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INTRODUCTION — While M-mode and 2D echocardiography create ultrasonic 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.
==Introduction==
While M-mode and 2D echocardiography create ultrasonic 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.


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:
==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]
   Doppler shift (F[d]) = F[r] - F[t]
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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.
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, and color flow.
==Doppler Modalities==
There are several Doppler methods used for cardiac evaluation — continuous wave, pulsed, and color flow.


Continuous wave Doppler — Continuous wave Doppler employs two dedicated ultrasound crystals, one for continuous transmission and a second for continuous reception. 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.
===Continuous Wave Doppler===
Continuous wave Doppler employs two dedicated ultrasound crystals, one for continuous transmission and a second for continuous reception. 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 (figure 4). 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. Although the maximum frequency shift depends on ø, the profile, onset, and termination of the Doppler signal are not dependent upon this value, resulting in inappropriate underestimation of true velocity. For this reason, continuous wave Doppler positioning is often integrated with 2D and color flow imaging to allow for good alignment with flow, ie, ø <20º.
An ideal Doppler profile is one with a smooth "outer" contour, well-defined edge and maximum velocity, and abrupt onset and termination (figure 4). 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. Although the maximum frequency shift depends on ø, the profile, onset, and termination of the Doppler signal are not dependent upon this value, resulting in inappropriate underestimation of true velocity. For this reason, continuous wave Doppler positioning is often integrated with 2D and color flow imaging to allow for good alignment with flow, ie, ø <20º.
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Continuous wave Doppler is typically used to measure higher velocities as in pulmonary hypertension and aortic stenosis (figure 5 and figure 6).
Continuous wave Doppler is typically used to measure higher velocities as in pulmonary hypertension and aortic stenosis (figure 5 and figure 6).


Pulsed Doppler — In contrast to continuous wave Doppler which records signal along the entire length of the ultrasound beam, pulsed Doppler permits sampling of blood flow velocities from a specific region. This modality is particularly useful for assessing the relatively low velocity flows associated with transmitral or transtricuspid blood flow, pulmonary venous flow, left atrial appendage flow, or for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation (figure 7 and figure 8).
===Pulsed Doppler===
In contrast to continuous wave Doppler which records signal along the entire length of the ultrasound beam, pulsed Doppler permits sampling of blood flow velocities from a specific region. This modality is particularly useful for assessing the relatively low velocity flows associated with transmitral or transtricuspid blood flow, pulmonary venous flow, left atrial appendage flow, or for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation (figure 7 and figure 8).


To permit this, 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. 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.
To permit this, 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. 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.
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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.
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 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 pulse repetition frequency (PRF) is determined by the maximum depth of the Doppler signals.
===Color flow imaging===
Doppler color flow imaging is based upon the principles of pulsed 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 pulse repetition frequency (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 (figure 9A-B and figure 10A-B and figure 11 and figure 12).
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 (figure 9A-B and figure 10A-B and figure 11 and figure 12).
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It is important to remember that this simplified Bernoulli formula measures the pressure difference, not 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".)
It is important to remember that this simplified Bernoulli formula measures the pressure difference, not 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".)


References:
==References==
 
<biblio>
www.uptodate.org
# www.uptodate.org
Otto
#Otto isbn=978-1-4160-5559-4
Feigenbaum
#Feigenbaum1 isbn=0781795575
</biblio>
Glori
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