Doppler Ultrasonography

The Doppler effect, named after the Austrian mathematician and physicist Christian A. Doppler (1803-1853) who first hypothesized it, simply states that for a stationary observer, the apparent frequency of a wave emitted from a moving source changes proportional to the relative velocity of the source with respect to the observer (Fig. 78-3 is a conceptual drawing of wave fronts generated by a moving source). In simple terms, relative velocity is the rate of change of the distance between the source and the observer. In that sense, it could be the observer that is moving. The Doppler frequency shift solely depends on the relative velocity regardless of who or what is moving.

FIGURE 78-3 A moving source generates a shorter wavelength (corresponding to higher frequency) ahead of itself and a longer wavelength (corresponding to lower frequency) behind itself compared with the wavelength it would generate while stationary.

Although Doppler developed his hypothesis for light waves, he also noted that the same hypothesis applied to sound waves. The Doppler effect is observed most easily with sound waves because, in the audible range, our ears can hear and detect changes in frequency (pitch). Imagine (or rather remember) a vehicle passing by while honking, or an ambulance, fire truck, or police car with sirens blaring. The pitch of the siren is noted to change while passing by: the siren starts with a high pitch; when getting close, the pitch starts to drop; the siren attains the actual pitch right when passing by and continues to drop thereafter. Figure 78-4 shows a simulated example, in which the solid and dashed curves show the frequency spectrum of the transmitted and received signals.

FIGURE 78-4 Doppler frequency shift observed for a target moving toward the observer: The solid curve represents the frequency spectrum of the transmitted signal, whereas the dashed curve represents the frequency spectrum of the received signal. The difference between these two spectra is a result of the Doppler shift.

In ultrasonography, the Doppler effect is applied to identify tissue motion, blood flow, and vessel structures. The simplest application of Doppler effect in ultrasonography is fetal heart rate monitoring. These devices detect the motion of the fetal heart and convert it into audible sound. More sophisticated Doppler instruments that visualize blood flow and vessel structures are now integrated into most modern ultrasound imaging systems.¹

In blood flow imaging, the Doppler signal is generated by blood cells that back-scatter the transmitted ultrasound wave. The back-scattering coefficient of the cells is a quadratic function of their relative size with respect to the wavelength.² The blood is mostly composed of red and white blood cells and water. In B-mode imaging, blood is almost anechoic because of the very small back-scattering coefficient of blood cells. The echoes coming from the surrounding tissue dominate the image. In Doppler imaging, in which the surrounding tissue is stationary, the moving blood cells generate the Doppler signal. Although white blood cells are larger, red blood cells are more numerous in the blood and generate most of the Doppler flow signal.

The Doppler frequency shift f_D is formulated as:

where f₀ is the frequency of the transmitted ultrasound wave, v_s,r is the relative velocity of the moving target with respect to the transducer, and c is the speed of sound in tissue (on average 1540 m/s); θ is the angle of the direction of the moving target with respect to the transducer. This angle is zero when the target is moving head on toward the transducer, and it is 90 degrees when the target is moving parallel to the transducer surface. The factor 2 comes from the fact that the targets (e.g., blood cells in this case) are reflectors, and the transmitted signal is twice subjected to Doppler shift.

The Doppler signal is proportional to this frequency shift f_D. The previous formula captures some of the attributes of Doppler imaging. The Doppler signal is proportional to the transmitted ultrasound frequency, so higher frequencies are preferred. Another reason for preferring higher frequencies is the back-scattering coefficient of blood cells. As mentioned previously, the back-scattering coefficient of cells is a quadratic function of the size of the cells in relation to the wavelength. Because wavelength is inversely proportional to the frequency, using higher ultrasound frequencies (shorter wavelength) increases the back-scattering coefficient significantly. Attenuation in tissue, which is an exponential function of frequency, ultimately limits the frequency and penetration depth, however. Stated in another way, for a target depth, there is always an optimal Doppler frequency dictated by these three factors. The cosine term accounts for the angular dependence of the Doppler signal. It is strongest when the blood flow is parallel to the ultrasound beam, and is zero when the blood flow is at right angles to the ultrasound beam. The Doppler signal is also proportional to the velocity of the blood cells and the number of blood cells in the sample volume interrogated by the ultrasound beam. Because the number of blood cells per unit blood volume is more or less constant, it is the spot volume of the ultrasound beam that determines the strength of the Doppler signal.³

