obtainable, with effort, in nearly all imaging situations.
Continuous wave (CW) Doppler uses two US crystals, one as a transmitter and the other as a receiver, to continuously record all Doppler shift information along the entire path of the Doppler beam. CW Doppler is non-selective and combines all Doppler shift information from all blood vessels in its path. CW Doppler is used commonly in obstetrics to monitor fetal heart tones.
Pulsed wave Doppler is commonly used in conjunction with real-time gray-scale US imaging to perform duplex Doppler US. Duplex Doppler combines routine imaging with Doppler interrogation of visualized vessels. Excluding all signals obtained along the Doppler
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beam line-of-sight except those obtained from a small time window creates a Doppler sample volume (Fig. 11.2A). This sample volume can be precisely positioned within any vessel visualized to obtain selective blood flow information from one vessel or even just a portion of one vessel. The combination of gray-scale US, color Doppler, and spectral Doppler is sometimes called triplex Doppler US (Fig. 11.2).
Spectral Doppler displays Doppler shift information as a graph of velocity, or frequency shift, information displayed over time. Blood flow velocity can be displayed interchangeably with Doppler frequency shift information by solving the Doppler equation shown previously. The appearance of the Doppler spectrum remains the same; only the scale changes.
Color Doppler uses a larger sample volume to detect mean Doppler shifts within a larger visualized area. Moving blood is displayed in color superimposed upon the gray-scale image.
Doppler Spectral Display
Echoes returning from the Doppler sample volume are analyzed for frequency shift information. A Fast Fourier Transform sorts the range and mixture of Doppler shift information into individual components and displays them as a function of time. Analysis is performed rapidly enough to display the information in real time corresponding to heartbeat (Fig. 11.2B).
  • The horizontal scale (x-axis) is time in seconds.
  • The vertical scale (y-axis) is flow velocity in m/sec or cm/sec, or is the Doppler frequency shift in kHz.
  • Brightness of pixels in the spectrum corresponds to the relative number of RBCs moving at given velocity at a specific instant in time. The more RBCs moving at that specific velocity and time, the brighter the pixel.
  • Flow relatively toward the Doppler beam is displayed above the zero baseline.
  • Flow relatively away from the Doppler beam is displayed below the zero baseline.
  • Spectral waveforms vary over time with cardiac contraction with highest flow velocities during systole and lowest flow velocities during diastole. Many vessels show reversed flow or no flow during diastole.
Color Doppler Imaging Display
Color Doppler imaging (CDI) superimposes Doppler flow information on a standard gray-scale real-time US image. Color Doppler displays colors based on measurement of mean Doppler shifts (Fig. 11.3) [1].
  • A color map is displayed adjacent to the image to indicate the colors used to display flow information [2]. A wide variety of color maps are available. The map in use must be analyzed to interpret the color information (Fig. 11.4).
  • The color map is divided into two parts by a black bar that corresponds to the baseline or zero flow point on the Doppler spectral display. The color on the top of the map, above the baseline, is used to show flow relatively toward the Doppler beam. The color on the bottom of the map, below the baseline, shows flow relatively away from the Doppler beam. Red is commonly used to indicate flow toward the Doppler beam, whereas blue indicates flow away from the Doppler beam. Obviously, red and blue colors provide no direct indication of whether vessels are arteries or veins. Knowledge of anatomy, appearance of the vessel, and Doppler waveform analysis are used to differentiate various arteries and veins.
  • Brighter colors are used to display higher mean velocities. Darker colors indicate lower mean velocities. Some color maps use different color shades for higher and lower velocities (Fig. 11.3). For instance, red may transition to yellow and blue may transition to green for higher flow velocities.
  • Numbers at the top and bottom of the color map indicate the velocity scale setting the Nyquist limit. The velocity scale must be set appropriately to the blood flow velocity
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    encountered. A scale set too high will obscure slow flow. A scale set too low will alias. Aliasing and the Nyquist limit are discussed later in this chapter.
  • A color Doppler sample volume is specified and positioned on the gray-scale image by the US operator. The color Doppler sample volume box is usually etched in white. Only the tissue within the box will be analyzed for Doppler shift information. Keeping the sample volume small optimizes Doppler information gathering.
  • Note that in CDI the format of the transducer determines the direction of the Doppler beam. The Doppler angle may change with vessel orientation and produce color changes related only to changes in the Doppler angle and not to changes in blood flow.
  • As with spectral Doppler, color flow information will not be obtained when the Doppler angle is at or near 90 degrees.
  • The color displayed within blood vessels on CDI is a function of
    • - Flow velocity
    • - Doppler angle
    • - Presence of aliasing
    • - Color map utilized
    • - Phase of the cardiac cycle
Figure 11.3 Color Doppler Imaging Display. The color map is shown on the left side of this image. See text for detailed explanation of the color Doppler display (see Color Figure 11.3).
Color Doppler and spectral Doppler are complementary to each other. Color flow shows flow information based on measurement of mean Doppler shifts, whereas spectral Doppler displays the full range of detected Doppler shifts. Color flow imaging can be used to detect blood vessels and confirm presence and direction of blood flow, whereas spectral Doppler provides more detailed characterization of blood flow and precise velocity measurements to estimate stenosis.
Color Doppler Energy Display
Color Doppler energy (CDE) displays color flow information obtained from integration of the power of the Doppler signal, rather than the Doppler frequency shift itself [3]. Another name for this technique is power Doppler. CDE relates more directly to the number of moving
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RBCs than to their velocity. CDE is relatively angle-independent and is more sensitive to slow flow than is CDI. CDE is a useful adjunct to CDI, especially in technically challenging situations. CDE significantly improves evaluation of parenchymal flow and assessment of tumor vascularity (Fig. 11.5) [4, 5].
Figure 11.4 Color Maps. Inversion of the color map radically changes the appearance of the color image of a carotid artery. Choice and orientation of the color map are at the option of the US operator (see Color Figure 11.4).
Figure 11.5 Color Doppler Energy Display. A. The blood vessels within and supplying a renal transplant are displayed on this color Doppler energy (CDE) (power Doppler) image. CDE shows the presence of blood flow with high sensitivity. However, direction of blood flow is not determined and adjacent arteries and veins with blood flow in opposite directions are shown in the same color. The CDE sample volume is adjusted by the operator to include the tissues of interest. B. CDE image of the liver shows flow in the portal vein and hepatic artery. In this example, the Doppler sample volume is shaded in color (see Color Figure 11.5).
  • CDE provides no information on flow direction or velocity.
  • Artifacts related to patient motion are significantly increased with CDE compared to CDI. Patients unwilling or unable to hold their breath or remain motionless may not be accurately imaged with CDE.
  • Visualization of gray-scale anatomy is often limited within the CDE sample volume.
  • CDE also displays a color map adjacent to the image. As with CDI, the colors chosen are arbitrary. However, no information on flow direction is obtained with CDE.
  • The CDE sample volume may be shaded in a color different than that used to indicate blood flow.
Blood Flow Dynamics
The major types of blood flow are laminar (parabolic), plug, disturbed, and turbulent [6].
Laminar Blood Flow
Laminar blood flow is the normal blood flow found in arteries and in large veins (Fig. 11.6) [7]. The highest velocity RBCs are in the center of the blood vessels. Blood flow velocity progressively decreases closer to the vessel wall. Blood at the vessel wall is hardly moving. Laminar blood flow is sometimes called parabolic blood flow because a line connecting the levels of flow at differing velocities has the shape of a parabola. The characteristics of laminar flow are:
  • Spectral Doppler shows a narrow range of velocities within the sample volume at any given instance in time. This is the normal “narrow spectrum” (Fig. 11.6B).
  • A well-defined window is seen beneath the Doppler spectrum in systole reflecting the fact that detected RBCs accelerate uniformly during systole. This is called the “systolic window.”
  • Highest flow velocities are mid-stream with decreasing flow velocities closer to the vessel wall.
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  • Color Doppler shows the brightest color mid-stream with darker colors toward the vessel wall (Fig. 11.6C).
  • Cardiac contraction causes flow velocity to increase rapidly to peak values during systole, then flow velocity decreases with diastole during which there may be no flow or flow may reverse in direction.
  • The sound of laminar flow has a whistling quality.
Figure 11.6 Normal Laminar Blood Flow. A. Arrows represent the orderly layers of red blood cells moving at different velocities that characterize laminar blood flow. The highest velocities are found in mid-stream. The lowest velocities are found adjacent to the vessel wall. B. Doppler spectral display shows the narrow spectrum characteristic of laminar flow in the common femoral artery. The systolic window (arrow), characteristic of laminar flow, is seen as the triangular space beneath the Doppler spectrum in systole. C. Color Doppler image of the common carotid artery at mid-systole shows bright color in mid-stream, indicating faster flow, and darker color near the vessel wall, indicating slower flow (see Color Figure 11.6C).
Plug Flow
Plug flow is a type of normal flow seen in large diameter vessels such as the aorta. In mid-stream a wide band of RBCs is moving at the same speed, forming a plug-shaped velocity profile.
  • The Doppler spectral is very narrow, reflecting RBCs all moving at the same speed throughout the cardiac cycle.
Disturbed Blood Flow
Disturbed blood flow occurs at vessel bifurcations and in areas of vessel stenosis. Disturbed flow no longer travels in a straight line but generally continues in a forward direction [8].
  • Flow velocity is increased (Fig. 11.7).
  • “Spectral broadening” indicates the disorganization characteristic of disturbed flow. The thickness (vertical height) of the Doppler spectrum is increased, indicating a broader range of the velocities within the sample volume at any given instant of time.
