DOI: 10.1148/rg.243035139
RadioGraphics 2004;24:657-675
© RSNA, 2004
Optimizing Doppler and Color Flow US: Application to Hepatic Sonography1
Jonathan B. Kruskal, MD, PhD,
Peggy A. Newman, RVT,
Laurie G. Sammons, BA, RDMS and
Robert A. Kane, MD
1 From the Department of Radiology, Beth Israel Deaconess Medical Center, 1 Deaconess Rd, Boston, MA 02215. Recipient of a Certificate of Merit Award for an education exhibit at the 2002 RSNA scientific assembly. Received June 4, 2003; revision requested July 7 and received July 24; accepted July 28. Address correspondence to J.B.K. (e-mail: jkruskal@bidmc.harvard.edu).
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Abstract
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In the imaging of patients with chronic liver disease or portal hypertension or who have undergone liver transplantation or surgery, accurate evaluation of the hepatic vasculature is usually necessary. Because Doppler ultrasonography (US) is capable of accurately characterizing the nature of flow within the major hepatic arteries and the portal and hepatic veins, it is widely used for imaging the liver vasculature. An informed choice of transducer and scanning techniques is important in the evaluation of the liver vasculature. In addition, there are a variety of operator-dependent technical parameters (eg, baseline, frame rate, wall filters, gain, velocity range, angle correction, gate size and position) that must be optimized when performing Doppler US of the liver. Changes in these parameters independently influence both the color and spectral components of the Doppler US examination; therefore, the parameters should be optimized separately for each patient. Failure to appropriately adjust these parameters may result in artifacts or misinterpretation of the study, which will frequently affect patient treatment. In contrast, knowledge of these operator-dependent parameters will permit optimization of the study and improve the overall utility of liver Doppler US.
© RSNA, 2004
Index Terms: Hepatic arteries, US, 952.1298 • Hepatic veins, US, 95.1298, 957.1298 • Ultrasound (US), Doppler studies, 95.12983, 95.12984 Ultrasound (US), pulsed, 95.1298
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Introduction
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When properly performed, Doppler ultrasonography (US) provides rapid, comprehensive, and accurate evaluation of the entire hepatic vasculature. If need be, this versatile technique can be performed at the patient’s bedside, intraoperatively, or immediately postoperatively. Hepatic hemodynamic evaluation is most commonly performed (a) in patients with unexplained ascites or with cirrhosis or portal hypertension, (b) for surveillance of the patency and function of transjugular intrahepatic portosystemic shunts (TIPS), and (c) after liver surgery, especially transplantation. In these clinical situations, when Doppler US is performed properly, it provides accurate hemodynamic information concerning vessel patency and flow direction, which facilitates clinical management. Although abnormalities of Doppler flow are well described for many disorders that involve the hepatic vasculature (1–3), little attention has been given to describing techniques for optimizing the Doppler US examination.
In this article, we discuss the transducers and scanning techniques that are best for evaluating the liver vasculature. We also describe the essential technical and operator-dependent parameters (Fig 1) that can be modified to optimize the performance and interpretation of Doppler US of the liver and illustrate how each parameter influences detection of flow and should be adjusted for optimal depiction and characterization of the hepatic vasculature. Some of these parameters influence both the color and spectral components of the Doppler US examination (baseline, velocity scale, wall filters, inversion of flow), some are specific for the spectral Doppler component (angle correction, spectral gain, gate size and position), and some are specific for the color Doppler component (color gain, color bar, color box or overlay, color velocity scale, color priority).
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Figure 1. Photograph shows the Doppler panel that appears on the console of many contemporary US imagers. Each parameter can be adjusted to optimize the color or spectral Doppler components of the examination.
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Choice of Transducer
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The choice of transducer depends in part on the indications for the study, the body habitus of the patient, and operator preference. It is not unusual for more than one transducer to be used during a study. However, for evaluating the hepatic vasculature, the selected transducer should be equipped for both the color and spectral components of the Doppler US examination (as most modern transducers are). A shrunken cirrhotic liver is frequently seen in a high subcostal location that necessitates an intercostal scanning approach, in which case a small transducer footprint is ideal.
The frequency of the transducer determines axial image resolution and usable image depth. As the frequency increases, so does the resolution, but the depth of penetration decreases. Because tissue penetration is inversely related to transducer frequency, a compromise is frequently required to optimize liver imaging. In essence, the highest frequency that will achieve the best resolution with sufficient penetration should be chosen. Typically, a frequency of approximately 3.5 MHz is best for imaging of the liver vasculature, but lower frequencies may be necessary for larger patients. Because hepatic veins should be imaged down to their confluence with the vena cava and vessels supplying posterior segments are often deeply located, lower-frequency transducers may also be required for imaging and evaluating these deeper segments and vessels in the liver. The depth of field can be reduced with appropriate patient positioning or rotation.
Linear-array transducers with higher frequencies are best for imaging of superficial structures. The high frequency of these transducers results in images with high resolution and less spatial distortion. Thus, improved spatial resolution, image detail, and definition are achieved. However, the higher frequency results in poor penetration and a smaller field of view. In hepatic imaging, these transducers are best reserved for characterizing superficial abnormalities, imaging the liver surface, and evaluating superficial nodules within a few centimeters of the liver.
Curved-array transducers have a large field of view, penetrate soft tissue well, and have less spatial distortion (ie, better spatial resolution) at depth or in the far field than mechanical sector scanners. However, the resolution of curved-array transducers is lower than that of linear-array transducers. Although curved-array transducers have a larger footprint than do phased-array scanners, they can also be used to image with an intercostal approach.
Phased-array transducers have small footprints that are ideally suited for intercostal or subcostal access, which is frequently used in patients with chronic liver disease. The transducer beam is mechanically steered, producing a wide field of view. These transducers also have less spatial distortion in the near field, which facilitates visualization during needle insertion. However, the resolution of these scanners at depth is inferior to that of curved-array transducers.
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Scanning Techniques
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The scanning window selected for imaging of hepatic vessels depends on the specific vessel being studied. Window selection is especially important when the direction of flow is being determined and when the Doppler angle requires optimization for estimating flow velocities. The liver is scanned with a combination of transabdominal, subcostal, and intercostal approaches, depending on the size of the liver, prior liver surgical procedures, and the presence of underlying liver disease (Fig 2). Intercostal scanning, performedwith a small transducer head positioned parallel to the right ribs, is often facilitated by placing the patient on the left side in the decubitus position. This approach is usually required to obtain appropriate Doppler angles and to visualize specific vessels in the posterior segment of the right lobe. Different vessels are best studied with different scanning orientations, allowing better Doppler angles to be created between the transducer and the direction of blood flow. A subcostal oblique position with cephalad angulation of the transducer head is best for imaging the hepatic veins as they converge into the vena cava.