Although we have been talking about Doppler frequency shift, in modern ultrasound systems the phase shift, not the frequency shift, is measured. Making true frequency measurements requires the system to use continuous wave ultrasound signals, which means complete loss of axial resolution for an ultrasound image. Because of this, ultrasound systems now use pulsed wave ultrasound techniques, which measure the phase shift in the received signal using a mathematical process called autocorrelation. This technique is achieved by interrogating the same sample volume multiple times (at least 3, typically 10 to 20 times), and correlating the echoes.¹ Because measuring the phase shift is essentially equivalent to measuring the Doppler frequency shift within certain limitations, all the arguments we made before about Doppler frequency shift remain the same.

In B-mode imaging, the ultrasound pulse length is typically kept at minimum for best axial resolution. In Doppler imaging, shortest possible pulse length is insufficient to extract phase shift information, however, and several cycle pulses are used. As mentioned before, the Doppler signal strength is proportional to the ultrasound beam volume, which is proportional to the pulse length—more blood cells generate larger signal. In that sense, it is better to use long pulses in Doppler imaging, although at the expense of axial resolution. Two types of Doppler ultrasound equipment are clinically available: continuous wave and pulsed Doppler devices.

Continuous Wave Doppler Imaging

The continuous wave Doppler device, which continuously transmits and receives signals, requires two separate sections mounted in the ultrasound probe; one is the transmitting transducer, and the other is the receiving transducer. This is a simple process and it works well when only the magnitude of the Doppler shift frequency is required.⁷ It does not provide directional information about the flow, however, and does not have any axial resolution. The Doppler signal comes from a large sample volume, which is essentially the intersection of the transmit and receive beams. Vascular structures at different depths are sampled simultaneously.² The major advantage of this system is that aliasing does not occur, and continuous wave Doppler is more sensitive to slow flow than pulsed Doppler.

B-Flow Imaging

In Doppler ultrasonography, long pulses are used to be able to visualize weak flow signals, which compromise axial resolution. Besides, generating an image line typically takes 10 to 20 firings along the same line, which severely limits the frame rate. Finally, Doppler images are overlaid on B-mode images, which sometimes results in the obstruction of vessel walls and important diagnostic information.

B-flow imaging uses coded excitation to enhance the flow signal, and it equalizes the tissue signal to display tissue and flow signals simultaneously.⁶ There is no loss of information because of overlaying of images. In addition, coded excitation allows the use of long pulses without degrading the axial resolution, and these pulses are done close to the frame rate of the usual B-mode image.⁸ In all, B-flow imaging provides a precise depiction of the blood flow, vessel structures, and surrounding tissue simultaneously at the resolution and frame rate of B-mode images. Because of the ability of B-flow to scan at high frame rates, it is also used to visualize the interaction of blood flow with anatomic structures inside the vessel.⁶

With all the above-mentioned advantages, B-flow imaging allows the depiction of plaque characteristics even better than color Doppler and B-mode imaging, and is useful in carotid imaging. It can also be used to show small venous thrombi as a filling defect of the vessel lumen. It is useful in evaluating complex flow states, such as with bypass grafts, arteriovenous fistulas, pseudoaneurysm, and dialysis fistulas, in which color Doppler artifacts may obscure flow information.⁹

Major disadvantages are that B-flow imaging does not provide velocity information and gives only visual and qualitative information about flow. Ultrasound attenuation is another limitation of the system, which means depiction of flow characteristics in deep vessels such as of the abdomen is not as good as the depiction in superficial vessels, so it is used mainly in superficial vascular imaging.

