Direction Of Flow In Color Doppler

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Çj Speed = 3 revolutions/minute

Velocity / Direction

Sampling rate

4 X/ minute

Start 15 sec 30 sec 45 sec 60 sec G" 270" 180° 90" 0s

Direction will be perceived incorrectly as counterclockwise and velocity, incorrectly, as 1 revolution per minute

location (depth) to be interrogated. On the ultrasound machine, this is accomplished by placing a cursor over a specific area on the 2D image (Fig. 10). What the ultrasound machine is actually doing is emitting a pulse, then waiting the exact amount of time it would take that pulse to travel to the cursor location and return to the transducer. In PW Doppler, the time between pulses cannot be less than that round-trip transit time and the PRF will be inversely related to that time. Figures 12 and 13 illustrate the aliasing principles in Doppler echocardiography.

Blood Flow Profiles in the Heart

Blood flowing through the heart and blood vessels can be either laminar or turbulent. Laminar flow occurs when the majority of flow is moving in the same direction and at similar velocities. Turbulent flow occurs when flow is disturbed, by a stenosis or in the setting of significant regurgitation. The type of flow can be discerned from the PW Doppler waveform. With laminar flow, the waveform will appear "hollow" because the majority of blood cells will be moving at similar velocities (and close to the maximal velocity). With turbulent or nonlaminar flow, the velocities will cover a wider spectrum, with some blood cells moving very rapidly and some moving very quickly. Thus, the waveform will appear "filled" (Fig. 14).

Practical Aspects of PW Doppler

PW Doppler is used primarily to obtain velocity information for relatively low velocity flows at a specific

Echocardiography Images

Fig. 13. The problem of aliasing occurs when sampling a sound wave as well as when the sampling frequency is less than twice the frequency of the wave that is being sampled. In the example, when the sampling rate is twice the frequency of the original wave (in black), it is possible to reconstruct the wave accurately. However, when the sampling frequency is less than twice the frequency of the wave, the wave is reconstructed incorrectly, as is shown in the bottom panel, where the reconstructed wave is shown in gray. In Doppler echocardiography, we are "sampling" the Doppler shift. The sampling rate is determined by the pulse repetition frequency (PRF). The Doppler shift is reflective of the velocity of the blood flow (by the Doppler equation). Thus, higher PRFs are able to discern higher velocities of blood flow.

Fig. 13. The problem of aliasing occurs when sampling a sound wave as well as when the sampling frequency is less than twice the frequency of the wave that is being sampled. In the example, when the sampling rate is twice the frequency of the original wave (in black), it is possible to reconstruct the wave accurately. However, when the sampling frequency is less than twice the frequency of the wave, the wave is reconstructed incorrectly, as is shown in the bottom panel, where the reconstructed wave is shown in gray. In Doppler echocardiography, we are "sampling" the Doppler shift. The sampling rate is determined by the pulse repetition frequency (PRF). The Doppler shift is reflective of the velocity of the blood flow (by the Doppler equation). Thus, higher PRFs are able to discern higher velocities of blood flow.

location within the heart or blood vessels. Examples of Doppler assessments that are typically made with PW Doppler include assessing the left ventricular outflow tract velocity (except in conditions in which they outflow tract velocity is markedly elevated in which case CW Doppler would be needed), assessment of mitral inflow velocities, and assessment of pulmonary venous velocities. These are all relatively low velocity flows within the heart.

CW Doppler

Unlike PW Doppler, in which individual pulses are emitted and reflected back to the transducer, ultrasonic beeps, CW Doppler emits a continuous tone from the transducer. Reflections from this continuous ultrasound tone are then received by the transducer continuously as well (Fig. 11). Because the machinery is not waiting for a pulse to reflect and return, it is impossible for the ultrasound equipment to determine the location of the reflection. Nevertheless, moving blood cells will reflect the continuous ultrasound tone and this reflection will be subject to the Doppler shift as a function of the velocity of the blood flow (just as with pulsed Doppler). The advantage of CW Doppler is that because "sampling" is occurring continuously, the ability to detect particular frequencies is not subject to the Nyquist limit, and we can thus interrogate much higher velocities than is possible with PW Doppler. The disadvantage of CW Doppler, however, is that because we are not sampling, we cannot "gate" the returning ultrasound pulse and thus cannot listen for a reflection that is coming from a particular depth. Thus, CW Doppler tells us the maximal velocity along the line of the ultrasound beam. We cannot, however, determine the location of the maximal velocity. CW Doppler is particularly useful then for assessing high velocity blood flow, for example, the velocity across the aortic valve in aortic stenosis, or the velocity of tricuspid regurgitation.