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Figure 2a. Clinical photographs show transducer placement for imaging of the hepatic vasculature. Depending on which vessels are being evaluated, the transducer can be placed in a transabdominal (a), subcostal (b), or intercostal (c) location.
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Figure 2b. Clinical photographs show transducer placement for imaging of the hepatic vasculature. Depending on which vessels are being evaluated, the transducer can be placed in a transabdominal (a), subcostal (b), or intercostal (c) location.
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Figure 2c. Clinical photographs show transducer placement for imaging of the hepatic vasculature. Depending on which vessels are being evaluated, the transducer can be placed in a transabdominal (a), subcostal (b), or intercostal (c) location.
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Prior to commencing the Doppler component of the liver examination, the basic parameters for the B-mode (gray-scale) examination should be optimized, not only for higher-quality images but also to facilitate the subsequent Doppler component of the study. These basic parameters include (a) the location and number of focal zones, (b) the depth of field for the specific segment, lobe, or vessel of the liver being imaged, (c) the two-dimensional (2D) gain setting, (d) the scan orientation, (e) the image zoom settings, and, where possible and depending on the equipment being used, (f) the presets for the specific transducer being used and the type of study being performed. Because color Doppler flow data are superimposed on the 2D image, a high 2D gain setting suppresses color information and a low setting highlights color information. The frame rate varies inversely with the depth of field: Sampling from a deeper segment slows the frame rate. Similarly, a single focal zone should be used because the frame rate also varies with the number of focal zones selected.
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Liver Doppler US
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For quantitative and qualitative evaluation of flow within the major hepatic arteries and veins, the Doppler US examination should include both a color and a spectral waveform analysis. The basic principles of Doppler US have been well described in excellent reviews (3–5). The color Doppler flow US examination measures and color codes the mean Doppler frequency shifts that occur in moving blood and superimposes a color depiction of these data on the gray-scale image, thereby providing a global depiction of the presence and direction of blood flow. The spectral Doppler US examination requires manual placement of an operator-defined sample volume in a vessel (as depicted on either the gray-scale or color image), and a spectral waveform of all velocities is then plotted against time. The waveform morphology of these velocities illustrates the hemodynamics of the vessel within the sample volume being studied. For duplex or triplex Doppler US, the gray-scale image is simultaneously displayed with the color flow or spectral waveform. Given that a finite quantity of data are acquired with the transducer, the more components that are displayed, the more these data are divided up, which compromises the quality of the displayed image. Power Doppler US depicts the amplitude shift without estimating velocity and is thus more sensitive to slow flow than is color Doppler flow US.
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Color and Spectral Parameters
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Adjustment of the parameters described in the following sections will alter both the color and spectral components of the Doppler US examination, depending on which scanning mode is active at the time of optimization. Thus, the operator should be aware of which scanning mode is active at the time of adjustment; if the color component is active, adjustments will not affect pulsed Doppler parameters and vice versa.
Baseline
The baseline is depicted on both the spectral waveform and the color bar. The baseline divides the color bar into positive and negative Doppler shifts (Fig 3). Adjustment of the baseline alters the velocity range that is displayed and is therefore used to prevent aliasing (Fig 4). Aliasing is one of several imaging artifacts that can be prevented with appropriate understanding and adjustment of the Doppler parameters. Pulsed Doppler US samples the Doppler frequency shifts at a user-defined rate known as the pulse repetition frequency (PRF). When this sampling rate is too low, the reconstructed color or spectral depiction falls outside the selected range, resulting in an aliasing artifact. This artifact is more likely to occur with high-velocity flow (eg, in the hepatic artery—especially following transplantation when stenosis may develop—or when studying deeper vessels of the right lobe, because the PRF becomes slower as the depth of sampling increases). Technically, aliasing occurs at flow velocities corresponding to Doppler shifts above the Nyquist frequency, which is twice the PRF. The depicted color or spectral signal "folds over" into the reversed-flow portion of the display, not allowing any meaningful quantitative flow data to be obtained. Several adjustments can be made to parameters to avoid aliasing (4,5), including moving the color or spectral baseline up or down (Figs 5, 6), increasing the velocity scale (also referred to as the sampling rate or PRF), decreasing the Doppler shift frequency (changing the angle of insonation), increasing the scanning angle, and using a lower-frequency transducer. Aliasing can also play a useful diagnostic role in evaluating the liver. For example, adjusting the velocity range to fit below the expected velocity of the hepatic artery and above that of the portal vein allows the presence of aliasing at color Doppler flow US to be used to help identify the hepatic artery, especially when this is challenging (eg, immediately following liver transplantation).
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Figure 3. Color baseline. The position of the baseline on the color bar is indicated by a horizontal black line (yellow circles). When the baseline is adjusted, the relative position of this horizontal black line changes. Note that when the position of the baseline is changed, the color velocity range that is displayed on the color bar also changes (in this example, from 15.3 to 46.1 cm/sec above or below the baseline). The range of depicted velocities remains constant, but different flow velocities will be emphasized depending on their relative position on the color bar.
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Figure 4. Aliasing of the spectral waveform. Duplex US image shows aliasing of the spectral waveform with wraparound of the highest flow velocities into the negative part of the graph. Note that the color Doppler flow US image shows normal antegrade portal venous flow with no aliasing. To eliminate or reduce this artifact, spectral Doppler US must be active before different parameters can be modified.
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Figure 5a. Changing the color baseline to avoid aliasing. (a) On a color Doppler flow US image, flow within the portal vein appears green, the color equivalent of aliasing on the selected color bar. The color baseline (arrow) is positioned too high on the color bar. Although the US image helps confirm the presence of flow, the baseline should be lowered to obtain meaningful directional flow data. (b) On a color Doppler flow US image obtained after the baseline was lowered (arrow), accurate directional flow data have been obtained from the main portal vein: Appropriate antegrade portal venous flow toward the transducer appears red.