Contrast-Enhanced Harmonic Imaging

Ultrasound propagation in tissue is nonlinear by nature. Nonlinear wave propagation in tissue generates harmonics of the transmitted signal. If a 5-MHz signal is transmitted, part of the signal is transferred to 10 MHz. The received signal is filtered around 10 MHz to generate the harmonic image. Normally, high-frequency signals are attenuated severely, and this happens in the transmit and receive directions. By harmonic imaging, one can transmit at lower frequency (5 MHz) and receive at the harmonic frequency (10 MHz), and save one-way attenuation.

Aside from having a high back-scattering coefficient, ultrasound contrast agents are also known to have a highly nonlinear response.^10,11 That is, when excited by an ultrasound beam, the back-scattered echo from a contrast agent contains harmonics of the transmitted signal. Because contrast agents are introduced into the bloodstream and perfuse into the vessel structure completely, contrast-enhanced harmonic imaging is a powerful tool for vascular ultrasonography, and it generated great interest in early tumor angiogenesis detection. One of the pitfalls of harmonic imaging is the harmonic signal generated by the tissue itself. Because contrast agents have a unique nonlinear response, using specific complex pulse sequences that suppress tissue harmonics and enhance contrast agent harmonics mitigates this pitfall.¹²

Choice of Instrumentation

Color Doppler imaging permits rapid assessment of flow over a large region, and highlights gross circulation abnormalities, such as stenosis, aneurysm, and turbulent flow. Analysis of these specific sites by spectral Doppler provides detailed information about flow characteristics and enables quantification of flow. The two modes are complementary to each other, and combined use of them increases the diagnostic information about blood flow.

Choice of Probe and Frequency

The choice of the transducer depends on the clinical application and on the body habitus of the patient. Frequently, more than one transducer is used during an examination. Many factors influence the choice of frequency. The frequency determines the axial resolution and depth of the image. As the insonating frequency increases, attenuation of the ultrasound wave by the tissue increases, and penetration depth decreases. Detecting deep flow requires the use of lower frequencies. Also, low frequencies are less susceptible to aliasing and allow examination of high-velocity flow without aliasing. Sensitivity to flow increases with high frequencies because Doppler shift increases as the insonating frequency increases. Axial resolution is higher with higher frequencies.^2,7 There is a compromise between axial resolution, sensitivity to flow, and depth. The highest frequency that would achieve the best resolution and highest Doppler shift with sufficient penetration should be chosen.

On the basis of this principle, examination of superficially situated vascular structures as in peripheral vasculature is performed by steered linear array transducers with frequencies of 5 MHz or greater. Curvilinear or convex probes in which insonating frequency ranges from 2.5 to 5 MHz are well suited for abdominal scanning.^7,15 Trapezoid field of view convex array is a common configuration for transvaginal and transrectal applications.⁷

Sample Volume

The effective sample volume size in color Doppler is influenced by the size of the color box. The color box or color region of interest defines the volume of tissue in which color processing occurs. The shape, size, and location of the box are adjustable, and image resolution and quality are affected by the box size. In principle, the size of the color box should be kept as small as possible, and the location of it should be as superficial as possible, while still providing the necessary information. The box size is kept small: as the width and depth of the color box increase, more color processing is needed, which reduces the frame rate. A small-sized color box also allows a higher scan line density, which provides better spatial resolution. The color box should be located as superficial as possible because a deeper box demands a low PRF and is more susceptible to aliasing.

In spectral Doppler, sample volume, which is also referred to as the gate size, defines the size and location of the area from which the Doppler information is obtained. Although larger sample volume results in higher signal-to-noise ratio, it may include erroneous signal arising from the adjacent vessels and from the movement of the vessel wall (Fig. 78-5).^7,16 The gate size or sample volume in spectral Doppler should be kept as small as possible. A smaller sample volume also increases the frame rate and the spatial resolution. If it is kept too small, it may give the false impression of reduced or even absent flow.¹⁶

FIGURE 78-5 A, Gate or sample volume (open arrow) includes signals from the artery and the vein. Arterial flow is displayed above the spectrum (arrow); venous flow is displayed below the spectrum. B, Sample volume or gate (open arrow) size is reduced, and signal only from the artery is included, which is displayed clearly on the spectrum above the baseline.