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  • Increased spectral broadening indicates increased disturbance of flow. The systolic window characteristic of laminar flow is reduced in size and may be obliterated.
Figure 11.7 Disturbed Blood Flow. Spectral display of the waveform obtained in an area of high-grade stenosis of the internal carotid artery (ICA). Peak systolic velocity is increased to 270 cm/sec. The spectrum is broadened and ill-defined.
Figure 11.8 Turbulence. Severe turbulence within a venous shunt is seen on spectral Doppler, A, and on color Doppler, B. C. Marked turbulence downstream from a severe renal artery stenosis shows marked spectral broadening and a waveform that is barely recognized as pulsatile. The arrows indicate systolic peaks (see Color Figure 11.8B).
Turbulent Blood Flow
Turbulent blood flow is random and chaotic, with RBCs flowing in all directions. Turbulence is found downstream from high-grade stenosis and in areas where flow velocities are very high, such as within shunts and fistulas [8].
  • Spectral broadening is severe and the systolic window is obliterated (Fig. 11.8A).
  • With severe turbulence, flow is concentrated at lower velocities.
  • With severe stenosis, flow is less pulsatile.
  • Flow may be detected in both directions simultaneously, owing to the formation of eddy currents. Eddy currents occur downstream from high-grade obstructions where the blood vessel lumen widens after the stenosis.
  • Flow velocities fluctuate with time.
  • Color Doppler shows a mixture of colors with aliasing reflecting the high velocities (Fig. 11.8B).
Individual blood vessels may be characterized by their Doppler “signatures,” a recognizable Doppler spectrum relatively unique to the vessel (Fig. 11.6B). A major determinant of the appearance of the Doppler spectrum is downstream resistance to blood flow. Blood vessels may be characterized as being “high resistance” or “low resistance” vessels.
High Resistance Blood Vessels
When vessels show a high resistance pattern, small arteries and arterioles downstream are contracted, increasing resistance to blood flow and maintaining blood pressure at high
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levels. Pulse pressures traveling down the arterial tree are highly reflected resulting in little flow to the capillary bed during diastole. Arteries that supply systemic muscles are characteristically high resistance when the muscles are at rest. With exercise, arterioles open, blood flow increases, and the high resistance pattern converts to low resistance.
Figure 11.9 High Resistance Spectra. Triphasic high resistance patterns are shown in the spectra of the superficial femoral artery and an arterial graft. The external carotid artery shows a relatively high resistance pattern with low velocity flow in diastole.
  • A triphasic waveform characterizes arteries supplying skeletal muscles at rest (Fig. 11.9). Examples include the femoral artery, the external iliac artery, and the radial artery.
    • - Velocity rises sharply with onset of systole and falls rapidly with cessation of ventricular contraction.
    • - Flow reverses for a short time in early diastole.
    • - The remainder of diastole shows little or no forward flow.
  • Some arteries show relatively high resistance patterns. Examples include the external carotid artery (ECA) (Fig. 11.9) and the superior mesenteric artery (SMA) during fasting.
    • - The rise to peak systolic velocity (PSV) is sharp with a rapid fall of velocity at the termination of systole.
    • - Flow in diastole remains forward in direction but is at very low velocity.
Low Resistance Blood Vessels
Low resistance blood vessels are characterized by forward flow throughout the cardiac cycle. These arteries supply vital organs like the brain and kidneys that demand a continuous supply of oxygenated blood. Arterioles within these organs are generally kept wide open.
  • Low resistance arteries have characteristic biphasic waveforms. Examples include the internal carotid artery (ICA), the renal artery, the umbilical artery, and the SMA after eating (Fig. 11.10).
    • - Velocity rises more slowly and falls more gradually during systole.
    • - Flow is forward throughout the cardiac cycle.
    • - Flow never reaches the zero velocity baseline.
Figure 11.10 Low Resistance Spectra. The renal artery and vertebral artery show relatively high velocity flow in diastole characteristic of low resistance spectra seen in arteries that supply vital organs. Forward flow toward the organ is present throughout the cardiac cycle.
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Velocity Ratios
Many times a Doppler signal can be obtained from tiny blood vessels that cannot be discretely visualized. Intrarenal arteries are a common example. At other times the vessel is so tortuous that a Doppler angle cannot be accurately estimated, such as in the twisted umbilical artery in the umbilical cord. Without an accurate Doppler angle estimation, an accurate flow velocity cannot be determined. In these instances the Doppler equation can be solved using the measured Doppler frequency shift and a cosine θ value of 1. The velocity values will not be accurate, but the Doppler spectrum characterizes flow within the blood vessel. A variety of velocity ratios can be calculated from the displayed Doppler spectrum. The formulas for the commonly used velocity ratios are displayed in Box 11.1.
  • Resistance index is also called the Pourcelot index. It is calculated by subtracting end diastolic velocity from PSV and dividing the result by PSV. A high resistance index indicates high resistance to blood flow within the vessel. High resistance to blood flow may be produced by arteriole constriction or by limited arterial distensibility such as may be produced in an edematous obstructed kidney [9].
  • Systolic/diastolic ratio (A/B ratio) is calculated by dividing PSV by end-diastolic velocity.
  • Pulsatility index requires determination of temporal mean velocity and thus is more difficult to calculate. US computers allow the operator to trace the spectrum and the computer will calculate a mean velocity (Fig. 11.11). Pulsatility index is then calculated by subtracting end diastolic velocity from PSV and dividing the result by mean velocity.
Figure 11.11 Pulsatility Index. The operator manually traces the outline of one cardiac cycle on the Doppler spectrum. From this tracing, the US computer determines the temporal mean velocity and calculates the pulsatility index (PI). The result is displayed in the upper left-hand corner of the image (PI = 0.63). ICA, internal carotid artery.
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Doppler Changes Caused by Stenosis
A common use of Doppler US is to detect and characterize arterial stenosis. Gray-scale US will identify atherosclerotic plaques and vessel narrowing. The severity of stenosis is determined by correlation of Doppler findings with real-time imaging.
  • Proximal to the stenosis, laminar flow is generally present unless the vessel is seriously diseased upstream (Fig. 11.12).
  • Within the stenotic zone, flow velocity is increased but usually remains laminar. PSV correlates best with the severity of stenosis. The highest velocity may be found in a very small region of the stenosed vessel lumen. A careful search of the vessel with a small sample volume is needed.
  • In the post-stenotic zone, spectral broadening occurs as flow spreads out to occupy the widened vessel lumen. Turbulence and eddy currents form downstream to severe stenosis. Maximum flow disturbance is usually found within 1 cm of the maximum stenosis.
  • Downstream, the Doppler signal is dampened by severe stenosis. This results in the tardus parvus waveform.
Tardus Parvus Waveform
The tardus parvus waveform is a sign of marked arterial stenosis that does not require direct evaluation of the stenosis [10]. The waveform is detected in arteries downstream from the
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stenosis. It is particularly useful in situations where visualization of the supplying artery is commonly difficult because of location of the artery or obscuration by bowel gas. Evaluation for stenosis of the renal artery or stenosis of an artery supplying a transplant are examples (Fig. 11.13) [11].
Figure 11.12 Velocity Changes Across a Vessel Stenosis. To maintain flow volume across an area of stenosis, velocity must increase through the stenotic segment. The maximal change in velocity occurs at the site of greatest narrowing. Doppler velocity measurements made at this site correlate best with the severity of stenosis. Beyond the stenosis, flow is disturbed and sampling shows spectral broadening, turbulence, and areas of flow reversal.
Figure 11.13 Tardus Parvus Waveform. Doppler of an intrarenal artery in a patient with surgically proven, renal artery stenosis shows a characteristic tardus parvus waveform. The acceleration time (δT) (short arrow) is 0.142 seconds. The acceleration index (Accl) (long arrow) is 2.4 m/sec2. Note the blunted appearance of the waveform in systole.
  • Tardus refers to a slowed systolic upstroke. This can be measured by acceleration time, the time from end diastole to the first systolic peak. An acceleration time >0.07 sec correlates with >50% stenosis of the renal artery [12].
  • Parvus refers to decreased systolic velocity. This can be measured by calculating the acceleration index, the change in velocity from end diastole to the first systolic peak. An acceleration index <3.0 m/sec2 correlates with >50% stenosis of the renal artery [12].
Venous Flow
Blood flow within veins is typically low velocity and non-pulsatile. However, venous spectra may be phasic when influenced by respiration, may vary in velocity when influenced by motion of the vein itself, or may be pulsatile when transmitting motion from the right side of the heart. These normal variations in venous flow must be recognized to avoid diagnostic errors (Fig. 11.14).
Portal Vein
The portal vein is formed by the confluence of the splenic and superior mesenteric veins. It provides approximately 70% of the incoming blood to the liver.
  • Normal blood flow velocity is 13-23 cm/sec with an average of 18 cm/sec.
  • Flow velocity is commonly somewhat phasic because rocking motion of the liver caused by motion of the heart moves the portal vein under the Doppler sample volume (see Fig. 11.24).
  • Slight phasicity may also be evident related to respiration.
  • Normal blood flow direction is into the liver. Any reversal of blood flow direction is abnormal and usually indicative of portal hypertension.
  • The portal vein is normally <13 mm in diameter. Increased diameter suggests portal hypertension.
Figure 11.14 Venous Waveforms. Doppler spectra from the portal, splenic, hepatic, and femoral veins are shown. See text for discussion of the waveforms.