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Figure 5b. Changing the color baseline to avoid aliasing. (a) On a color Doppler flow US image, flow within the portal vein appears green, the color equivalent of aliasing on the selected color bar. The color baseline (arrow) is positioned too high on the color bar. Although the US image helps confirm the presence of flow, the baseline should be lowered to obtain meaningful directional flow data. (b) On a color Doppler flow US image obtained after the baseline was lowered (arrow), accurate directional flow data have been obtained from the main portal vein: Appropriate antegrade portal venous flow toward the transducer appears red.
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Figure 6a. Changing the spectral baseline to avoid aliasing. (a) Duplex US image demonstrates aliasing of the spectral waveform, which results in the production of inaccurate waveform data and an inability to obtain accurate quantitative flow data. (b) On a duplex US image obtained after the spectral baseline was lowered, the spectral waveform falls within the range of velocities being evaluated, so that accurate quantitative data can be obtained. Note that changing the baseline does not change the velocity scale (PRF = 1,515 Hz), making this adjustment a logical initial change when reducing aliasing.
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Figure 6b. Changing the spectral baseline to avoid aliasing. (a) Duplex US image demonstrates aliasing of the spectral waveform, which results in the production of inaccurate waveform data and an inability to obtain accurate quantitative flow data. (b) On a duplex US image obtained after the spectral baseline was lowered, the spectral waveform falls within the range of velocities being evaluated, so that accurate quantitative data can be obtained. Note that changing the baseline does not change the velocity scale (PRF = 1,515 Hz), making this adjustment a logical initial change when reducing aliasing.
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Color Baseline.—The color baseline, which appears as a horizontal black line on the color bar, can be adjusted to emphasize certain aspects of flow (Fig 3). By lowering the color baseline, the emphasis will be on flow toward the transducer (Fig 5). If flow occurs away from the transducer with a low baseline setting, aliasing will occur. Note that changing the baseline will not result in any change in the color scale (range of velocities) that is shown. Although the overall range of depicted velocities does not change, the depicted flow that is emphasized (ie, where flow falls on the color range) will change. When color Doppler flow US is the active scanning mode, adjusting the baseline will not alter depiction of the presence of flow but will optimize the qualitative depiction of flow. In this situation, it may be important to reduce aliasing, not only to determine flow direction but also to characterize flow. Doing so is especially important in liver transplant recipients, in whom increases in focal velocity may be the only hint of an anastomotic stenosis.
Spectral Baseline.—
Like the color baseline, the spectral baseline can be altered to improve depiction of the spectral waveform (Fig 6). This adjustment, which is commonly made as one way to prevent aliasing, allows the waveform to be brought down onto the baseline and helps prevent wraparound. In patients who are undergoing TIPS surveillance or who have undergone liver transplantation, accurate measurements of flow velocity may influence management decisions; therefore, it is important to prevent aliasing so that accurate flow velocity measurements can be obtained.
Velocity Scale
The velocity scale is the range of flow velocities that are depicted with either the color or spectral component. If the measured flow velocity falls outside the selected scale, aliasing of the currently active scanning mode will occur. This problem can be avoided by expanding the scale to allow the actual velocity to fall within the selected range (Figs 7, 8). Depending on which scanning mode is active, the spectral scale (Fig 7) or color velocity scale (Fig 8) should be optimized separately to prevent aliasing. A high scale increases the range of Doppler shifts that are displayed and is depicted on the monitor as a higher rate of data sampling (higher PRF). The velocity scale also depends on depth of imaging: In acquisition of data from a deeper segment of the liver, it takes longer for the sound waves to traverse tissue in both directions, with an increased duration from pulsing to sampling. The PRF indicates the rate (frequency) at which data are sampled. This variable is directly related to the velocity range because higher flow velocities will require more rapid sampling to obtain accurate measurements. A higher PRF increases the frame rate.
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Figure 7a. Adjusting the spectral scale. (a) Color duplex US image demonstrates that when the spectral scale (or the sampling rate) is set too high (in this example, PRF = 14,286 Hz), flow is more difficult to appreciate and characterize on the scale. (b) On a color duplex US image obtained after the scale was reduced (PRF = 3,731 Hz), the range of depicted velocities is reduced and the appearance of the spectral waveform is improved, providing more visible quantitative and qualitative data. Note that the color Doppler flow US image, color bar, and color scale all remain unchanged because the spectral component is active.
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Figure 7b. Adjusting the spectral scale. (a) Color duplex US image demonstrates that when the spectral scale (or the sampling rate) is set too high (in this example, PRF = 14,286 Hz), flow is more difficult to appreciate and characterize on the scale. (b) On a color duplex US image obtained after the scale was reduced (PRF = 3,731 Hz), the range of depicted velocities is reduced and the appearance of the spectral waveform is improved, providing more visible quantitative and qualitative data. Note that the color Doppler flow US image, color bar, and color scale all remain unchanged because the spectral component is active.
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Figure 8a. Adjusting the color velocity scale. (a) Color Doppler flow US image obtained with the color velocity scale set too high (69.2 cm/sec) demonstrates apparent absence of flow in the portal vein. (b) Color Doppler flow US image obtained after the scale was reduced to 30.7 cm/sec demonstrates normal flow in a widely patent portal vein. (c) Color Doppler flow US image obtained after the scale was set even lower (2.3 cm/sec) shows aliasing of color flow in all branches of the portal vein, which results in meaningless data concerning flow direction. Thus, the color velocity scale should be increased to increase the sampling rate.
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Figure 8b. Adjusting the color velocity scale. (a) Color Doppler flow US image obtained with the color velocity scale set too high (69.2 cm/sec) demonstrates apparent absence of flow in the portal vein. (b) Color Doppler flow US image obtained after the scale was reduced to 30.7 cm/sec demonstrates normal flow in a widely patent portal vein. (c) Color Doppler flow US image obtained after the scale was set even lower (2.3 cm/sec) shows aliasing of color flow in all branches of the portal vein, which results in meaningless data concerning flow direction. Thus, the color velocity scale should be increased to increase the sampling rate.
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Figure 8c. Adjusting the color velocity scale. (a) Color Doppler flow US image obtained with the color velocity scale set too high (69.2 cm/sec) demonstrates apparent absence of flow in the portal vein. (b) Color Doppler flow US image obtained after the scale was reduced to 30.7 cm/sec demonstrates normal flow in a widely patent portal vein. (c) Color Doppler flow US image obtained after the scale was set even lower (2.3 cm/sec) shows aliasing of color flow in all branches of the portal vein, which results in meaningless data concerning flow direction. Thus, the color velocity scale should be increased to increase the sampling rate.