Choice of Pulse Repetition Frequency/Velocity Scale

The most important factor that has an influence on PRF is the velocity of flow. As a rule of thumb, a low PRF value should be used to look at low velocities, and a high PRF value should be used to look at high velocities. Low PRF in the presence of a high-velocity flow produces aliasing (Figs. 78-6 and 78-7). High PRF reduces the sensitivity to flow, and an examination with high PRF may miss the signals from low-velocity flow (see Figs. 78-6 and 78-7).¹⁴ When searching for signals, examination should be started with the lowest PRF. When the flow signal is detected, PRF can be increased.⁷ Depth of the image is another factor affecting PRF. As the depth increases, it takes a longer time for the sound waves to traverse tissue, which increases the time interval between pulsing and sampling. Deeply situated flow should be examined with low PRF values. Using lower PRF values decreases the frame rate.

FIGURE 78-6

Effect of PRF/velocity scale setting on spectral Doppler. A, Aliasing occurs because of low PRF/velocity scale setting (30 cm/s [arrow]). The systolic peak terminates abruptly and wraps around to project below the baseline. B, A clear spectrum without aliasing is obtained by increasing PRF/velocity scale to 40 cm/s (arrow). C, PRF/velocity scale is increased further to 100 cm/s (arrow), and the spectrum is compressed along the baseline.

FIGURE 78-7 Effect of PRF/velocity scale setting on color Doppler. A, Aliasing in color Doppler is shown as a mixture of red and blue color mimicking the appearance of flow reversal. PRF/velocity scale is 7 cm/s (arrow). B, With an appropriately set PRF/velocity scale, aliasing disappears, and flow direction is correctly displayed. C, PRF/velocity scale is increased further to 77 cm/s (arrow). The lumen filling with color is incomplete.

Angle of Insonation and Angle of Correction

The angle of insonation is the angle between the transducer and the vessel being examined. Remember the formulation of Doppler frequency shift f_D, which is:

As easily understood from this equation, Doppler frequency shift is directly proportional to the cosine of the angle θ. As the transducer gets more aligned with the vessel, which means as θ gets smaller, the Doppler shift frequency increases. When the transducer is aligned with a vessel, the largest Doppler shift is obtained, but at such small angles technical difficulties occur in obtaining signal. Theoretically, Doppler shift frequency is zero, and no signal can be detected as the angle reaches 90 degrees. At angles approaching 90 degrees, directional knowledge of flow is lost, and flow appears equally above and below the zero line forming the mirror image artifact (Fig. 78-8). The ideal angle of insonation is between 30 degrees and 60 degrees.²

FIGURE 78-8 Angle of insonation is 90 degrees, and the spectrum shows a mirror pattern in which flow appears equally above and below the baseline.

Angle correction refers to adjustment of Doppler angle in spectral Doppler. It is used to calculate the velocity of flow as Doppler devices are equipped to calculate flow velocity from the Doppler frequency shift if the angle between the sound beam and the flow direction is measured and indicated to the machine by the operator. For accurate flow measurement, Doppler angle should not be greater than 60 degrees. At angles exceeding 60 degrees, small errors in angle measurement cause large errors in angle-corrected velocity computations.^2,7

Power and Gain

Acoustic power generated by the Doppler device and transmitted to the tissue is an important factor affecting the sensitivity to flow. Color Doppler and spectral Doppler use higher average power than B-mode ultrasonography. Most devices allow the operator to control the power output according to the specific application. It must be set as low as possible and should be increased only if there is no other way to eliminate noise and detect a Doppler signal.⁷ Power output control is crucial, especially in obstetric Doppler applications.