Color Flow Doppler

Color flow Doppler imaging uses the same general technology as PW Doppler imaging. However, color flow Doppler samples multiple locations along a scan line simultaneously and determines the velocity of individual locations. These velocities are then "color encoded" utilizing a color map in which particular colors are used to represent particular velocities (Fig. 15). The color map is displayed on the ultrasound image so that the relationship between particular colors and velocities are visible. By convention, flow that is moving away from the transducer

Doppler Color Flow Map

Fig. 14. Pulsed-wave Doppler profiles of (A) laminar and (B) turbulent flow. (C) Demonstrates flow away from the transducer. Notice that the flow profile of laminar flow is "hollow" indicating that the most of the blood cells are traveling at similar velocities. When flow is turbulent, there is a wider range of velocities, and the flow profile appears filled in.

Fig. 14. Pulsed-wave Doppler profiles of (A) laminar and (B) turbulent flow. (C) Demonstrates flow away from the transducer. Notice that the flow profile of laminar flow is "hollow" indicating that the most of the blood cells are traveling at similar velocities. When flow is turbulent, there is a wider range of velocities, and the flow profile appears filled in.

is encoded in blue, and flow that is moving toward the transducer is encoded in red (Fig. 16; please see companion DVD for corresponding video). Because color flow Doppler utilizes the same basic principles as PW Doppler, it is also subject to sampling issues and the problem of "aliasing." Doppler shift frequencies (and hence velocities) that are above the Nyquist limit are encoded as a "green mosaic." The Doppler information is superimposed on the 2D image, providing a very powerful visual assessment of blood movement in the heart. Because color flow Doppler displays velocity information on top of anatomical information, it is possible to visualize even very fast flows in the heart, although color flow Doppler does not allow accurate assessment of velocities beyond the Nyquist limit. As with standard pulsed Doppler, the PRF is an important setting in color flow Doppler, and can be adjusted by the operator. The PRF can be lowered or raised to decrease or increase the aliasing velocity.

Color Doppler Flow Direction
Fig. 15. Color Doppler scale depicting flow direction and relative velocities.

From Velocity to Pressure: Measuring

Gradients in the Heart

Doppler echocardiography measures the velocity of blood movement within the heart and blood vessels. From this velocity information, it is possible to estimate pressure gradients utilizing the Bernoulli equation. The Bernoulli principle states that the velocity of flow through a fixed orifice will be dependent on the pressure gradient across the orifice. Intuitively, this principle states that the higher the pressure gradient, the faster the blood flow. The full form of the Bernoulli equation is relatively complex:

Fig. 16. Apical view with superimposed color Doppler showing flow direction during early systole. (Please see companion DVD for corresponding video.)

Direction Blood Flow Color Doppler

Fig. 16. Apical view with superimposed color Doppler showing flow direction during early systole. (Please see companion DVD for corresponding video.)

In general ultrasound use, the following simplification can be made:

where p = pressure gradient, v12 = proximal velocity, v22 = distal velocity. With most velocities that are greater than 1 m/s, proximal velocities can often be ignored, leaving the following simplified modified Bernoulli equation: P = 4V22. This equation is useful for translation of velocities to gradients in most clinical circumstances.

For example, a maximal CW velocity across a stenotic aortic valve of 4 m/s is equivalent to a pressure gradient of 64 mmHg across the valve. Likewise, a maximal CW velocity of tricuspid regurgitation of 3 m/s is equivalent to a systolic gradient between the right ventricular and the right atrium of 36 mmHg. It is important, however, to recognize clinical situations when the proximal flow velocities cannot be ignored. For example, if there is significant flow acceleration proximal to the aortic valve—as, for example, in the case of a patient with aortic stenosis and subaortic stenosis (caused by a membrane or by septal hypertrophy), it would be necessary to use the longer form of the Bernoulli equation, thus, taking into account the increased proximal flow velocities.

Doppler Cautions and Caveats

Doppler cannot measure pressure directly. Nor can Doppler measure "flow." Doppler measures the velocity of blood flow. For this reason, much of the information that we ultimately derive from Doppler measurements has to be inferred. For example, although we can measure the gradient between the right ventricle and the right atrium by looking at the tricuspid regurgitant velocity, to estimate pulmonary artery systolic pressures we need to make the following assumptions: first, we need to assume that there is no pressure gradient between the right ventricle and the pulmonary artery. Obviously, in patients with pulmonic stenosis, we would be unable to calculate the pulmonary systolic pressures from the tri-cuspid regurgitant velocity without taking into account the gradient across the pulmonic valve. In addition, because we know the gradient between the right ventricle and the right atrium, to calculate right ventricular systolic

Fig. 17. Aliasing on color Doppler reflecting high velocity turbulent flow in a patient with severe mitral regurgitation. (Please see companion DVD for corresponding video.)