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The spectral waveform typically makes use of 256 pulse cycles per scan line and contains qualitative and quantitative diagnostic information for interpretation. On the other hand, the color map contains only eight pulse cycles per scan line, thereby providing considerably less information than does the spectral waveform. Color Doppler flow US depicts the presence and direction of flow, vessel anatomy, and flow disturbances.
Wall Filters
Wall filters selectively filter out all acquired information below an operator-defined frequency threshold. Filters eliminate the typically low-frequency–high-intensity noise that may arise from vessel wall motion (Fig 9). Filter settings are usually preset by the manufacturer, and a high, medium, or low filter setting may be applied separately to spectral, color, and power imaging. Filters operate at variable frequencies to eliminate signal from low-velocity blood flow. "High" refers to the higher range of frequency shifts that are filtered out and thus are not depicted on the color image or spectral waveform. Wall filters do not usually compromise hepatic Doppler US studies. However, in patients with very slow portal venous flow, a high filter setting may cause this flow to be inadvertently obscured or missed. To avoid the loss of signal that characterizes slow flow, filter settings should be kept at the lowest possible setting (typically in the 50–100-Hz range). The cutoff frequencies vary with the velocity scale; the lowest filter cutoff frequencies cannot be used with the highest velocity ranges and vice versa. On the color bar, the filter setting is indicated by black areas on both sides of the baseline. Expansion of the filter shows up as a widening of the black band (Fig 10). On the spectral waveform, a high wall filter setting will result in loss of depicted spectral information immediately above the baseline. Reducing the wall filter setting results in filling in of spectral data toward the baseline. Because filters can be applied separately to color images and spectral waveforms and are thus displayed separately on the console, the operator must make sure that the scanning mode with a filter applied is currently active.
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Figure 9a. Changing the wall filter. (a) Color duplex US image obtained with a high wall filter setting shows loss of the low-velocity-flow component of the spectral waveform immediately above the baseline. Higher-velocity flow is well depicted, and accurate flow quantification can still occur. In the evaluation of the liver vasculature, this is likely to become relevant only when flow velocity is very low and falls within the range of velocities that are filtered out. (b) Color duplex US image demonstrates how the spectral waveform progressively fills in toward the baseline as the wall filter is sequentially reduced from high (left arrow) to medium (middle arrow) to low (right arrow).
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Figure 9b. Changing the wall filter. (a) Color duplex US image obtained with a high wall filter setting shows loss of the low-velocity-flow component of the spectral waveform immediately above the baseline. Higher-velocity flow is well depicted, and accurate flow quantification can still occur. In the evaluation of the liver vasculature, this is likely to become relevant only when flow velocity is very low and falls within the range of velocities that are filtered out. (b) Color duplex US image demonstrates how the spectral waveform progressively fills in toward the baseline as the wall filter is sequentially reduced from high (left arrow) to medium (middle arrow) to low (right arrow).
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Figure 10a. Changing the color Doppler wall filter. (a) Color Doppler flow US image obtained with the highest possible wall filter setting shows how color signal arising from low-velocity flow may be filtered out. (b) Color Doppler flow US image obtained with a low filter setting demonstrates filling in of flow in the hepatic veins (blue), which indicates minimal filtering of color signal. The change in the filter setting appears as a change in the width of the horizontal black line in the center of the color bar.
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Figure 10b. Changing the color Doppler wall filter. (a) Color Doppler flow US image obtained with the highest possible wall filter setting shows how color signal arising from low-velocity flow may be filtered out. (b) Color Doppler flow US image obtained with a low filter setting demonstrates filling in of flow in the hepatic veins (blue), which indicates minimal filtering of color signal. The change in the filter setting appears as a change in the width of the horizontal black line in the center of the color bar.
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Inversion of Flow
Inversion refers to the ability to electronically invert the direction of flow as depicted on both the color flow and spectral waveforms. As such, color inversion will result in a blue-red reversal and may lead to misinterpretation of the direction of flow in the vessel being evaluated (Fig 11). Similarly, inversion of the spectral waveform will switch the depicted flow curve from above to below (or below to above) the baseline (Fig 12). Adjustment of the inversion button will alter either the color or the spectral scale, depending on which is active at the time of adjustment. Flow toward the transducer typically appears red (at color Doppler flow US) or above the baseline (positive flow) on the spectral waveform and can be changed simply by adjusting the inversion button. In evaluating the patency of TIPS, it is especially important to document the direction of flow within the contralateral limbs of the intrahepatic portal veins. Misinterpretation of flow direction can easily suggest a change in shunt function and affect patient treatment.
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Figure 11a. Inversion of color flow. (a) On a color Doppler flow US image obtained with color Doppler flow US as the active scanning mode and inversion of the color bar, portal venous flow appears blue, which falsely suggests reversal of flow (ie, away from the transducer). (b) On a color Doppler flow US image obtained with reversal of this inversion, appropriate directional flow is noted.
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Figure 11b. Inversion of color flow. (a) On a color Doppler flow US image obtained with color Doppler flow US as the active scanning mode and inversion of the color bar, portal venous flow appears blue, which falsely suggests reversal of flow (ie, away from the transducer). (b) On a color Doppler flow US image obtained with reversal of this inversion, appropriate directional flow is noted.
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Figure 12a. Inversion of spectral and color flow falsely suggesting reversal of portal venous flow. (a) On a color duplex US image obtained with spectral Doppler US as the active scanning mode, the spectral waveform is below the baseline, with appropriate color flow. (b) Color duplex US image obtained after the inversion button was reversed demonstrates appropriate directional flow, with the spectral waveform now appearing above the baseline. Note that the color bar does not change when the Doppler spectrum is inverted.
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Figure 12b. Inversion of spectral and color flow falsely suggesting reversal of portal venous flow. (a) On a color duplex US image obtained with spectral Doppler US as the active scanning mode, the spectral waveform is below the baseline, with appropriate color flow. (b) Color duplex US image obtained after the inversion button was reversed demonstrates appropriate directional flow, with the spectral waveform now appearing above the baseline. Note that the color bar does not change when the Doppler spectrum is inverted.
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Spectral-Specific Parameters
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The spectral waveform contains a host of hemodynamic information describing the velocity and character of the blood flow in the specific vessel being insonated. The waveform depicts the spectrum of frequency shifts of all blood traversing the sampled volume during the period of data acquisition (6). With use of the computer to apply an angle correction, a range of time-dependent velocities are displayed on the vertical axis (spectral display).