Gain in spectral and color Doppler determines the overall sensitivity to flow. In color Doppler examinations with appropriate gain settings, color should occupy the full anteroposterior diameter of the vessel (Fig. 78-9A). The spectrum should be clearly visible and free of noise with optimal spectral gain; this can be achieved by first increasing receiver gain until noise is displayed, then after a signal is obtained, gain is reduced until noise disappears. Gain settings that are too high cause loss of directional information and produce mirror image artifact in spectral Doppler, whereas aliasing and color noise in the surrounding tissues occur in color Doppler (see Fig. 78-9B). If gain is set too low, slow flow cannot be detected (see Fig. 78-9C).¹⁵

FIGURE 78-9 A, Color Doppler image obtained with appropriately set color gain in which color occupies the full anteroposterior diameter of the vessel. B, Color noise occurs in the surrounding soft tissues because of high color gain. C, Color gain is so low that most of the flow knowledge is lost, and luminal filling with color is incomplete.

Appropriately set B-mode gain is also important in color Doppler imaging. If it is set too high, it may suppress color within the vessel. When examining slow flow, decreasing B-mode gain helps to display the vessel in color flow imaging.

High-Pass Filter

Doppler frequency shift is generated also by soft tissue motion, such as the movement of vessel wall and cardiac structures or movement owing to respiration. Such signals have higher amplitude than signals generated by blood flow and may overwhelm the signal of flow.^2,17,18 Doppler equipment compensates for this effect by employing filters that cut out the high-amplitude, low-frequency Doppler signals generated by tissue movement. Because the dominant contribution to this signal is from the vessel wall movement, the filter is referred to as a wall filter. It is actually a high-pass filter. Filtering reduces the background noise and provides a clearer signal.¹⁹ Depending on the flow characteristics and desired clinical application, the operator can vary the cutoff frequency of the wall filter. A high-pass filter limits the minimum velocities that can be measured by the system, and it removes not only high-amplitude, low-frequency soft tissue components, but also slow flow components with frequency shifts below the cutoff frequency of the filter. If the filtration is set too high, diagnostic velocity information can be lost.²⁰

Baseline Shift

Color baseline appears as a horizontal black line on the color bar. Spectral baseline is the time axis of the spectral display. If aliasing occurs, and PRF cannot be increased because of the depth of the investigation, lowering the color or spectral baseline reduces aliasing and optimizes the display of flow.

Color and Spectrum Invert

Flow toward the transducer typically appears red at color Doppler and above the baseline (positive flow) on the spectrum. The Doppler system allows the operator to change the hue assigned to the direction of flow and to invert the spectral display simply by adjusting an inversion button. By this function, it is possible always to display arteries in red and veins in blue in color Doppler, and arterial flow above the baseline in spectral Doppler.

Dwell Time

Dwell time, which is also known as ensemble length or packet size, is a parameter of color Doppler. In color Doppler imaging as mentioned earlier, several pulses are necessary to obtain each line of Doppler information. Number of pulses used for each line is referred to as packet size and determines the duration of Doppler sampling for each line. As it increases, sensitivity to flow and color spatial resolution increases. However, the frame rate becomes slower, which may limit the ability of color Doppler to depict rapidly changing hemodynamics.

Focal Zone

In most duplex systems, the focal zone is automatically placed at the depth of the sample volume. In color Doppler systems, the operator determines the location of the focus. Spatial resolution is best at the focal zone, and flow is better detected here.⁷

Aliasing

Aliasing is a fundamental limitation of pulsed wave Doppler systems. There is no such limitation in continuous wave Doppler. As mentioned earlier, the system transmits a pulse to the target and waits for the received echo to obtain a Doppler signal. The sampling frequency of the Doppler signal is referred to as PRF. The sampling frequency should provide sufficient time for the system to collect all signals from one pulse before the next pulse is generated. PRF limits the maximum Doppler shift that can be measured by the pulsed wave Doppler system. When the frequency of Doppler signal exceeds one half of the PRF (Nyquist sampling rate), this results in the ambiguity of the Doppler signal that is termed aliasing.^9,20 In spectral tracing, this ambiguous signal is “wrapped around” and depicted as if flow is in the reversed direction (see Fig. 78-6A). Color aliasing projects the color of reversed flow in the central areas of higher laminar velocity (see Fig. 78-7A).²⁰ Awareness of some technical and physical issues (Table 78-2) helps to build strategies that avoid aliasing (Table 78-3).