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Splenic Vein
The splenic vein drains the spleen and receives inflow from the inferior mesenteric vein. The splenic vein joins the superior mesenteric vein posterior to the neck of the pancreas to form the portal vein.
  • The splenic vein shows low velocity forward flow toward the liver. Reversal of blood flow direction is seen with advanced portal hypertension.
  • Slight respiratory variation is common.
  • Normal diameter of the splenic vein is <10 mm. Increase in diameter is a sign of portal hypertension.
Hepatic Veins
Most individuals have three major hepatic veins that run a straight course to converge in the inferior vena cava just below the diaphragm and only approximately 1 cm from the right atrium. These three hepatic veins drain all of the liver except the caudate lobe. The appearance of the right, middle, and left hepatic veins and inferior vena cava has been likened to a Playboy bunny, a crow’s foot, or a caribou with antlers.
  • Pulsations of the right atrium are transmitted directly to the inferior vena cava and hepatic veins. None of these veins contains valves.
  • The normal hepatic vein waveform is undulating and mirrors motion of the right heart. The A-wave (reversed flow) is produced by atrial contraction. Forward flow of atrial filling is interrupted by the C-wave caused by bulging of the tricuspid valve with onset of ventricular contraction. The dominant S-wave represents high velocity atrial filling during ventricular systole. The V-wave represents the end of atrial filling and the D-wave is produced by opening of the tricuspid valve and simultaneous filling of the right atrium and right ventricle.
  • Color Doppler accurately demonstrates the normal flow direction changes characteristic of the hepatic vein (Fig. 11.15).
Figure 11.15 Normal Hepatic Vein Flow. Blood flow in the hepatic veins is normally toward the heart during atrial filling and away from the heart during atrial contraction. Note that a static color Doppler image represents only an instant in time during the cardiac cycle. Compare to the hepatic vein waveform in Figure 11.14 (see Color Figure 11.15).
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Peripheral Veins
Venous flow from the extremities occurs in response to the effects of gravity and muscular contraction.
  • Blood flow in peripheral veins is low velocity and may be appear to be absent if the Doppler velocity scale is set too high or if a large wall filter is used (see Fig. 11.21).
  • Flow shows respiratory variation (Fig. 11.14) caused by changes in intra-abdominal pressure produced by motion of the diaphragm. With inspiration the diaphragm descends, increasing intra-abdominal pressure and decreasing venous flow from the legs. With expiration the diaphragm rises, intra-abdominal pressure decreases, and venous flow from the legs increases. If the patient holds his breath, venous flow usually stops. A Valsalva maneuver will reverse flow in the lower extremity veins. Squeezing the patient’s calf will augment flow and improve detection of veins.
Doppler Artifacts
Artifacts produce confusing alterations of the Doppler signal and affect both spectral Doppler and color flow imaging. Artifacts are produced by physical limitations of Doppler instrumentation and by incorrect instrument settings [13]. Recognition of artifacts and proper correction prevents misdiagnosis.
Aliasing
Aliasing is a performance limitation of pulsed Doppler US that is related to the pulse repetition frequency. Pulse repetition frequency is limited by depth. The greater the distance to the vessel of interest, the longer it takes to transmit and receive echoes from that vessel. The Doppler signal that must be measured is the echo of the transmitted Doppler pulse that has been changed in frequency by reflection from moving RBCs. To accurately measure the frequency of the Doppler echo, the pulse repetition frequency must be at least twice the frequency of the Doppler echo. For any given setting of the Doppler US instrument, the Nyquist limit is defined as the maximal Doppler shift frequency that can be accurately measured. Because Doppler shift frequency and blood flow velocity can be substituted for each other mathematically by use of the Doppler equation, the Nyquist limit can also be expressed as the maximal blood flow velocity that can be detected by Doppler for a given set of instrument settings.
  • Aliasing produces a wrap-around effect on the Doppler display. On spectral Doppler, the peaks of the spectrum are cut off and displayed on the opposite side of the baseline (Figs. 11.16, 11.17).
  • On color Doppler, aliasing projects the color of reversed flow within central areas of highest velocity (Fig. 11.18). The key to recognizing color aliasing is to observe that no black stripe surrounds the color change. Color change of aliasing involves the brightest
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    shades of the color display. Color change produced by true reversal of blood flow direction, or by changes in the Doppler angle, are etched in black and involve the darkest shades of the color display.
  • On most Doppler instruments the pulse repetition frequency cannot be adjusted directly.
  • Increasing the velocity scale, which increases the pulse repetition frequency, can eliminate aliasing. Changing the baseline setting or using a lower Doppler US frequency can also eliminate aliasing.
  • Aliasing is not a feature of CDE (power Doppler) [14].
Figure 11.16 Aliasing in Spectral Doppler. A. Spectral Doppler shows the peaks (curved arrow) of the spectral display are cut off the top and are displayed on the bottom (straight arrow). B. With readjustment of the baseline to allow display of the 80 cm/sec peak systolic velocity, the aliasing is eliminated.
Spectral Broadening
Spectral broadening is an important spectral Doppler sign of abnormal blood flow. However, spectral broadening has a number of other causes that must be recognized and excluded before spectral broadening can be interpreted as a sign of abnormal blood flow.
  • When the Doppler sample volume is large compared to the size of the blood vessel, the sample volume will include the full range of blood flow velocities from slow flow near the vessel wall to the fastest flow in midstream. Inclusion of all flow velocities will broaden the spectrum. Because the smallest sample volume available on most Doppler instruments is 1.0-1.5 mm, vessels of this size and smaller will inevitably show broadening of the displayed velocity spectrum.
  • Placing the sample volume near the vessel wall instead of mid-stream will produce spectral broadening by inclusion of the slow-moving RBCs near the vessel wall. The highest flow velocities in mid-stream may be missed.
  • Excessive Doppler gain falsely broadens the spectrum.
Figure 11.17 Aliasing in Spectral Doppler. A. In an example of more severe aliasing, both the top and bottom (arrows) of the spectrum are cut off. Direction of blood flow cannot be determined from this spectrum. B. Adjustment of baseline and scale results in appropriate display of the spectrum.
Figure 11.18 Aliasing in Color Doppler. A. The dominant color displayed within this vessel is yellow from the top side of the color map. Yellow indicates flow toward the Doppler beams, or in this case, from right to left. The splotches of green color are areas of aliasing. Note the absence of a black border. The color velocity scale is set low with a Nyquist limit of 0.040 m/sec. When the detected mean blood flow velocity exceeds this limit, aliasing occurs and color from the bottom portion of the color map is displayed. Aliasing must be recognized, but in this case may be useful by providing identification of highest velocity flow. B. This color Doppler image of the internal carotid artery demonstrates the appearance of true reversal of blood flow direction as indicated by the black border around the region of color shift. A small area of flow reversal is a normal finding opposite the flow divider at the bifurcation of the common carotid artery. Note that the direction of the Doppler beams is different than the direction of the gray-scale image beams (see Color Figure 11.18).
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Incorrect Doppler Gain
When the color or spectral Doppler gain is set too low, Doppler information may be lost and blood flow may not be detected. The color image with gain set too high demonstrates color in non-flow areas and random color noise. Correct gain settings are attained by turning up the gain setting until noise appears on the color image or spectral display, then gain is reduced slowly until the noise disappears.
  • Gain set too low shows no spectral display or color flow on the image.
  • Color gain set too high shows color bleed beyond the limits of the vessel, color signal in non-flow areas, and random color noise.
  • Spectral gain set too high shows random noise on the spectral display, false spectral broadening, and commonly displays a mirror image of the spectrum on the opposite side of the baseline (Fig. 11.19).
Velocity Scale Errors
Errors in the setting of the Doppler velocity scale may obscure slow flow or cause aliasing. Velocity scale and baseline settings must be adjusted to be appropriate to the velocity of blood flow in the interrogated vessel.
  • Too high settings of velocity scale obscure low velocity flow on both color flow and spectral Doppler (Fig. 11.20). The small signal may be obliterated by the wall filter (Fig. 11.21). Vessels with slow flow may be judged to be thrombosed.
  • When velocity scale settings are too low, aliasing occurs (Figs. 11.16, 11.17 and 11.18).
Figure 11.19 Gain Error. Doppler gain set too high causes artifactual spectral broadening of the true Doppler spectrum and displays a mirror image of the Doppler spectrum on opposite sides of the baseline. Turning down the Doppler gain to an appropriate setting corrects the artifacts.
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Incorrect Wall Filter
Motion of the vessel wall produced by pulsatile blood flow causes a low-velocity but very high-intensity Doppler shift. To prevent wall motion from being displayed as blood flow, wall filters are included in Doppler instrumentation for both spectral and color Doppler. The spectral Doppler wall filter produces a thin black line on either side of the baseline. The black bar in the color map represents the color Doppler wall filter.
  • A wall filter set too high will obscure low-velocity blood flow (Fig. 11.21).
  • The wall filter on color Doppler is included in the black bar in the center of the color map (Fig. 11.3) and may not be obvious as a cause of absent color signal in a vessel.
Figure 11.20 Velocity Scale Error. Setting the Doppler velocity scale too high shrinks the Doppler spectrum and may make flow undetectable. The appropriate velocity scale setting reveals a diagnostic Doppler spectrum.
Figure 11.21 Wall Filters. A. Wall filters (arrow) set at three different levels obliterate all low-velocity spectral signals near the baseline. B. Spectral waveform of low-velocity flow within a vein is obliterated by a wall filter set too high (arrow).
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Doppler Mirror Image Artifact
Mirror image duplication of the color flow US display may be produced by strong reflectors such as the surface of the air-filled lung or even the wall of a blood vessel.