Angle Correction
Angle correction refers to adjustment of the Doppler angle and is used to calibrate the velocity scale for the angle between the US beam and the blood flow being measured.
Spectral Doppler US provides both qualitative and quantitative data about flow velocity. Flow velocity is calculated from the Doppler frequency shift according to the Doppler equation V = 
fC/2focos
, where V = velocity, f = Doppler frequency shift, C = speed of sound in soft tissue, fo = original transmitted frequency, and = the angle between the transducer and blood flow.
Ideally, the direction of flow should be at an approximately 45°–60° angle relative to the transducer. Within this range, a linear relation exists between velocity and the Doppler shifts. Outside this range, the unreliable signal produces inaccurate estimates of flow (Figs 13, 14). The calculated velocity is inversely related to the cosine of the angle between the transducer and flowing blood. Because the cosine of 90° is 0, the calculated velocity approaches 0 as the angle approaches 90°.
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Figure 13a. Angle correction. (a) Color duplex US image obtained with no angle correction shows how no meaningful velocity data can be obtained from the portal venous waveform because the computer automatically assigns an angle of 0° (cos 0° = 1). Without angle correction, the measured flow velocity is 18.0 cm/sec. (b) Color duplex US image obtained with correct definition of the angle between the transducer and the direction of portal venous flow demonstrates a flow velocity of 29.3 cm/sec.
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Figure 13b. Angle correction. (a) Color duplex US image obtained with no angle correction shows how no meaningful velocity data can be obtained from the portal venous waveform because the computer automatically assigns an angle of 0° (cos 0° = 1). Without angle correction, the measured flow velocity is 18.0 cm/sec. (b) Color duplex US image obtained with correct definition of the angle between the transducer and the direction of portal venous flow demonstrates a flow velocity of 29.3 cm/sec.
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Figure 14a. Angle correction. (a) Color duplex US image obtained with a 30° corrected angle, which is too low, demonstrates a flow velocity of 21.3 cm/sec in the portal vein. This figure represents an underestimation of the true flow velocity. (b) Color duplex US image obtained with a 70° corrected angle, which is too high, demonstrates a flow velocity of 52.8 cm/sec in the portal vein, which represents an overestimation of flow velocity. Note that the measured flow velocity increases as the corrected angle increases.
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Figure 14b. Angle correction. (a) Color duplex US image obtained with a 30° corrected angle, which is too low, demonstrates a flow velocity of 21.3 cm/sec in the portal vein. This figure represents an underestimation of the true flow velocity. (b) Color duplex US image obtained with a 70° corrected angle, which is too high, demonstrates a flow velocity of 52.8 cm/sec in the portal vein, which represents an overestimation of flow velocity. Note that the measured flow velocity increases as the corrected angle increases.
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The differences between angle correction and angle of insonation are important to understand. Angle correction specifies the true Doppler angle by placing the cursor parallel to the direction of blood flow (Fig 13). Manually applying this correction allows the computer to solve the Doppler equation. The angle of insonation is the angle between the transducer and the vessel being studied (Fig 15). The angle of insonation should also be between 45° and 60°. When applied to the Doppler angle, undercorrection will result in a falsely low flow estimate. This result comes directly from the Doppler equation, in which the smaller the angle between flow direction and transducer position, the greater the denominator of the equation. Flow may appear to be reversed when the beam-flow angle changes about 90°. Complete loss of flow may be evident when the beam-flow angle is 90°.
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Figure 15a. Angle of insonation. (a, b) Color duplex US images of the anterior branch of the right portal vein obtained with the transducer positioned in an intercostal (a) and subcostal (b) location depict flow as moving toward and away from the transducer, respectively. (c) On a color duplex US image obtained with the transducer positioned perpendicular to flow (arrow), no color is assigned, yielding a false finding of absent flow. The angle of insonation of the vein depends entirely on the position of the transducer.
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Figure 15b. Angle of insonation. (a, b) Color duplex US images of the anterior branch of the right portal vein obtained with the transducer positioned in an intercostal (a) and subcostal (b) location depict flow as moving toward and away from the transducer, respectively. (c) On a color duplex US image obtained with the transducer positioned perpendicular to flow (arrow), no color is assigned, yielding a false finding of absent flow. The angle of insonation of the vein depends entirely on the position of the transducer.
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Figure 15c. Angle of insonation. (a, b) Color duplex US images of the anterior branch of the right portal vein obtained with the transducer positioned in an intercostal (a) and subcostal (b) location depict flow as moving toward and away from the transducer, respectively. (c) On a color duplex US image obtained with the transducer positioned perpendicular to flow (arrow), no color is assigned, yielding a false finding of absent flow. The angle of insonation of the vein depends entirely on the position of the transducer.
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Spectral Gain
The spectral gain setting enhances the intensity of depicted flow in the spectral display. Gain should be adjusted to outline the contour of the depicted waveform (Fig 16). Too low a setting falsely suggests absent flow. Too high a setting artificially fills in the spectral waveform, resulting in falsely increased flow with little meaningful quantitative flow data. The gain settings function independently of other parameters, so that changing the percentage of gain applied to an acquired signal will not alter any other parameter. A change in the color gain does not alter the spectral gain and vice versa, with the PRF remaining unchanged.
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Figure 16a. Optimization of gain settings. (a) Duplex US image obtained with spectral Doppler US as the active scanning mode and too low a gain setting (0%) falsely suggests absent flow. (b-d) Duplex US images obtained with a gain setting of 38% (b), 77% (c), and 100% (d) demonstrate gradual artificial filling in of the spectral waveform, yielding a false finding of increased flow with little meaningful quantitative flow data. The gain settings function independently of other parameters; therefore, changing the gain setting will not alter any other parameter. Changing the color gain does not alter the spectral gain (and vice versa), and the PRF remains unchanged. Whether the color or spectral component is active, the gain setting should be adjusted to outline the contour of the depicted waveform or color flow depiction.
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Figure 16b. Optimization of gain settings. (a) Duplex US image obtained with spectral Doppler US as the active scanning mode and too low a gain setting (0%) falsely suggests absent flow. (b-d) Duplex US images obtained with a gain setting of 38% (b), 77% (c), and 100% (d) demonstrate gradual artificial filling in of the spectral waveform, yielding a false finding of increased flow with little meaningful quantitative flow data. The gain settings function independently of other parameters; therefore, changing the gain setting will not alter any other parameter. Changing the color gain does not alter the spectral gain (and vice versa), and the PRF remains unchanged. Whether the color or spectral component is active, the gain setting should be adjusted to outline the contour of the depicted waveform or color flow depiction.
