Doppler interrogation: Difference between revisions

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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.


==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 0º and 180º (parallel with blood flow), cosine ø = 1
For ø of 90º (perpendicular to blood flow), cosine ø = 0 and the Doppler shift is 0
For ø up to 20º, cos ø results in a minimal (<10 percent) change in the Doppler shift
For ø of 60º, cosine ø = 0.50
The value of ø is particularly important for accurate assessment of high velocity jets, which occur in aortic stenosis or pulmonary artery hypertension. It is generally assumed that ø is 0º 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. Shifts toward the transducer are represented as "positive" deflections from the "zero" baseline, and shifts away from the transducer are displayed as "negative" deflections.
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.
===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.
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).
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.
The distance from the transmitter to the region of interest is called the sample volume, with the width and length of the sample volume varied by adjusting the length of the transducer "receive" interval. In contrast to continuous wave Doppler, which is sometimes performed without 2D guidance, pulsed Doppler is always performed with 2D guidance to determine the sample volume position
Because pulsed 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 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 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).
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 green.
Color flow imaging is typically used in the screening and assessment of regurgitant flows (figure 13 and figure 14A-B 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.
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 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) can be calculated from:
  ΔP = [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 enter the constants, the formula is simplified to:
  ΔP (mmHg) = 4 x (V2 x V2 - V1 x V1)
where both V2 and V1 are squared. In most settings, V2 is greater than V1 by a factor of three or more. Thus, the terms (V2 x V2 - V1 x V1) is 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 (figure 15A-C):
  ΔP (mmHg) = 4 x V2 x V2
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==
<biblio>
# www.uptodate.org
#Otto isbn=978-1-4160-5559-4
#Feigenbaum1 isbn=0781795575
</biblio>

Revision as of 10:53, 5 March 2012

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