  • Carotid ghost artifact duplicates the color image of the carotid artery in deeper cervical tissue [15].
  • Mirror images of the CDI of the subclavian artery and vein may be produced by reflection from the aerated apex of the lung (Fig. 11.22) [16].
  • Color display of hepatic vessels may be included in the gray mirror image of the liver displayed above the diaphragm.
  • Mirror images of the Doppler spectrum are displayed on the opposite side of the baseline by Doppler gain set too high or by large Doppler angles near 90 degrees (Fig. 11.19).
Tissue Vibration Artifact
Vibration may produce color display in perivascular solid tissue, indicating blood flow where none is present. Tissue vibration artifact is produced in non-flow areas by turbulence caused by severe stenosis, arteriovenous fistulas, and shunts [17]. A color bruit at the site of arterial stenosis is indicative of a severe stenosis.
  • The artifact appears as a mixture of red and blue colors in perivascular soft tissues (Fig. 11.23). The artifact is more prominent during systole and less prominent during diastole.
Figure 11.22 Color Doppler Mirror Image. Intense reflection from the surface of the lung causes a mirror image reflection (arrow) of the subclavian artery to be displayed over the lung where no blood vessel is present (see Color Figure 11.22).
Figure 11.23 Tissue Vibration Artifact. Turbulent blood flow in a hemodialysis shunt produces a “visible bruit” of tissue vibration artifact seen as a random pattern (white arrow) of red and blue color displayed over the soft tissues adjacent to the shunt. The random color pattern within the two limbs of the shunt (black arrows) is indicative of turbulent blood flow (see Color Figure 11.23).
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Vessel Motion Artifact
Artifactual pulsatility may be introduced into a spectral Doppler tracing when the vessel under interrogation is moving. The portal vein and its branches move with cardiac contraction that rocks the liver. The rocking motion of the vessel displaces the Doppler sample volume from regions of higher velocity to regions of lower velocity, simulating pulsatility or periodic motion in the venous flow spectrum (Fig. 11.24). This artifact is decreased by increasing the size of the Doppler sample volume or by changing the Doppler angle.
Directional Ambiguity
When a Doppler interrogation beam intercepts a vessel at a Doppler angle near 90 degrees, the direction of blood flow is difficult to determine (Fig. 11.25). The Doppler spectrum is commonly displayed both above and below the baseline. The ambiguity is corrected by adjusting the approach to the vessel to create a more acute Doppler angle.
Figure 11.24 Vessel Motion Artifact. Rocking of the liver produced by contraction of the heart produces a phasic pattern of blood flow in the portal vein as it moves areas of faster and slower blood flow into the Doppler sample volume. MPV, main portal vein.
Figure 11.25 Directional Ambiguity. A. Doppler spectrum of the splenic vein obtained with a Doppler angle of 87 degrees shows signal on both sides of the baseline. Direction of blood flow cannot be determined. B. By readjusting position and angulation of the transducer, an acute Doppler angle of 5 degrees is created, and an unambiguous spectrum is produced. Blood flow in the normal direction toward the liver is confirmed.
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Color Flash—Color in Non-Vascular Structures
Any motion of non-vascular tissue will produce a Doppler shift and may result in false color display. Cardiac contraction and highly pulsatile vessels, such as the aorta, cause motion of adjacent tissues which produces a splash of color that obscures blood flow information (Fig. 11.26). Most color Doppler instruments incorporate motion discriminators to suppress this artifact. However, in hypoechoic regions, such as within cysts or ducts, the artifact suppression is limited. Thus color flash may be particularly prominent, or be seen solely, within these lucent structures, falsely simulating blood flow. Color flash is particularly prominent with CDE (power Doppler) imaging. Color flash is reduced by lowering color sensitivity, increasing the velocity scale, and by lowering the gain setting.
Figure 11.26 Color Flash—Fluid Motion. A. Cardiac motion causes color flash artifact and obscures the junction of hepatic veins and inferior vena cava. B. A kicking motion by a baby in utero moves the amniotic fluid to produce a prominent color flash artifact (see Color Figure 11.26).
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Fluid Motion
Color signal can be produced during CDI by motion of fluids other than flowing blood. Motion by fluid within cysts, by moving bowel contents, and by ureteral peristalsis may be misinterpreted as blood flow.
  • Bowel peristalsis with moving gas bubbles can result in rectangular or comet tail areas of false color display.
  • Movement of a fetus stirs the amniotic fluid and produces color artifact (Fig. 11.26).
  • Visualization of ureteral jets from squirts of urine into the bladder caused by ureteral peristalsis is good evidence of ureteral patency (see Fig. 3.4) [18].
Color Doppler Interpretation
Interpretation of Color Doppler Images
To interpret the meaning of the colors displayed on a color Doppler image, the following parameters must be analyzed:
  • Color Map. The colors chosen for forward flow (top of the map) and for reversed flow (bottom of the map) must be noted (Figs. 11.3, 11.4). The velocity scale is indicated by the Nyquist limit velocities displayed at the top and bottom of the map. When these mean velocities are exceeded, the color display will be aliased. The velocity scale must be set appropriately for the blood flow velocity in the vessel imaged.
  • Transducer Format and Color Doppler Sample Volume. The format of the transducer in use determines the direction of color Doppler interrogation beams. Sector and curved array transducers have diverging beams that may intersect a vessel at continuously changing Doppler angles (Fig. 11.27). With linear array transducers, the color Doppler beams may be aligned parallel to the vertical beams of the gray-scale image or be steered to create more acute Doppler angles (Figs. 11.18B, 11.28). In this circumstance the gray-scale beams are vertical whereas the color Doppler
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    beams are angled parallel to the sides of the color sample volume.
  • Doppler Angle. The Doppler angle must be analyzed for each portion of the color image (Figs. 11.27, 11.28). The Doppler angle commonly changes because of divergence of the Doppler US beams or because the vessel curves within the field of view.
  • Blood Flow Physiology. The physiology of blood flow (laminar, disturbed, turbulent) must be interpreted in concert with the technical analysis of the color image. Color images will change constantly throughout the cardiac cycle (Figs. 11.15, 11.29).
  • True color changes from red to blue shades are etched in black and result from either changes in the Doppler angle or from reversal of blood flow direction (Figs. 11.18B, 11.27, 11.28).
  • Color changes caused by aliasing are not etched in black and involve the brightest colors on the color scale (Figs. 11.18, 11.27, 11.28).
Figure 11.27 Color Change Caused by Changing Doppler Angle. A sector transducer is used to produce a color image of the splenic vein as it curves through the pancreas (PANC). The sector transducer sends diverging Doppler beams (tiny white arrows) through the pancreas. On the right side of the image, the red color in the splenic vein indicates flow toward the Doppler beams. In the mid-portion of the image (black arrow), the Doppler beams intersect the moving blood at a 90-degree angle; therefore no color is displayed in this portion of the vein. On the left side of the image, the blue color indicates blood flow away from the Doppler beams. The yellow color without a black border indicates aliasing caused by an unbalanced velocity scale as shown on the color map. In summary, the color changes indicate normal flow in the splenic vein toward the portal confluence and the liver (see Color Figure 11.27).
Figure 11.28 Linear Array—Vertical Doppler Beams. In this color image, the Doppler beams are vertical (tiny white arrows) and parallel to the gray-scale US beams. The common carotid artery (CCA) has a gently curving course through the color field of view. On the right side of the image, the blue color indicates flow relatively away from the Doppler beams. On the left side of the image, the red/yellow color indicates flow relatively toward the Doppler beams. In summary, blood flow is from left to right, indicating normal flow direction toward the brain. Note the black border of transition between the red and blue colors (black arrow) where the color changes because of change in Doppler angle. Aliasing is indicated by patches of green without a black border in the yellow colored flow (see Color Figure 11.28).
The final interpretation of the color image is based upon piece-by-piece analysis of each of the items listed.
To Optimize Color Doppler Images
Because the Doppler signal is intrinsically weak, color Doppler images are frequently difficult to obtain. Gray-scale and color Doppler information is obtained sequentially, each taking a finite length of time that depends upon the depth of the image and the width of the field of view. To produce a duplex color and gray-scale image requires 10-20 sweeps through the color field of view for every sweep through the gray-scale field of view. To optimize the color image, adjust the following instrument settings:
  • Doppler Gain Setting. Turn the Doppler gain up until noise, seen as a random pattern of red and blue dots, covers the image. Then slowly decrease the gain setting until the noise is eliminated.
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  • Power. Color Doppler commonly requires that the Doppler power setting be set to the maximum setting (Fig. 11.30).
  • Transducer Frequency. Higher transmit frequencies (5.0-10.0 MHz) provide the greatest Doppler shift. However, high frequency is very limited in tissue penetration. Particularly when encountering difficulties in obtaining Doppler signals from deep in the abdomen, a lower transmit frequency must be utilized to obtain adequate penetration (Fig. 11.30).
  • Doppler Angle. Doppler signals are very weak at large angles (>60 degrees) and are absent at 90 degrees. Therefore, even in color Doppler, the color signal will be increased and the color image will be improved by adjusting the angle of scanning to produce more acute Doppler angles relative to the vessel imaged.
  • Color Doppler Field of View. To limit color information gathering time and to improve the quality of the color image, narrow the color field of view and reduce its depth to the minimum needed to visualize the vessel of interest.
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  • Gray-Scale US Field of View. Narrowing the gray-scale field of view and reducing its depth provides a minor improvement in the color image by decreasing the gray-scale information gathering time.