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Figure 16c. Optimization of gain settings. (a) Duplex US image obtained with spectral Doppler US as the active scanning mode and too low a gain setting (0%) falsely suggests absent flow. (b-d) Duplex US images obtained with a gain setting of 38% (b), 77% (c), and 100% (d) demonstrate gradual artificial filling in of the spectral waveform, yielding a false finding of increased flow with little meaningful quantitative flow data. The gain settings function independently of other parameters; therefore, changing the gain setting will not alter any other parameter. Changing the color gain does not alter the spectral gain (and vice versa), and the PRF remains unchanged. Whether the color or spectral component is active, the gain setting should be adjusted to outline the contour of the depicted waveform or color flow depiction.
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Figure 16d. Optimization of gain settings. (a) Duplex US image obtained with spectral Doppler US as the active scanning mode and too low a gain setting (0%) falsely suggests absent flow. (b-d) Duplex US images obtained with a gain setting of 38% (b), 77% (c), and 100% (d) demonstrate gradual artificial filling in of the spectral waveform, yielding a false finding of increased flow with little meaningful quantitative flow data. The gain settings function independently of other parameters; therefore, changing the gain setting will not alter any other parameter. Changing the color gain does not alter the spectral gain (and vice versa), and the PRF remains unchanged. Whether the color or spectral component is active, the gain setting should be adjusted to outline the contour of the depicted waveform or color flow depiction.
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Gate Size
The operator-adjusted gate defines the size and location of the area from which Doppler information is obtained. The gate is delineated as a pair of cross-hairs within the 2D image and should be as small as possible to exclude erroneous signal arising from adjacent vessels or marginal flow. Too large a gate may admit erroneous signal fromadjacent vessels (Fig 17) or may lead to acquisition of data from extraneous parenchyma. Too small a gate may give the false impression of reduced or even absent flow. A smaller gate also reduces computation time and increases the frame rate, thereby allowing more accurate depiction of flow.
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Figure 17. Optimizing gate size and position. Color duplex US image obtained with a wide gate placed in a suboptimal location shows sampling of flow in both the portal (above the baseline) and hepatic (below the baseline) veins. Too large a gate size may result in sampling from too large an anatomic region. By reducing the gate size and improving the position for sampling, a normal spectral waveform is obtained.
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Gate Position
To maximize depiction of flow, the gate should be positioned over the central part of the vessel being studied. In central portions of the liver, where portal and hepatic veins course in proximity to one another, a small gate should be carefully placed on the desired vessel to avoid obtaining flow patterns from adjacent vessels (Fig 17). When helical flow occurs in the portal vein, as is often seen following liver transplantation, gate placement over the central part of the vein will demonstrate expected flow above and below the spectral baseline.
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Color-Specific Parameters
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Several operator-adjustable parameters are specific for the color Doppler component. These parameters include the color gain, color bar, color box or overlay, and color velocity scale. When color velocities are depicted, the beam-vessel angle is assumed to be 0° throughout the image. However, velocity determinations are highly dependent on the angle between the direction of flow and the beam. Colors that are depicted on the image are thus equally dependent on the angle of insonation.
Color Gain
Gain refers to amplification of the sampled information for purposes of improving the depiction of acquired data (Fig 18). The computer is unable to amplify data that are not present. For example, increasing the gain will not facilitate depiction (or identification) of slow flow in the portal vein; it will only improve the apparent intensity of flow on the monitor. If one is struggling to identify flow, the gain should not be independently increased without optimizing other parameters and ensuring that the gain is sufficiently high to depict flow. If flow is present and the gain is set too low, it is possible that no flow will be depicted on the monitor. Both color and spectral gain should be adjusted to alter sensitivity to Doppler flow. Doppler sensitivity varies according to tissue interfaces and composition. Increasing the gain amplifies the appearance of the acquired signal; however, an overly high color gain setting produces noise, obscuring the true Doppler signal. Too low a setting also results in underestimation of flow disturbances. Color gain should be set as high as possible without displaying random color speckles.
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Figure 18a. Adjustment of color gain with color flow US as the active scanning mode. (a, b) Color Doppler flow US images obtained with a gain setting of 44% (a) and 100% (b) show underadjustment and overadjustment, respectively. (c) Color Doppler flow US image obtained with an optimal gain setting of 65% demonstrates normal-appearing wall-to-wall flow in the main portal vein. Note that, although the color gain changes, no change occurs in the color velocity scale (23 cm/sec) or sampling rate (PRF = 1,500 Hz).
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Figure 18b. Adjustment of color gain with color flow US as the active scanning mode. (a, b) Color Doppler flow US images obtained with a gain setting of 44% (a) and 100% (b) show underadjustment and overadjustment, respectively. (c) Color Doppler flow US image obtained with an optimal gain setting of 65% demonstrates normal-appearing wall-to-wall flow in the main portal vein. Note that, although the color gain changes, no change occurs in the color velocity scale (23 cm/sec) or sampling rate (PRF = 1,500 Hz).
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Figure 18c. Adjustment of color gain with color flow US as the active scanning mode. (a, b) Color Doppler flow US images obtained with a gain setting of 44% (a) and 100% (b) show underadjustment and overadjustment, respectively. (c) Color Doppler flow US image obtained with an optimal gain setting of 65% demonstrates normal-appearing wall-to-wall flow in the main portal vein. Note that, although the color gain changes, no change occurs in the color velocity scale (23 cm/sec) or sampling rate (PRF = 1,500 Hz).
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Color Bar
The color bar displayed on the monitor shows the color assignment for mean Doppler shift frequencies (velocities) at the selected gate location. The scale or range of operator-selected velocities is thereby displayed. By convention, positive Doppler shifts (ie, those toward the transducer) appear red. Many manufacturers place a horizontal black line across the color bar; this line represents the baseline, and its position will shift as the baseline is changed. The width of this horizontal black line depicts the wall filter setting. Any increase or decrease in the color range will be depicted on the color bar, both as a change in the color spectrum and as the actual velocity range that is shown (usually in centimeters per second). It is important to remember that the velocity shifts for a given PRF.
