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Principles of Doppler echocardiography

Principles of Doppler echocardiography
Literature review current through: Aug 2023.
This topic last updated: Jan 05, 2022.

INTRODUCTION — While M-mode and two-dimensional (2D) echocardiography allow for creation of anatomic images of the heart, Doppler echocardiography utilizes ultrasound to record blood flow within the cardiovascular system. Doppler echocardiography is based upon the changes in frequency of the backscatter signal from small moving structures (ie, red blood cells) intercepted by the ultrasound beam.

The principles of Doppler echocardiography will be reviewed here. The principles of other echocardiographic techniques, as well as the normal views and protocol for an echocardiogram, are discussed elsewhere. (See "Echocardiography essentials: Physics and instrumentation" and "Tissue Doppler echocardiography", section on 'Technical aspects' and "Transthoracic echocardiography: Normal cardiac anatomy and tomographic views".)

BASIC PRINCIPLES — A moving target will backscatter an ultrasound beam to the transducer so that the frequency observed when the target is moving toward the transducer is higher and the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency (figure 1). This Doppler phenomenon is familiar to us as the sound of a train whistle as it moves toward (higher frequency) or away (lower frequency) from the observer. This difference in frequency between the transmitted frequency (F[t]) and received frequency (F[r]) is the Doppler shift:

Doppler shift (F[d])  =  F[r]  -  F[t]

Blood flow velocity (V) is related to the Doppler shift by the speed of sound in blood (C) and ø, the intercept angle between the ultrasound beam and the direction of blood flow. A factor of 2 is used to correct for the "round-trip" transit time to and from the transducer.

F[d]  =  2  x  F[t]  x  [(V  x  cos ø)]  /  C

This equation can be solved for V, by substituting (F[r] – F[t]) for F[d]:

V  =  [(F[r]  -  F[t])  x  C]  /  (2  x  F[t]  x  cos ø)

Note that the angle of the ultrasound beam and the direction of blood flow are critically important in the calculation (figure 2):

For ø of 0° and 180° (parallel with blood flow), cosine ø = 1

For ø of 90° (perpendicular to blood flow), cosine ø = 0 and the Doppler shift is 0

For ø up to 20°, cos ø results in a minimal (less than 10 percent) change in the Doppler shift

For ø of 60°, cosine ø = 0.50

The value of ø is particularly important for accurate assessment of high velocity jets, which occur in aortic stenosis or tricuspid regurgitation in persons with pulmonary artery systolic hypertension. It is generally assumed that ø is 0° and cos ø is therefore 1.

Spectral analysis — When the backscattered signal is received by the transducer, the difference between the transmitted and backscattered signal is determined by comparing the two waveforms with the frequency content analyzed by fast Fourier transform (FFT). The display generated by this frequency analysis is termed spectral analysis. By convention, time is displayed on the x (horizontal) axis and frequency shift on the y (vertical) axis. On spectral Doppler, shifts toward the transducer are represented as "positive" deflections from the "zero" baseline, and shifts away from the transducer are displayed as "negative" deflections (figure 3).

Multiple frequencies exist at every time point. Each received frequency is displayed, with the magnitude (or amplitude) shown as the "brightness" of each frequency shift component.

DOPPLER MODALITIES — There are several Doppler methods used for cardiac evaluation: continuous wave, pulsed wave, and color flow [1,2].

Continuous wave Doppler — Continuous wave Doppler employs two dedicated ultrasound crystals: one for continuous transmission and a second for continuous reception of ultrasound signals. This permits measurement of very high frequency Doppler shifts or velocities. The "cost" is that this technique receives a continuous signal along the entire length of the ultrasound beam. Thus, there may be overlap in certain settings, such as stenoses in series (eg, left ventricular outflow tract gradient and aortic stenosis) or flows that are in close proximity/alignment (eg, aortic stenosis and mitral regurgitation). Differentiation of the signal from each component may still be determined from the characteristic timing and/or profile.

An ideal Doppler profile is one with a smooth "outer" contour, well-defined edge and maximum velocity, and abrupt onset and termination (waveform 1). The continuous wave Doppler profile is usually "filled in" because lower-velocity signals proximal and distal to the point of maximum velocity are also recorded. Since the maximum frequency shift depends on ø, inappropriate underestimation of true velocity may occur if the angle of interrogation is 20 degrees or greater. For this reason, continuous wave Doppler positioning is often integrated with two-dimensional (2D) and color flow imaging to allow for good alignment with flow (ie, ø less than 20°).