  • Color-Write Priority. Many color Doppler US instruments allow the operator to manually adjust the color-write priority. The quality of the gray-scale image may be sacrificed to produce a better quality color image. Increasing the color-write priority setting devotes more time to color Doppler information gathering and less time to gray-scale information gathering.
Figure 11.29 Color Changes in the Cardiac Cycle. A. Spectral Doppler shows the characteristic changes of the Doppler spectrum of the common femoral artery during the cardiac cycle. B. Color Doppler images show the corresponding changes in color images that correspond to phases of the cardiac cycle (see Color Figure 11.29).
Figure 11.30 Effect of Transducer Frequency and Doppler Power Settings. This series of transvaginal images of a uterus containing a first trimester molar pregnancy provides graphic example of the effect of transducer frequency and power settings on the color Doppler image. The best color image is obtained using the lowest transducer frequency (4 MHz) with the highest power setting (500 dB) (see Color Figure 11.30).
Carotid Ultrasound
A major use of vascular US is in the detection of significant stenosis of the ICA that can lead to stroke. Approximately 76% of all strokes are caused by atherosclerotic disease at the carotid bifurcation. Because it is non-invasive and relatively inexpensive, carotid US is the initial study of choice for detection and characterization of extracranial carotid disease.
Ultrasound Technique
Carotid sonography requires a thorough and methodical examination utilizing gray-scale US, color flow US, and spectral Doppler. Examination should always include both carotid systems and both vertebral arteries.
  • The patient is positioned supine with head turned slightly away from the side being examined. The neck may be hyperextended slightly by placing a pillow under the patient’s shoulders. Both patient and examiner must be comfortable because the examination routinely takes 20-40 minutes.
  • The examiner sits at the head of the table and is able to rest the examining arm on the table for stability and comfort. High-frequency (7.5-10 MHz) linear array transducers are used for the examination.
  • The carotid bifurcation is identified usually 1-2 cm below the angle of the jaw. The bifurcation is easiest to locate by scanning in the transverse plane.
  • Gray-scale US is used to detect and evaluate the appearance of carotid plaque. Plaques are always evaluated in both longitudinal and transverse planes.
  • Color Doppler or power Doppler is used to identify vessels, assess the presence of blood flow, and to identify the presence and location of plaque and carotid wall thickening.
  • Spectral Doppler is used to evaluate the severity of stenosis. The Doppler angle must be kept at <60 degrees to minimize measurement error. All Doppler spectra must be angle corrected to obtain accurate velocity measurements. The Doppler angle indicator is aligned parallel to the wall of the vessel at the location of Doppler sampling. The Doppler sample volume must be kept at less than half of the vessel diameter. Usually the sample volume is kept at 1.5 mm and is positioned in the middle of the flow channel. Doppler spectra are obtained from the common carotid artery (CCA), ECA, and ICA. A careful search is made to ensure that the highest velocity in the area of stenosis is detected and recorded. Doppler spectra are compared to spectra obtained upstream and downstream to the narrowed area of the artery.
  • Both vertebral arteries are routinely examined to assess patency and direction of blood flow (Fig. 11.31) [19].
Normal Carotid Ultrasound
Findings that differentiate the ICA from the ECA are listed in Table 11.2. Characteristic Doppler spectra are shown in Figure 11.32.
  • The CCA is the source of blood for both the ICA and ECA. The Doppler spectrum of the CCA combines characteristics of both ICA and ECA proportional to their relative blood
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    flow (70% to the ICA and 30% to the ECA). Normal PSV is 50-100 cm/sec. Symmetric bilateral low CCA velocity is caused by low cardiac output (congestive heart failure) or wide diameter arteries. Symmetric bilateral high CCA velocity is caused by hypertension, bradycardia, hyperthyroidism, or small diameter arteries.
    Figure 11.31 Normal Vertebral Artery. A normal vertebral artery (straight arrow) is seen on color Doppler US between the acoustic shadows cast by the transverse processes (curved arrow) of the cervical spine. The vertebral artery is imaged by aligning the transducer with the common carotid artery and then looking deeper for the shadows of the transverse processes. The vertebral arteries are evaluated for patency and flow direction.
  • The ECA supplies the head and face. Its Doppler spectrum is relatively high resistance. Flow velocity returns to baseline during diastole. PSV is greater than the ICA and less than the CCA. The ECA is the smallest of the carotid arteries and has visible branches. Tapping the temporal artery with a finger distorts the ECA spectrum and confirms its identification (Fig. 11.33).
  • The ICA supplies the brain and has a low resistance Doppler spectrum. PSV is lower than both the CCA and ECA. Flow is antegrade toward the brain throughout the cardiac cycle. The artery is larger than the ECA and is usually lateral to the ECA.
  • Normal reversed flow is seen in the proximal ICA opposite the flow divider at the bifurcation.
Table 11.2: Characteristics of the Internal versus the External Carotid Artery
Internal Carotid Artery External Carotid Artery
Gray-scale US Gray-scale US
   Larger vessel lumen (~6 mm)    Smaller vessel lumen (~3-4 mm)
   Postero-lateral location    Antero-medial location
   Courses posteriorly toward mastoid    Courses anteriorly toward face
   No branch vessels    Has branch vessels
   Carotid bulb is at origin Color flow US
Color flow US    Color flow is intermittent during the cardiac cycle
   Continuous color flow is seen throughout the cardiac cycle Spectral Doppler US
Spectral Doppler US    High systolic velocity
   Low systolic velocity    Sharp, narrow systolic peak
   Broad systolic peak    Low-velocity diastolic flow
   High-velocity diastolic flow    Diastolic velocity approaches zero baseline
   Diastolic velocity does not return to baseline    Tapping on the temporal artery disturbs the Doppler spectrum
   Tapping on the temporal artery has no
effect on the Doppler spectrum
 
Figure 11.32 Normal Doppler Spectra of the Carotid Arteries. See text for description. Arrows indicate end-diastole for each carotid waveform. CCA, common carotid artery; ECA, external carotid artery; ICA, internal carotid artery.
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Thickness of the Carotid Wall
Thickness of the wall of the carotid artery is a physiologic marker for atherosclerotic disease and may be used to follow the progression of atherosclerosis and to assess the effectiveness of medical therapies (Fig. 11.34) [20, 21].
  • Diffuse thickening of the carotid wall, measurable by the intima-media thickness, may be atherosclerotic or non-atherosclerotic in origin. By definition, intima-media thickness is measured on the far arterial wall as the distance between the echogenic lumen-intima interface and the hypoechoic media-adventitia interface [22]. This US appearance of the intima and media has been called the double line pattern. Because of gain-dependent reverberation artifact, measurements of the near arterial wall are not accurate.
  • Diffuse increase in intima-media thickness, as measured in the CCA or ICA, has been correlated with symptomatic coronary artery disease, obstructive peripheral artery disease, and increased risk of stroke [23, 24, 25, 26, 27]. This measurement is useful in assessing the effects of lifestyle intervention (cessation of cigarette smoking, low-fat diet, etc.) and in the use of medications (antihypertensives and lipid-lowering drugs) [28, 29]. Significant thickening is defined differently in various studies. Thickening >1.0-1.3 mm is generally considered abnormal. An increase of 0.1-0.2 mm in thickness over time is considered evidence of significant disease progression.
  • Hypertension is the primary cause of non-atherosclerotic diffuse thickening of the vessel wall. Thickening results primarily from hypertrophy of the media [30].
Figure 11.33 Tapping the External Carotid Artery (ECA). A finger tap on the ipsilateral temporal artery distorts blood flow (TAP) in the external carotid artery to confirm its identification.
Figure 11.34 Carotid Wall Thickness. Images of the common carotid artery demonstrate normal thickness and appearance of the arterial wall in A, mild intima-media thickening in B, and severe intima-media thickening in C.
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Plaque Characterization
Atherosclerotic plaque may be homogeneous, heterogeneous, or calcified (Fig. 11.35). Heterogeneous plaque is associated with intraplaque hemorrhage and an increased risk of subsequent stroke [27].
  • Focal thickening of the arterial wall, found primarily near vessel bifurcations, corresponds to atherosclerotic plaque.
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  • Homogeneous plaque consists of uniform, low-level echoes comparable in echogenicity to muscles in the neck. The surface of the plaque is smooth. Homogeneous plaque corresponds pathologically to dense, fibrous, laminated, connective tissue.
  • Heterogeneous plaque is complex in appearance and contains at least one distinct focal sonolucent area that represents intraplaque hemorrhage or amorphous lipid deposits. An irregular plaque surface classifies plaque as heterogeneous. However, heterogeneous plaque may have a smooth or irregular surface. Although an irregular surface suggests the possibility of plaque ulceration, it is not a reliable criterion for ulceration. No US sign is reliable in detecting ulceration.
  • Calcification may be seen in either homogeneous or heterogeneous plaque and is not considered as a criterion for classifying plaque.
Figure 11.35 Carotid Plaques. A. A plaque is seen as a focal thickening (straight arrow) of the arterial wall. This plaque would be classified as homogeneous because of its uniform echogenicity. The presence of calcifications (curved arrow) within the plaque does not affect its classification. B. Image of another common carotid artery shows a heterogeneous plaque with central lucency (white arrow) and a focal densely calcified plaque (black arrow).