Color Box or Overlay
The color box is an operator-adjustable area within the US image (displayed on the monitor) in which all color Doppler information is displayed. The size, shape, and location of the box are adjustable and define the volume of tissue from which color data are acquired. All velocity information from this user-defined volume of tissue is presented as color-encoded Doppler shifts in the image field. Because the frame rate decreases as box size increases, image resolution and quality are affected by box size and width. Increasing the size or width will reduce the frame rate and increase the required processing power and time. Thus, the overlay should be as small and superficial as possible while still providing the necessary information, thereby maximizing the sampling or frame rate. Too big an overlay reduces the frame rate and thus results in inferior depiction of flow (Fig 19). A deep color box will result in a slower PRF, which may produce aliasing of the depicted color flow.
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Figure 19a. Color box or overlay. (a) Color Doppler flow US image obtained with an oversized color box results in an increased frame rate and the inclusion of extraneous data. (b) Color Doppler flow US image obtained with the box size reduced demonstrates a decreased frame rate and improved image quality.
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Figure 19b. Color box or overlay. (a) Color Doppler flow US image obtained with an oversized color box results in an increased frame rate and the inclusion of extraneous data. (b) Color Doppler flow US image obtained with the box size reduced demonstrates a decreased frame rate and improved image quality.
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The frame rate is the rate (per second) at which complete images are produced. It is influenced by the number of scan lines and the width and depth of the region being imaged. The frame rate is an important specification of a scanner because it affects the temporal detail that can be resolved. With pulse-echo imaging alone, the frame rate can exceed 50 images per second. However, the time required to produce color flow images is much longer, which significantly lowers the frame rate. The frame rate in color imaging is dependent on several factors. For example, the size and position of the color box have a great effect on the frame rate. The width of the box is especially important: The wider the box, the more scan lines are required and the longer it will take to acquire the data to produce the image. If box width is kept constant, adjusting the depth will not make any noticeable difference in the image depicted on the monitor. Increasing the imaging depth also decreases the frame rate because it results in a longer wait time or delay for returning echoes. The PRF also influences the frame rate: Increasing (or decreasing) the PRF increases (or decreases) the frame rate. A compromise must be reached between the area over which color information is acquired and the time needed to acquire it.
Color Velocity Scale
The color velocity scale can be changed separately from the scale depicted on the spectral baseline and should be adjusted for the anticipated range of velocities to be studied. The color velocity range depicts the range of velocities that are represented in the color overlay. Too low a range will result in color aliasing, which may complicate interpretation of flow direction.
Color Priority
Although color priority is unlikely to affect a liver Doppler US examination, adjusting this parameter changes the emphasis of displayed color information. Because each pixel is displayed either as gray-scale or color, increasing the color priority will permit color information to be displayed where low-intensity signals may be present, such as at the periphery of vessels. Alternatively, increasing the gray-scale priority will result in gray-scale information being depicted and displacing color data. Depending on the manufacturer, many US imagers permit adjustment of the color priority on a scale that is often depicted adjacent to the color bar.
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Clinical and Tissue-Specific Presets
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Commercially available transducers possess a variety of preset scanning parameters. The system is optimized for imaging by selecting a tissue-specific preset. Once a transducer head is selected, preset choices include a clinical option (eg, general, adult, obstetric) and a tissue-specific preset (eg, abdomen, renal, transplant). Presets are also specific to the transducer that is selected. To optimize for slow flow in the hepatic vasculature, most US imagers permit the operator to define a specific set of presets. For example, to optimize for slow portal venous flow, a transducer can be preset to have a low color and spectral velocity range, sampling rate, and filter and a small color overlay.
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Hepatic Vascular Anatomy
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The arterial and venous anatomy of the liver is complex and variable. Indeed, exceptions are the rule where the hepatic arterial anatomy is concerned. For optimal evaluation of flow velocity and direction, it is essential that the relationship between transducer position and individual vessels be known.
Hepatic Artery
Identification and characterization of hepatic arterial flow is essential after liver transplantation. In healthy individuals, hepatic arterial flow velocity varies from 30 to 60 cm/sec. Note should be made of accessory or replaced left or right hepatic arteries, which arise from the left gastric and superior mesenteric arteries, respectively. Hepatic arterial flow typically has a pulsatile low-resistance waveform with a broad systolic peak, antegrade diastolic flow, and spectral broadening (Fig 20). The artery can be identified adjacent to the main portal vein with color flow US, which is used for localization of the artery; flow within the artery is characterized with spectral Doppler US.
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Figure 20. Hepatic arterial waveform. Color duplex US image obtained with a small gate placed over the hepatic artery adjacent to the main portal vein shows a normal spectral waveform and a low-resistance profile with systolic velocities ranging from 30 to 40 cm/sec.
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Hepatic artery occlusion may occur following cadaveric or living donor liver transplantation. This complication can be devastating because the biliary endothelium is supplied by the hepatic artery. In addition, hepatic artery stenosis may occur at or near the anastomotic site. Depending on the extent of stenosis, spectral waveforms obtained at or distal to the stenosis may demonstrate elevated peak systolic velocity, spectral broadening, or a tardus parvus waveform (Fig 21). Indications for stenosis of the hepatic artery that exceeds 50% include focal velocity increases (>200 cm/sec) associated with turbulence, a low resistive index (<0.5), and systolic acceleration times greater than 0.8 seconds (Fig 21).
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Figure 21. Tardus parvus hepatic arterial waveform. Color duplex US image obtained in a patient who had undergone liver transplantation 24 hours earlier shows slow upslope, broadening of the spectral waveform, and low-peak-velocity flow. This waveform is commonly seen in liver transplant recipients and resolves by 24-48 hours after surgery. When this finding is seen more than 48 hours after the procedure, hepatic artery stenosis or even dissection should be excluded.
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Portal Vein
The portal vein confluence is best identified with a midline transverse or oblique approach. Intercostal scanning is frequently required to assess flow in the intrahepatic branches of the right portal vein. Flow in the portal vein is typically hepatopetal, mildly undulating, and laminar, displaying respiratory variation. Normal portal venous flow velocity is 20–30 cm/sec (Fig 22). Pulsatility is typically absent, unless there is right-sided heart failure with or without pericarditis, but the portal and hepatic veins can both demonstrate periodicity (ie, normal variations in velocity resulting from cardiac motion). Helical portal venous flow is seen in 20% of patients with severe liver disease or shunting and is an expected observation following liver transplantation (Fig 23) (7). Intrahepatic helical flow may be seen in patients with TIPS or with liver tumors that abut the portal vein. Color Doppler flow US will show flow both toward and away from the transducer because normal laminar flow is replaced by flow with a spiral appearance. The spectral waveform may demonstrate hepatopetal, fugal, or bidirectional flow (Fig 23). In patients with well-functioning TIPS, reversal of flow (ie, toward the stent) typically occurs in the contralateral intrahepatic portal veins (Fig 24).