TABLE 78-2 Doppler Signal

TABLE 78-3 Aliasing

Wall Filter Setting

Diagnostically significant flow information, especially about low-velocity flow, may be lost if wall filter is set too high.²⁰ When the area of investigation has a low-velocity flow pattern, no filtering or little filtering should be used. There are other system parameters such as PRF and frame rate that have an effect on filtering. In some systems, wall filter automatically increases as the PRF is increased.^7,14 The processing time and Doppler sensitivity decrease as the frame rate increases; if high frame rates are used, wall filter should be reduced.

Gain Setting

When color or spectral gain is too low, it gives a false impression of absence of flow. Too high B-mode gain may suppress color in the vessel, especially when the flow velocity is low. While examining vessels with slow flow velocities, color and spectral gain should be high enough so that signal free of noise is detected, and B-mode gain should be as low as possible.

Directional Ambiguity

Directional ambiguity can result when the insonating angle is 90 degrees. It may display absence of flow in color Doppler. With linear transducers, this is a more significant problem, but beam steering can improve the angle and alleviate this problem. With sector transducers, the same artery is coded in different colors in different segments of the vessel, and a small colorless region exists at the point where the flow is perpendicular to the beam (Fig. 78-10).^2,20

FIGURE 78-10 Color Doppler image of abdominal aorta obtained by a convex probe. The vessel is coded by different colors in different segments. Doppler shift is absent at the small colorless region (arrow) where flow is perpendicular to the angle of insonation.

Grating or Side-Lobe Artifact

Electronically focused transducers concentrate the insonating beam toward the sample volume. Because of design of the transducers, weak secondary lobes of focused sound may insonate areas unrelated to the primary beam, and if reflected echoes from these regions are strong enough, they are displayed in the image. Convex probes are more susceptible to this artifact than linear probes. These off-axis lobes can insonate vessels unrelated to Doppler sample volume, and if Doppler signal is strong enough, it can be displayed in the spectrum in regions where no Doppler signal is expected.²⁰

Spectral Broadening Artifact

Angle of insonation and velocity of flow are the two factors that may cause spectral broadening. It is mainly observed in stenotic arterial segments, but may also occur in a normal arterial segment owing to the insonating angle approaching 90 degrees.

Mirror Image Artifact

Mirror image artifact, which is commonly observed in B-mode imaging, can also occur in color Doppler and spectral Doppler imaging. In color Doppler imaging, it occurs in a vessel that is adjacent to a highly reflective surface, such as lung.²¹ The subdiaphragmatic region of the liver and supraclavicular region are also notorious for this artifact.²⁰

In spectral Doppler, mirror image artifact is displayed as a similar time-velocity spectrum appearing below and above the zero line (see Fig. 78-8). It occurs at large angles of insonation especially when large receiver gain is used to detect weak Doppler signals.²

Flow Direction Artifact

In color Doppler imaging, flow toward and away from the transducer is encoded in different hues, so the veins and arteries are coded in different colors. The operator assigns the hue to the direction of flow. In some anatomic regions where vessels diverge, the color is not an absolute indicator of the vessel type (artery or vein) (Fig. 78-11). In such regions, the device codes arteries in red and blue. Use of spectral Doppler helps alleviate this problem.

FIGURE 78-11 Color Doppler image of the renal artery. The vessel bifurcates, and branches of the artery diverge and are coded in different colors.

Color in Nonvascular Structures and Color Modulation of Biologic Movement

In color flow devices, any motion of a reflector relative to the transducer produces a Doppler shift and is modulated into color by the device. An artifactual impression of flow is produced and is referred to as the color flash artifact.^2,20,22 To compensate for this problem, most color flow devices are equipped with motion discriminators that separate true flow from random motion of soft tissues.