Figure 11.36 Methods of Measuring Diameter Stenosis. Different methods of measuring diameter stenosis on arteriograms lead to different calculations of percent diameter stenosis. The traditional method, used by the European Carotid Surgery Trial (ECST), uses the diameter of the carotid bulb, B, as the diameter of reference. Percent diameter stenosis is calculated as (B - A)/B. The North American Carotid Endarterectomy Trial (NASCET) used C, the diameter of the internal carotid artery (ICA) distal to the bulb as the diameter of reference. Percent diameter stenosis is calculated as (C - A)/C. If A is 4 mm, B is 10 mm, and C is 8 mm, ECST would calculate diameter stenosis as 60% [(10 - 4)/10], while NASCET would calculate diameter stenosis as 50% [(8 - 4)/8]. A, diameter of the internal carotid artery (ICA) lumen at the point of maximal narrowing. B, diameter of the disease-free ICA distal to the bulb. C, estimated normal diameter of the bulb of the ICA.
Table 11.3: University of Washington Criteria for Internal Carotid Artery Stenosis
Diameter Stenosis (Percent)a Peak Systolic Velocity (cm/sec) End Diastolic Velocity (cm/sec) Spectral Broadening Plaque Visualized
0 <125 <140 No No
1-15 <125 <140 During systolic deceleration Yes
16-49 <125 <140 Throughout systole Yes
50-79 >125 <140 Extensive Yes
80-99 >125 >140 Extensive Yes
Occlusion No flow detected      
Note that each category of stenosis is characterized by presence of an additional criterion.
aPercent diameter stenosis is calculated in the traditional manner using the diameter of the bulb of the internal carotid artery as the diameter of reference. These criteria are not applicable to the 60% and 70% stenosis cutoffs of the North American Carotid Endarterectomy Trial and the Asymptomatic Carotid Atherosclerosis Study.
Source: Fell G, Phillips DJ, Chikos PM, et al. Ultrasonic duplex scanning for disease of the carotid artery. Circulation 1981;64:1191–1195 and Carotid Research Laboratory, University of Washington, Seattle, WA.
Table 11.4: Criteria for Internal Carotid Artery Stenosis Used at the University of Utah
Diameter Stenosis (Percent)a Peak Systolic Velocity (cm/sec) End Diastolic Velocity (cm/sec) ICA/CCA Ratio PSV-ICA/PSV-CCAb
≥60% >260 >70 >3.5
≥70% >325 >110 >4.0
aPercent diameter stenosis is calculated by the North American Carotid Endarterectomy Trial standard using the diameter of the distal internal carotid artery as the diameter of reference. These criteria are applicable to the 60% and 70% stenosis cutoffs of the North American Carotid Endarterectomy Trial and the Asymptomatic Carotid Atherosclerosis Study. bInternal carotid artery/common carotid artery ratio equals peak systolic velocity in the area of maximal stenosis in the internal carotid artery divided by peak systolic velocity in a normal segment of the common carotid artery. Source: Zwiebel WJ. New Doppler parameters for carotid stenosis. Semin Ultrasound CT MRI 1997;18:66-71; and University of Utah School of Medicine and Department of Radiology, Veterans Affairs Medical Center, Salt Lake City, UT [37].
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Carotid Stenosis
As a result of the North American Carotid Endarterectomy Trial (NASCET) and the European Carotid Surgery Trial, endarterectomy is currently routinely recommended for 70-99% diameter stenosis of the ICA in symptomatic patients [31, 32]. It is important to note that percent diameter stenosis was calculated differently in these two studies (Fig. 11.36) [33]. A 70% European Carotid Surgery Trial stenosis is equivalent to a 50% NASCET stenosis. Care must be taken to note the method used to calculate stenosis when evaluating any published study and when using published US parameters to determine stenosis (Tables 11.3, 11.4 and 11.5) [34]. The NASCET method yields a consistently lower percentage stenosis.
Because perioperative risk of stroke is high, prophylactic endarterectomy is routinely recommended for asymptomatic patients with >75% stenosis who are undergoing major surgery such as coronary artery bypass grafting. Management is controversial for asymptomatic patients with significant carotid stenosis (≥60%). The Asymptomatic Carotid Atherosclerosis Study (ACAS) demonstrated a decreased stroke risk for asymptomatic patients with ≥60% stenosis treated with endarterectomy [35]. Although the improvement in stroke incidence was not as dramatic as in the NASCET study, many vascular surgeons quote this
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study as justification for performing endarterectomy in asymptomatic patients. The ACAS used the NASCET method for measuring stenosis.
Table 11.5: Criteria for Internal Artery Stenosis Recommended by Grant et al.a
Percent Diameter Stenosisb Peak Systolic Velocity (cm/sec) ICA/CCA Ratio
PSV-ICA/PSV-CCAc
≥70%    
Symptomatic patients 175 2.5
≥60%    
Asymptomatic patients 200 3.0
aThese criteria separate patients into those with (symptomatic) and those without (asymptomatic) neurological symptoms. All patients were considered at risk for generalized atherosclerotic disease.
bPercent diameter stenosis is calculated by the North American Carotid Endarterectomy Trial standard using the diameter of the distal internal carotid artery as the diameter of reference. These criteria are applicable to the 60% and 70% stenosis cutoffs of the North American Carotid Endarterectomy Trial and the Asymptomatic Carotid Atherosclerosis Study.
cInternal carotid artery/common carotid artery ratio equals peak systolic velocity (PSV) in the area of maximal stenosis in the ICA divided by PSV in a normal segment of the CCA.
Source: Grant EG, Duerinckx AJ, Saden SE, et al. Doppler sonographic parameters for detection of carotid stenosis: Is there an optimum method for their selection? AJR Am J Roentgenol 1999;172:1123–1129.
Figure 11.37 Carotid Stenosis >70%. A. Color Doppler identifies a large plaque (arrows) in the left internal carotid artery (L ICA). Flow as shown by color display is narrowed to a trickle in the area of maximum stenosis (see Color Figure 11.37A). B. Doppler spectrum in the area of maximal stenosis shows spectral broadening and elevated peak systolic velocity measured at 3.57 m/sec. End diastolic velocity is approximately 1.30 m/sec. Using the criteria from the University of Utah (Table 11.4) Doppler assessment indicates significant stenosis of >70%. C. Downstream from the plaque, flow velocities have decreased but turbulence is prominent.
US parameters used to determine categories of carotid stenosis remain under debate. Historically, the University of Washington criteria (Strandness) (Table 11.3) have been widely accepted and utilized. However, these criteria do not match the 70% and 60% threshold levels of the NASCET and ACAS used to justify carotid endarterectomy in symptomatic and asymptomatic patients. In addition, the percent stenosis of the University of Washington criteria was determined by traditional angiographic methods using the diameter of the “normal” carotid bulb as the standard of reference. A wide range of new criteria matched to NASCET and ACAS criteria and using the NASCET method of determining percentage stenosis have been published and are being used (Tables 11.4 and 11.5) [36, 37, 38]. Each US laboratory must correlate the criteria used with patient outcomes (Fig. 11.37).
  • PSV in the area of maximum narrowing has the best statistical correlation with percent diameter stenosis as determined by carotid arteriography. Note that PSV continues to increase until critical stenosis is reached at approximately 60-70% diameter stenosis (Fig. 11.38). With increasing stenosis beyond this level, both blood flow volume and blood flow velocity fall rapidly.
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  • Carotid disease is classified in categories of stenosis (i.e., 50-79% or ≥60%), not as a specific percent diameter stenosis (i.e., 72%).
  • CCA stenosis has no established parameters. ICA criteria are usually used to estimate severity of CCA stenosis.
  • ECA stenosis has no established parameters and has limited clinical significance except for explanation of the presence of a carotid bruit. ECA stenosis can be estimated by using PSV. Mild stenosis is indicated by PSV >130 cm/sec, moderate stenosis by PSV >200 cm/sec, and severe stenosis by PSV >300 cm/sec.
Figure 11.38 Velocity vs. Flow and Percent Stenosis. As the diameter of the vessel decreases (from left to right), the velocity in the area of maximum stenosis increases. Peak velocities occur at approximately 60-70% diameter stenosis. Thereafter, the velocity falls off rapidly to zero. Blood flow, however, remains stable until diameter stenosis of 50-60% is reached. Thereafter, blood flow falls off rapidly to zero.
Occlusion of the Internal Carotid Artery
Occlusion of the ICA may be asymptomatic if blood supply to the brain is adequate through collaterals of the circle of Willis (Fig. 11.39).
  • Color and spectral Doppler signals are absent in the ICA.
  • The CCA spectrum is identical in appearance to the ECA spectrum with low flow velocity in diastole.
  • The ECA may show a Doppler waveform more characteristic of the ICA if it supplies collateral vessel connections to reconstitute the intracranial ICA.
  • The ICA lumen is filled with echogenic thrombus.
  • The ICA lumen is sub-normal in size.
  • The wall of the ICA does not expand with pulsations.
  • “Stump flow” may be seen at the proximal end of the occlusion. The spectrum shows brief systolic spikes caused by the “thumping” of blood against the occlusion.
  • Doppler US does not reliably differentiate high-grade stenosis with a trickle of flow from complete occlusion. This differentiation is critical because near-occlusion is potentially correctable by endarterectomy whereas total occlusion is not reversible. Angiography is needed to confirm total occlusion.
Subclavian Steal
Occlusion of the innominate or subclavian artery proximal to the origin of the vertebral artery will “steal” blood from the intracranial circulation to supply the affected arm. Flow in the ipsilateral vertebral artery is retrograde and the patient may experience symptoms of cerebral ischemia, especially with muscle activity of the arm.
  • Flow is reversed in the ipsilateral vertebral artery away from instead of toward the intracranial circulation (Fig. 11.40).