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Figures 22. Helical portal venous flow. On a color duplex US image of the main portal vein, the spectral waveform shows phasicity secondary to patient respiration. The color Doppler component shows flow as both blue (away from the transducer) and red (toward the transducer), a finding that is consistent with helical flow.
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Figures 23. Helical portal venous flow in a liver transplant recipient. On a color duplex US image, helical flow in the main portal vein appears both red and blue and is depicted as occurring both above and below the baseline. If a gate is too small and is placed on a single component of portal venous flow, the flow may inadvertently appear reversed.
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Figure 24. Reversal of left portal venous flow in a patient with a TIPS. Color Doppler flow US image shows flow toward the transducer (red) in the left hepatic artery (HA) and reversed flow (blue) in the left portal vein (LPV). These findings are expected when a functioning TIPS bridges the right portal and hepatic veins.
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When thrombus is identified in the portal vein, identification of pulsatile intrathrombus flow (typically low-resistance pulsatile or nonpulsatile) is used to distinguish bland thrombus from tumor thrombus. The waveform morphology differs from that of the hepatic artery and that of the nonthrombosed portions of the portal vein. Often, however, flow cannot be detected in tumor thrombus. Thus, absence of flow should not be used to distinguish tumor thrombus from bland thrombus. Given the importance of this observation and its potential impact on patient care, Doppler parameters must be fully optimized when characterizing flow within a thrombus.
Hepatic Vein
The hepatic veins possess a triphasic waveform, with both respiratory variation and cardiac pulsatility (Fig 25). The waveform is altered in the presence of hepatic or cardiac disease. Hepatic veins are imaged with a transverse midline or intercostal approach. Because liver disease may obscure or dampen flow patterns, hepatic veins should initially be sampled near their confluence with the vena cava. Doppler US may show abnormalities in the hepatic veins, such as absent flow or loss of the normal triphasic waveform in the portal vein (where flow may be reversed) and vena cava. Absent hepatic venous flow may be partial or complete, depending on the extent of intrahepatic collateral vessels. Reversed flow in adjacent hepatic vein branches may produce a "hockey stick" appearance (8). Collateral vessels may be capsular or communicate with the vena cava, or enhanced drainage via accessory hepatic veins may occur.
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Figure 25. Normal hepatic venous waveform. On a color duplex US image, the spectral waveform for a normal hepatic vein shows triphasic flow above and below the baseline. The waveform shows periodicity and is triphasic due to transmitted cardiac activity, similar to the waveform for the jugular vein. The component above the baseline corresponds to atrial systole; the components below the baseline correspond to ventricular systole and the filling phase during atrial diastole.
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Optimization for Detection of Slow Flow in the Liver
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A common challenge in evaluating the liver in patients with portal hypertension or liver tumors or who have undergone surgery is to document and characterize flow in the portal vein. This flow may not only be hepatofugal (reversed), but may be stagnant or slow, or the vein may be occluded. When thrombus is present in the portal vein, it is important to search for flow within the thrombus, because such a finding may be used to distinguish tumor thrombus from bland thrombus (1). When thrombus is identified, careful search should be made for tumor within the liver.
Most commonly, however, flow is slow. To accurately identify slow flow and not mistakenly consider flow to be absent, the Doppler parameters must be optimized for detection of slow flow. First, one must be sure that the B-mode parameters are optimized, including depth of field, location and number of focal zones, output power, and time-compensated gain.
Spectral or power Doppler US is more sensitive in the detection of slow flow than is standard color Doppler flow US. The wall filter setting should be as low as possible so that low-frequency signals arising from the portal vein are not eliminated. The Doppler angle should be optimized, not only by correcting the angle with the computer, but by positioning the patient so as to allow the transducer to be placed in an appropriate superficial location. This placement is often achieved by putting the patient on the left side in the decubitus position and the transducer in a high intercostal location in the region of the right midaxillary line. The strength of the color flow image decreases with the Doppler angle; if the angle is too small, the transducer should be moved to a more suitable position. In addition, the color box should be as small, narrow, and superficial as possible, whereas the gate used for pulsed Doppler sampling should be wide. A larger or wider box (ie, greater depth) requires a longer round-trip time for pulses, reducing the PRF, increasing the signal processing time, degrading the image, and diminishing the ability to depict slow portal venous flow. The velocity range should be adjusted to a level appropriate for flow within the portal vein.
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Conclusions
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Knowledge of the various operator-dependent Doppler controls will permit optimization of the study and improve the overall utility of liver Doppler US. This optimization is especially important given the alterations in hepatic flow that occur in chronic liver diseases (including portal hypertension) and the importance of evaluating flow following transplantation, TIPS insertion, or portosystemic shunt creation.
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Footnotes
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Abbreviations: PRF = pulse repetition frequency,
TIPS = transjugular intrahepatic portosystemic shunt,
2D = two-dimensional
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References
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- Grant EG, Schiller VL, Millener P, et al. Color Doppler imaging of the hepatic vasculature. AJR Am J Roentgenol 1992; 159:943-950.[Abstract/Free Full Text]
- Pozniak MA. Doppler ultrasound of the liver. In: Allan PL, Dubbins PA, Pozniak MA, McDicken WN, eds. Clinical Doppler ultrasound. London, England: Churchill Livingstone, 2000; 123-168.
- Taylor KJ, Holland S. Doppler US. I. Basic principles, instrumentation, and pitfalls. Radiology 1990; 174:297-307.
- Merritt CR. Doppler US: the basics. RadioGraphics 1991; 11:109-119.[Abstract]
- Rubin JM. Spectral Doppler US. RadioGraphics 1994; 14:139-150.[Abstract]
- Rosenthal SJ, Harrison LA, Baxter KG, Wetzel LH, Cox GG, Batnitzky S. Doppler US of helical flow in the portal vein. RadioGraphics 1995; 15:1103-1111.[Abstract]
- Kane R, Eustace S. Diagnosis of Budd-Chiari syndrome: comparison between sonography and MR angiography. Radiology 1995; 195:117-121.[Abstract/Free Full Text]
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R. H. Wachsberg
B-Flow Imaging of the Hepatic Vasculature: Correlation with Color Doppler Sonography
Am. J. Roentgenol.,
June 1, 2007;
188(6):
W522 - W533.
[Abstract]
[Full Text]
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Abstract
Introduction