Low level signals arising from hypoechoic areas such as cysts and collections do not trigger the motion discriminators, and color flash artifacts occur in such regions.²⁰ Artificial color signals may be observed in dilated bile ducts and gallbladder, especially if the color sensitivity settings are high.²³ Transient color flash artifact may also occur because of patient motion (e.g., respiration, cardiac pulsations, and bowel movement) or transducer motion.

Pseudoflow artifact, which is closely related to color flash artifact, occurs secondary to fluid motion. This artifact may be caused by motion of ascites or urine. Spectral analysis of these regions helps to differentiate color flash or pseudoflow artifact from real blood flow because it reveals a spectrum that is very atypical for a vessel.²²

Arterial Stenosis

Aneurysm and Pseudoaneurysm

Velocities decrease in the aneurysmatic region because of an increase in the vessel diameter. The spectrum is dampened, and color encoding is poor inside the lumen. A helical flow is displayed as a multiphasic spectrum and a mixture of red and blue color. Pseudoaneurysms have a communication with the arterial lumen via a narrow neck in which a to-and-fro flow pattern is observed.²⁴

Arteriovenous Shunts (Malformations and Fistulas)

Peripheral resistance is severely decreased in the region of the arteriovenous shunts. The enormous increase in velocities causes a very turbulent flow with transmission of mechanical vibrations to the perivascular tissue. Afferent and efferent vessel diameters increase. The afferent arterial flow is turbulent with high velocities in systole and diastole. The efferent venous flow has a clear cardiac modulation with increased velocities during systole.

Steal Phenomenon

Changes in the flow characteristics first appear at diastole as a decrease in the diastolic velocities. For that reason, the earliest finding is an increase in the resistive index. As the pathology advances, flow is reversed at diastole. Flow is totally reversed in systole and in diastole when the stealing effect increases.

Venous Obstruction and Venous Thrombosis

Flow is decreased, and cardiac or respiratory modulation is lost in the upstream region of venous obstruction. In venous thrombosis, no flow is shown inside the lumen, although the setting parameters of the device are optimal. Compressibility of the vein is lost if the thrombosed vein is superficial. Venous occlusion in the veins draining an organ, such as the renal vein, causes an increase in the arterial resistive index of that organ.

Velocity Measurements

When the beam/flow angle is known, measurement of mean velocity, EDV, and maximum flow velocity, which is the PSV for arterial flow, is possible from the spectrum.

Doppler Indices

Further analysis of the spectrum and calculating some indices help to describe the complex waveform in a simple way and to evaluate organ blood flow. Commonly used indices available on most scanners are resistive index, systolic/diastolic ratio, and pulsatility index (see Table 78-5 for calculations). The major advantage of these indices is that they are independent of the transmit frequency and Doppler angle.

TABLE 78-5 Doppler Indices

A, peak systolic frequency; B, end-diastolic frequency.

Acceleration Time and Acceleration Index

Acceleration time and acceleration index are also important parameters obtained from the Doppler spectrum. They quantify the flow.⁹ In systole, the flow velocity accelerates very rapidly in the arteries, and the time needed to reach the maximum systolic velocity is referred as the acceleration time.

Volume Blood Flow Measurement

Quantification of flow volume is possible with color Doppler and duplex Doppler instruments. Different techniques are used to calculate flow volume, but it is simply calculated by multiplying luminal area with the mean velocity. This calculation is done automatically by the ultrasound machine. Measurement of flow volume is fraught with difficulties even under ideal conditions. Errors may arise because of many factors, including shape of the vessel, inaccurate estimation of the mean velocity, and variation in the vessel diameter (e.g., artery diameters are smaller during systole).² Volume flow measurements have found surprisingly little use in a clinical setting. Parameters such as pulsatility index seem to be more helpful in defining many physiologic and pathologic conditions.^2,9 Increased resistance to flow may increase cardiac contractility, and flow may be maintained until late in the disease.

In color Doppler imaging, visualization of the stenotic vessel segment with its residual lumen is possible. Most modern instruments are equipped to calculate the percent stenosis of diameter and area from this image.