  • Partial subclavian steal, caused by high-grade stenosis of the innominate or subclavian artery, results in reversed flow in the vertebral artery (away from the brain) during systole and forward flow (toward the brain) during diastole.
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  • Early subclavian steal with continued antegrade flow in the vertebral artery is suggested when a sharp decline in flow velocity is noted in early systole [39].
Figure 11.39 Occlusion of the Internal Carotid Artery. A. Color Doppler image shows no flow in the internal carotid artery (arrow). The lumen is filled with hypoechoic thrombus (see Color Figure 11.39A). B. Doppler spectrum of the common carotid artery (CCA) shows a high resistance spectrum identical to that of the external carotid artery (ECA) shown in C. D. Doppler spectrum obtained at the origin of the internal carotid artery shows “stump flow.”
Figure 11.40 Subclavian Steal. Color Doppler image of the common carotid artery (CCA) (short arrow) and the vertebral artery (long arrow) shows colors on opposite sides of the color map indicating blood flow in different directions. The vertebral artery is flowing away from the brain while the CCA shows normal blood flow direction toward the brain (see Color Figure 11.40).
Peripheral Arteries
Peripheral Artery Stenosis
No criteria for characterizing stenosis of peripheral arteries are firmly established by clinical studies [25]. The following criteria are generally used [40, 41].
  • PSV are determined in area of maximal stenosis and compared as a ratio to PSV obtained upstream to the stenosis. As illustrated in Figure 11.12, the artery should be examined with gray-scale and Doppler US upstream from any visualized plaque, at the area of maximal narrowing, and downstream from the visualized plaque. Normal PSV in peripheral arteries is <150 cm/sec.
  • PSV (stenosis)/PSV (upstream) <2.0 indicates <50% stenosis. PSV is <200 cm/sec.
  • PSV (stenosis)/PSV (upstream) >2.0 indicates >50% stenosis. PSV is 200-400 cm/sec.
  • PSV (stenosis)/PSV (upstream) >3.5 indicates >70% stenosis. PSV is >400 cm/sec. Diastolic velocity in the area of stenosis is increased compared to upstream spectra. Flow reversal in diastole is lost because of the pressure drop across the severe stenosis. This feature is highly indicative of a severe stenosis. Severe downstream turbulence is usually present. Dampened waveforms may be characteristic of tardus parvus.
  • Occlusion is manifest by absence of blood flow and visualization of enlarged collateral vessels. Tardus parvus waveforms are common in reconstituted distal vessels.
Abdominal Aorta
Normal Aorta
The normal abdominal aorta is visualized from the diaphragm through the bifurcation into the common iliac arteries. The aorta lies on the left side of the lumbar spine.
  • The normal aorta gradually tapers as it proceeds distally giving off its branches. The origins of the celiac axis and SMA from the anterior aorta can usually be visualized (see Fig. 2.10).
  • Spectral Doppler of the infrarenal aorta shows a triphasic, high-resistance pattern with reversal of flow in early diastole.
Atherosclerotic Change of the Aorta
Atherosclerosis is the most common disease of the aorta.
  • Atheromatous plaques cause the wall of the aorta to appear irregular and focally asymmetric. Calcification in the plaques produces bright, discontinuous, linear echoes that may case acoustic shadows and obscure the aortic lumen (Fig. 11.41).
  • The aorta commonly becomes tortuous. Care must be taken to measure the aortic diameter in true transverse section and not be fooled by tortuosity simulating dilatation.
Aneurysm of Abdominal Aorta
US is the screening method of choice to document the presence of, and to follow, abdominal aortic aneurysms (AAA) because of low cost, high accuracy, and noninvasive nature [42]. Complications of AAA include rupture, which is often catastrophic, distal embolism, fistula to adjacent structures, thrombosis, and dissection.
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  • Focal or diffuse enlargement of the diameter of the abdominal aorta greater than 3 cm is an aneurysm (Fig. 11.42). The luminal diameter of the aorta is measured from inner wall to inner wall.
  • Enlargement of the distal aorta to 1.5 times the diameter of the adjacent, more proximal aorta is considered to be an aneurysm even if the aortic diameter is less than 3 cm.
  • Common iliac arteries are aneurysmal when the diameter exceeds 1.5 cm (Fig. 11.43) [43].
  • Mural thrombus is commonly present within AAA. Thrombus is hypoechoic and is best identified by using color Doppler to show the patent lumen.
  • Approximately 90-95% of AAA are confined to the infrarenal aorta.
  • Aneurysms that involve the aorta at the level of origin of the renal arteries change surgical management and must be diagnosed preoperatively. The renal arteries may be difficult to visualize; however, both arise within 2 cm of the origin of the SMA. Therefore, if a normal caliber aorta is documented at least 2 cm below the origin of the SMA, the AAA is confirmed to be infrarenal. Aneurysms that involve the renal artery origins are frequently extensions of aneurysms of the thoracic aorta.
  • AAA are usually not repaired until they exceed 4-5 cm in maximum diameter. The risk of rupture within 5 years is 25% at 5-cm diameter and rises to 75% at 7-cm diameter. AAA smaller than 5 cm have a 3% risk of rupture over 10 years [44].
  • US is used to monitor the rate of enlargement of AAA. The average increase in diameter is 2 mm per year.
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  • Leakage or rupture of an AAA is suggested by demonstration of fluid or hematoma around the aorta. However, spiral CT is the method of choice for diagnosis because US has unacceptably low sensitivity and specificity.
  • AAA may obstruct the ureters by direct compression or by perianeurysmal fibrosis. Routine survey of the kidneys for hydronephrosis is indicated when US is used to follow AAA.
Figure 11.41 Atherosclerotic Disease of the Abdominal Aorta. Longitudinal US image of the aorta (between cursors, +) shows an irregular wall and calcified atherosclerotic plaques, some of which cast acoustic shadows.
Figure 11.42 Aneurysm of the Abdominal Aorta. Longitudinal color Doppler image shows swirling blood flow within a 5-cm-diameter aneurysm (see Color Figure 11.42).
Figure 11.43 Aneurysms of the Common Iliac Arteries. A. Transverse plane color Doppler image (shown here in gray scale) below the aortic bifurcation (AO BIF) shows bilateral aneurysms of the common iliac arteries. B. Enlarged view of the right common iliac artery shows the large amount of intraluminal thrombus (T) commonly found in aneurysms of the aorta and iliac arteries. The patent lumen is indicated by the arrow.
Endovascular Aortic Stents
Endovascular repair of AAA is a new and rapidly expanding procedure that may replace standard surgical repair [45]. Aortic stent grafts are expandable intraluminal grafts that consist of a metal framework cage covered by synthetic graft material. These grafts are compressed within a catheter and guided under fluoroscopy to be placed within the AAA and expanded to the optimal size of the aorta. Ideally, all blood flow is excluded from the AAA allowing thrombosis of the AAA outside of the stent graft. Spiral CT or Doppler US are used to follow the integrity of stent grafts.
  • US examination must determine
    • - Patency of the stent graft
    • - Size of the AAA
    • - Presence of endoleaks, seen as blood flow within the aneurysm outside of the stent
    • - Change in diameter of the vessels at the sites of endograft attachment
    • - Presence of new aneurysms [45]
Ultrasound of Deep Venous Thrombosis
Compression US is the imaging procedure of choice for diagnosis of deep venous thrombosis (DVT) in the lower extremities [46]. US is highly accurate in the diagnosis of DVT involving the proximal leg veins with sensitivity exceeding 95% and specificity exceeding 98% [47]. DVT is the major risk factor for life-threatening pulmonary emboli.
Ultrasound Technique
US examination consists of visualizing deep veins with gray-scale US, supplemented by color flow and spectral Doppler, and then using the transducer to compress the vein until opposing walls touch.
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  • The patient is examined in supine position with the head of the bed raised 20 degrees to 30 degrees to promote venous pooling in the legs. The leg is rotated externally and flexed slightly at the knee.
  • A 7-10-MHz linear array transducer is utilized. Doppler capability greatly aids the examination but is not a requirement.
  • The common femoral vein is identified medial to the common femoral artery in the groin at the crease of the thigh.
  • Transducer pressure is used to compress and flatten the vein until the anterior and posterior walls are touching and the lumen is obliterated. Compression should always be performed transverse to the vein because the transducer may slide off the vein with compression in the longitudinal plane.
  • Thrombus makes compression of vein impossible with force less than that required to compress the adjacent artery.
  • The vein is followed distally and compressed every centimeter of its course to the bifurcation of the popliteal vein into posterior tibial and peroneal branches.
  • In the adductor canal, the superficial femoral vein is deep and may be difficult to compress. Placing one hand under the medial aspect of the thigh allows the vein to be pushed to a more superficial location where it can be compressed between the fingers and the transducer.
  • Having the patient perform a Valsalva maneuver will decrease venous return to the chest and distend the leg veins to make identification easier. Blood flow can also be augmented by gently squeezing the calf.
  • Evaluation of the calf veins is considerably more difficult and time consuming, and has a much higher rate of inadequate examination. Because calf vein thrombosis is not a direct risk factor in the development of pulmonary embolus, most clinicians do not treat calf vein thrombosis and consider examination of the calf veins to be unnecessary [48].
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  • If the initial US study is negative, but the patient remains symptomatic or highly suspect for DVT clinically, the US examination should be repeated in 1 week. This follow-up study is intended to detect patients with calf vein thrombosis that has propagated to involve the deep venous system [46].