Continuous wave Doppler is typically used to measure higher velocities as in pulmonary hypertension and aortic stenosis (image 1 and waveform 2).

Pulsed Doppler — In contrast to continuous wave Doppler, which records signal along the entire length of the ultrasound beam, pulsed wave Doppler permits sampling of local blood flow velocities at a specific region (or sample volume). This modality is particularly useful for assessing the relatively low velocity flows associated with mitral or tricuspid inflow, pulmonary venous flow, left atrial appendage flow, left ventricular outflow, or right ventricular outflow blood flows (figure 4 and image 2).

To permit this, an ultrasound pulse is transmitted and then the receiver "listens" during a subsequent interval defined by the distance from the transmitter and the sample site. This transducer mode of transmit-wait-receive is repeated at an interval termed the pulse-repetition frequency (PRF). The PRF is therefore depth-dependent, being greater for near regions and lower for distant or deeper regions.

The width and length of the sample volume is varied by adjusting the length of the transducer "receive" interval. Pulsed Doppler is always performed with 2D guidance to determine the optimal sample volume position.

Because pulsed wave Doppler echo repeatedly samples the returning signal, there is a maximum limit to the frequency shift or velocity that can be measured unambiguously. Correct identification of the frequency of an ultrasound waveform requires sampling at least twice per wavelength. Thus, the maximum detectable frequency shift or the Nyquist limit is one-half the PRF. If the velocity of interest exceeds the Nyquist limit, "wraparound" of the signal occurs first into the reverse channel, then back to the forward channel; this is known as aliasing.

Techniques that can minimize aliasing during pulsed Doppler include using a lower frequency transducer and shifting the baseline. Another solution is to increase the number of sample volumes, or high PRF. As noted above, when a pulse is transmitted, backscatter along the entire length of the beam is received. Depth resolution is achieved with pulsed Doppler using the duration of the "wait" period. However, signals from exactly twice (or 3x, 4x, etc) the distance will reach the transducer during the "receive" phase of the next (or subsequent) cycle. As a result, signals from 1x, 2x, 3x, 4x, 5x, etc have the potential for confounding the analysis.

The latter signals are generally of low amplitude and do not interfere with the spectral display. If, however, the sample volume is deliberately placed at one-half the depth of interest, backscattered signals from the 2x sample volume, the true depth of interest, will return to the transducer during the "receive" phase of the following cycle. This recording of signal at a higher PRF permits measurement of higher velocities without signal averaging. Even greater velocities could be achieved using additional sample volumes.

Color flow imaging — Doppler color flow imaging is based upon the principles of pulsed wave Doppler echocardiography. Along each scan line, a pulse of ultrasound is transmitted, and the backscattered signals are then received from each "gate" or sample volume along each line. In order to calculate accurate velocity data, several bursts along each scan line are used, known as the burst length. The process is performed for each scan line across the image plane. As with pulsed Doppler, the PRF is determined by the maximum depth of the Doppler signals.

With color flow imaging, velocities are displayed using a color scale, with flow toward the transducer typically displayed in orange/red and flow away from the transducer displayed as blue. Lighter shades are assigned higher velocities within the Nyquist limit (image 3 and image 4 and image 5A-B and image 6 and image 7).

At the Nyquist limit, and each multiple of the limit, aliasing is depicted as color reversal. Turbulent flow is characterized by varied blood velocities and directions. The variance of velocities within jets is usually color coded as a multicolored mosaic display.

Color flow imaging is typically used in the screening and assessment of regurgitant flows (image 8 and image 9 and movie 1 and movie 2 and movie 3 and movie 4). It is also useful in the assessment of intracardiac shunts (eg, atrial and ventricular septal defects) and pulmonary vein flow, and to assist in continuous wave Doppler alignment for tricuspid regurgitation velocities. Since the size of regurgitant jets varies with instrument settings, appropriate settings are required for accurate detection and assessment of valve lesions. Standard settings for assessment of valvular regurgitation include a Nyquist limit of 50 to 70 cm/sec and a high color gain that just eliminates random color speckle from nonmoving regions [3].

Tissue Doppler imaging — Tissue Doppler imaging is a form of pulsed wave Doppler that is used for recording myocardial tissue velocity. Tissue Doppler early diastolic signal from the mitral annulus in the apical view is used in evaluation of left heart diastolic function, while tissue Doppler systolic recording from the tricuspid annulus or basal right ventricular free wall in the apical four chamber view may be used to aid in the assessment of right ventricular systolic function (image 10).