Turbulent Flow Doppler Signal

Fig. 17. Aliasing on color Doppler reflecting high velocity turbulent flow in a patient with severe mitral regurgitation. (Please see companion DVD for corresponding video.)

pressure we need to know the estimated right atrial pressure. Assessment of right atrial pressure is indirect at best with echocardiography. Although certain echocardiography parameters, such as increased right atrial size and dilatation of the inferior vena cava, help us "guesstimate" right atrial pressure (see Chapter 18, Table 3), these are notoriously inaccurate, especially if pressures are high. By adding the gradient obtained from the tricuspid regurgitant velocity signal to our estimate of right atrial pressure we can obtain an estimate of right ventricular systolic pressure, which in turn should be equivalent to pulmonary artery systolic pressure.

Similarly, because echocardiography measures blood velocity, not blood flow, our estimate of the volumetric degree of regurgitation by echocardiography is limited. We cannot directly measure the volume traversing the aortic valve in, for example, aortic insufficiency. For this reason, Doppler is, in many ways, better for assessment of the severity of stenotic lesions than for assessment of the severity of regurgitant lesions.

Color flow Doppler utilizes the same concepts and technology as pulse wave Doppler and is, therefore, subject to the same limitations. The colors that we see in color flow Doppler are simply color encoded pixels that represent the velocity of blood flow at that particular spatial location. Colors that are pure red or blue in color flow Doppler represent velocities that are below the aliasing velocity of the Doppler signal (Fig. 16). Colors that appear to be yellow-green or mosaic, depending on the color map utilized, suggest high velocity (higher than the aliasing velocity) or turbulent flow (Fig. 17; please see companion DVD for corresponding video). The aliasing velocity, in centimeters per second, is usually listed on the scale present on the ultrasound image. Although we often estimate the volume of regurgitant lesions, including mitral and aortic regurgitation, from the color flow Doppler signal, we cannot do this directly because color flow Doppler provides limited assessment of the volumetric degree of regurgitation.

summary

The properties of ultrasound permit real-time generation of cardiac anatomical and hemodynamic data.

Improvements in transducer design and imaging modalities have led to improved image quality. The addition of Doppler ultrasound to 2D echocardiography provides reliable noninvasive determination of velocity shifts and pressure gradients within and across cardiac chambers. Echocardiography data is influenced by limitations intrinsic to ultrasound and Doppler technology, patient characteristics, and operator skill.

suggested reading

Cape EG, Yoganathan AP. Principles and instrumentation for Doppler. In: Skorton DJ, Schelbert HR, Wolf GL, Brundage BH, eds. Marcus Cardiac Imaging. A Companion to Braunwald's

Heart Disease, 2nd ed. Philadelphia: WB Saunders, 1996: 273-291.

Feigenbaum H. Echocardiography, Fourth ed. Lea and Febiger,

Malvern, PA: 1986. Geiser EA. Echocardiography: physics and instrumentation. In: Skorton DJ, Schelbert HR, Wolf GL, Brundage BH, eds. Marcus Cardiac Imaging. A Companion to Braunwald's Heart Disease, 2nd ed. Philadelphia: WB Saunders, 1996:273-291. Seghal CM. Principles of ultrasonic imaging and Doppler ultrasound. In: St. John Sutton MG, Oldershaw PJ, Kotler MN, eds. Textbook of Echocardiography and Doppler in Adults and Children. Cambridge, MA: Blackwell Science, 1996:3-30. Ultrasonography task force. Medical diagnostic ultrasound instrumentation and clinical interpretation. Report of the ultrasonography task force. Council on Scientific Affairs. JAMA 1991;265:1155-1159.

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Responses

  • emilia
    What is color depicted blood flow rate?
    6 months ago
  • leea pohjonen
    Is there higher velocity in the red and yellow color flow in echocardiography?
    1 month ago
  • Abbondio
    How to determine right to left direction of flow with color doppler?
    1 month ago
  • SELMA
    How is the direction of the flow represented on the doppler?
    1 month ago
  • silke baumgaertner
    How to Determine Blood Flow Direction with Ultrasound and Doppler?
    17 days ago

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