Figure 11.44 Deep Venous Thrombosis—Lower Extremity. A. Transverse power Doppler image with transducer compression applied shows flow in the femoral artery (A) and no flow in the femoral vein (V, arrow). The vein does not compress with transducer pressure, indicating intraluminal thrombus. B. Longitudinal color Doppler image of the junction of the greater saphenous vein (SAPH) with the common femoral vein (CFV, arrows) shows enlargement of the common femoral vein with intraluminal thrombus. Note the flow of blood around the thrombus above the venous junction (see Color Figure 11.44).
Acute Deep Venous Thrombosis
Lack of venous compressibility is the hallmark of US diagnosis of DVT (Fig. 11.44).
  • Acute thrombus is hypoechoic and is commonly indistinguishable from flowing blood. Inability to compress the vein and obliterate the lumen is prime evidence of the presence of thrombus.
  • The vein is distended when DVT is acute.
  • Color Doppler reveals absence of blood flow or a trickle of blood flow around the thrombus.
  • Duplication of the normally solitary deep veins of the thigh is a potential pitfall [49]. One of the paired veins may be patent while the other has thrombus. A clue to this diagnosis is unusually small size of the visualized patent vein.
  • The radiographic report must specify the diagnosis of DVT when present. Some clinicians are unaware that the superficial femoral vein is actually a deep vein.
Chronic Deep Venous Thrombosis
DVT is slow to resolve and is prone to recur. Involved deep veins return to normal appearance and compressibility in only 50% of patients by 12-24 months. In some patients, US evidence of DVT persists indefinitely. Diagnosis of new acute thrombus in a patient with known previous DVT is a challenge.
  • Thrombus becomes increasingly echogenic with time and may even calcify (Fig. 11.45).
  • The chronic clot is often discontinuous and only partially occlusive with intervening areas of normal appearing vein.
  • The chronic clot is usually adherent to the vein wall.
  • With chronic DVT, the wall of the vein thickens and stiffens, becoming more resistant to compression.
  • Re-examining high-risk patients after completion of their anticoagulant therapy is useful in establishing a new baseline on which to base diagnosis of recurrent DVT.
Figure 11.45 Chronic Deep Venous Thrombosis. Longitudinal image shows a very echogenic lobulated clot (arrow) in the common femoral vein. Increased echogenicity of the clot is evidence of chronicity.
Figure 11.46 Deep Venous Thrombosis—Upper Extremity. Color Doppler image of the subclavian vein (long arrow) shows that the lumen is distended with thrombus. No blood flow in the vein is evident. Flow is present in an adjacent artery (short arrow) (see Color Figure 11.46).
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Upper Extremity Deep Venous Thrombosis
Upper extremity DVT may also result in pulmonary embolus. Patients at risk include chronically ill or cancer patients with indwelling catheters in the upper extremity veins and young patients with idiopathic thrombus [50].
  • Compression US of the subclavian, axillary, basilic, and cephalic veins is performed in a manner similar to examination of the lower extremity. Doppler is exceptionally valuable in evaluating portions of the veins that are not easily accessible, such as the subclavian vein where covered by the clavicle [51].
  • Diagnostic findings are identical to those used in the lower extremity (Fig. 11.46).
Vascular Lesions of the Extremities
Pseudoaneurysm
Pseudoaneurysms occur as a complication of penetrating injuries to arteries. Many occur as complications of percutaneous interventional vascular procedures such as coronary or peripheral angioplasty [52]. A pseudoaneurysm is a perivascular hematoma that maintains a channel of flowing blood in communication with the parent artery. They are initially bounded by clotted blood but eventually form a fibrous capsule. Unlike true aneurysms, the wall does not contain any normal arterial wall layers (Fig. 11.47) [53].
  • Gray-scale US shows a complex perivascular mass with a variable amount of echogenic thrombus. Multiple compartments may be evident.
  • Color flow US shows swirling internal flow and a fistulous communication to the adjacent artery. The entire mass must be carefully examined because only a portion of the mass may show blood flow while the rest of the mass is thrombus.
  • Spectral Doppler confirms arterial pulsations within the mass and a distinctive “to and fro” spectral pattern at the communication. Blood flows into the pseudoaneurysm during systole and out of the pseudoaneurysm during diastole [54].
  • US-guided manual compression of the pseudoaneurysm is commonly curative, resulting in stable thrombosis [55]. The transducer is oriented to optimally visualize the neck of the pseudoaneurysm [56]. Firm compression is applied with the transducer with pressure sufficient to obliterate flow to the pseudoaneurysm. Compression is continued for
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    one or more 10-minute periods. Pressure is released after 10 minutes to assess for complete thrombosis. Patients on anticoagulant therapy commonly fail to respond to US-guided compression therapy. Surgery is usually required when guided compression fails.
Figure 11.47 Pseudoaneurysm. A. Gray-scale image shows a pseudoaneurysm (big arrows) largely filled with echogenic thrombus. The communication (small arrow) with the parent artery is evident. B. Color Doppler shows blood flow that enters a pseudoaneurysm via a large neck connecting to the parent artery (see Color Figure 11.47B). C. Spectral Doppler obtained at the neck of a pseudoaneurysm shows the characteristic “to and fro” pattern of blood flow.
Perivascular Hematoma
Hematomas may form adjacent to areas of vascular injury (Fig. 11.48) [52].
  • Hematomas are masses of variable echogenicity that change in appearance and shrink in size over time. Differentiation from pseudoaneurysm requires careful Doppler evaluation.
  • Internal blood flow is not present on spectral or color flow Doppler.
  • Tissue vibration artifact from adjacent blood vessels may be mistaken for flow in a pseudoaneurysm especially when the hematoma is echolucent.
Arteriovenous Fistula
Simultaneous arterial and venous puncture may result in a fistulous tract between the two vessels [53]. Arterial blood is preferentially shunted to the venous circulation and may result in distal limb ischemia [57].
  • High-velocity diastolic flow is seen in the artery proximal to the fistula.
  • Increased flow velocity with arterial pulsations are seen in the vein near the fistula. Turbulence is often prominent near the fistula.
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  • The fistula may be visualized with color flow US as a tiny track extending between the artery and vein (Fig. 11.49).
  • Perivascular tissue vibration artifact (a visible bruit) may be prominent on color Doppler US.
Figure 11.48 Hematoma. Color Doppler reveals no flow in a hematoma adjacent to a dialysis shunt graft in the arm (see Color Figure 11.48).
Arteriovenous Malformation
Arteriovenous malformation is a congenital vascular anomaly made up of a network of direct communications between arteries and veins (Fig. 11.50) [58].
  • Supplying arteries and draining veins are dilated.
  • Color Doppler shows the subcutaneous network of abnormal intercommunicating arteries and veins [59].
  • Tissue vibration artifact may be prominent in surrounding soft tissues.
  • The supplying artery shows a low-resistance spectrum.
Figure 11.49 Arteriovenous Fistula. Color Doppler shows a fistula between the greater saphenous vein (SAPH) and the common femoral artery (CFA). The common femoral vein (CFV) is also seen (see Color Figure 11.49).
Figure 11.50 Arteriovenous Malformation. A. Gray-scale US image of the soft tissues of the upper chest shows a network of tubular structures in the subcutaneous tissues. B. Color Doppler confirms blood flow in the tubular structures, representing a complex of vessels of an arteriovenous malformation (see Color Figure 11.50B).
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Peripheral Vascular Grafts
US is an ideal modality to survey peripheral vascular grafts for complications [60]. US examination includes real-time gray-scale survey of the entire graft and spectral and color Doppler examination of blood flow [61].
  • Focal fluid collections at the site of graft anastomoses are common, not pathologically significant, and generally resolve within a few months of surgery.
  • US-guided aspiration can easily be performed if infection of a perigraft fluid collection is suspected.
  • Grafts may show diffuse wall thickening caused by intimal hyperplasia or focal stenosis deemed significant if PSV in the area of stenosis is two times PSV in the more proximal graft.
  • Arteriovenous fistulas may develop as a result of surgery or graft puncture for arteriography.
  • Pseudoaneurysms are common at sites of graft anastomosis and are distinguished from hematomas and lymph nodes by use of color flow US.
Hemodialysis Access Grafts
Prosthetic and native vein arteriovenous grafts are in common use for vascular access for hemodialysis [62]. Progressive stenosis and thrombosis are common events with graft occlusion rates of 17-45% during the first year [63]. Routine US surveillance for developing graft stenosis with subsequent angioplasty of high-grade stenosis can prolong graft patency. The entire graft is inspected with gray-scale and color flow US. Any areas of stenosis are evaluated with spectral Doppler [62, 63, 64].
  • Spectral Doppler of normal grafts show low-resistance waveforms with PSV of 100-400 cm/sec with end-diastolic velocities of 60-200 cm/sec [62].
  • Focal elevation of PSV 2.0-2.9× PSV in the more proximal graft correlates with 50-79% stenosis. Focal 3× elevation of PSV indicates ≥75% stenosis [64]. Arteriography of the graft is routinely performed when US detects stenosis of ≥50% [63].
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  • Infection of the graft is common (10-15% of synthetic grafts) and necessitates removal of the graft [62]. US demonstrates perigraft fluid collections in patients with fever and positive blood cultures. US-guided aspiration is used to confirm graft infection.
  • Aneurysms and pseudoaneurysms are common complications of repeated graft puncture [62]. Small pseudoaneurysms (<5 mm) tend to be stable and not clinically significant. Pseudoaneurysms >5 mm usually enlarge progressively and should be embolized or repaired surgically. Pseudoaneurysms at the site of graft anastomosis tend to be complications of graft infection. Aneurysmal dilatation of the graft is common and is usually not treated.
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