RELATIONSHIP BETWEEN DOPPLER VELOCITY AND PRESSURE GRADIENT — One of the most powerful attributes of Doppler echocardiography is the ability to estimate the pressure difference across a stenotic valve (eg, aortic stenosis) or between two chambers (eg, estimation of the pulmonary artery systolic pressure from the tricuspid regurgitation velocity). This relationship is defined by the Bernoulli equation and is dependent on the velocity proximal to a stenosis (V1), velocity in the stenotic jet (V2), density of blood (p), acceleration of blood through the orifice (dv/dt), and viscous losses (R[v]). The pressure gradient (ΔP or delta P) can be calculated from:

Pressure gradient (mmHg) =  [0.5  x  p  x  (V2  x  V2  -  V1  x  V1)]  +  [p  x  (dv  /  dt)]  +  R[v]

If one assumes that the last two terms (acceleration and viscous losses) are small and then enters the constants, the formula is simplified to:

Pressure gradient (mmHg)  =  4  x  (V2  x  V2  -  V1  x  V1)

In most settings, V2 is greater than V1 by a factor of three or more. Thus, the terms (V2 x V2 - V1 x V1) are close to V2 x V2. As an example, 3 x 3 - 1 x 1 = 9 – 1, which is almost equal to 9.

Thus, the Bernoulli formula may be further simplified (image 11 and image 12 and image 13):

Pressure gradient (mmHg)  =  4  x  V2  x  V2

It is important to remember that this simplified Bernoulli formula measures the pressure difference, not the absolute pressure. In addition, it is imperative that accurate measurement of V2 be obtained. Due to the squaring of V2, a 10 percent error in V2 will result in a 20 percent error in the pressure estimate. (See "Aortic valve area in aortic stenosis in adults".)

SUMMARY

A moving target will backscatter an ultrasound beam to the transducer so that the frequency observed when the target is moving toward the transducer is higher and the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency (figure 1). This difference in frequency between the transmitted frequency and received frequency is known as the Doppler shift. This principle can be used in the measurement of blood velocity. (See 'Basic principles' above.)

There are several Doppler methods used for cardiac evaluation: continuous wave, pulsed wave, and color flow.

Continuous wave Doppler employs two dedicated ultrasound crystals: one for continuous transmission and a second for continuous reception of ultrasound signals. This permits measurement of very high-frequency Doppler shifts or velocities. The "cost" is that this technique receives a continuous signal along the entire length of the ultrasound beam, which may result in overlapping signals in certain settings (eg, left ventricular outflow tract gradient and aortic stenosis, aortic stenosis and mitral regurgitation). Continuous wave Doppler is typically used to measure higher velocities as in pulmonary systolic hypertension and aortic stenosis. (See 'Continuous wave Doppler' above.)

In contrast to continuous wave Doppler, which records signal along the entire length of the ultrasound beam, pulsed wave Doppler permits sampling of blood flow velocities from a specific region, a technique that is particularly useful for assessing relatively low velocity flows. A pulse of ultrasound is transmitted and then the receiver "listens" during a subsequent interval defined by the distance from the transmitter and the sample site. As a result, pulse wave Doppler is always performed with two-dimensional (2D) imaging to localize the same volume. (See 'Pulsed Doppler' above.)

Doppler color flow imaging is based upon the principles of pulsed wave Doppler echocardiography. Along each scan line, a pulse of ultrasound is transmitted, and the backscattered signals are then received from each "gate" or sample volume along each line. With color flow imaging, velocities are displayed using a color scale. Color flow imaging is typically used in the screening and assessment of regurgitant flows, intracardiac shunts and pulmonary vein flow, and to assist in continuous wave Doppler alignment for tricuspid regurgitation velocities. (See 'Color flow imaging' above.)

One of the most powerful attributes of Doppler echocardiography is the ability to estimate the pressure difference across a stenotic valve (eg, aortic stenosis) or between two chambers (eg, estimation of the pulmonary artery systolic pressure from the tricuspid regurgitation velocity). This relationship is defined by the Bernoulli equation and is dependent on the velocity proximal to a stenosis, velocity in the stenotic jet, density of blood, acceleration of blood through the orifice, and viscous losses. The modified Bernoulli equation

(Pressure gradient  =  4  x  V2  x  V2)

is the most commonly used application relating peak velocity to peak pressure gradient. It measures the pressure gradient across a valve (not absolute pressure). (See 'Relationship between Doppler velocity and pressure gradient' above.)

Topic 5315 Version 19.0

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