= Continuous-wave Doppler echocardiography
= CW Doppler echocardiography
{c}
{synonym}
\b[Continuous-wave Doppler echocardiography] measures the Doppler frequency distribution of continuously transmitted and received <ultrasound> through the <heart>. It can record high flow velocities without the pulse-repetition-frequency aliasing limit of <pulsed-wave Doppler echocardiography>, but it does not select one depth along the beam. Signals from different contributing locations can overlap.<Recommendations for Quantification of Doppler Echocardiography by Miguel A. Quiñones and others>[General Principles, printed p.167; CW Doppler, p.169]{ref}
For an aligned flow at speed $v$, the leading <Doppler ultrasound> relation is
$$
\Delta f\simeq\frac{2f_0v}{c}.
$$
With stipulated transmitted frequency $f_0=2\,\mathrm{MHz}$, sound speed $c=1540\,\mathrm{m/s}$ and $v=4\,\mathrm{m/s}$, the shift is approximately $10.4\,\mathrm{kHz}$. A pulsed measurement with <pulse repetition frequency> $4\,\mathrm{kHz}$ would have a Doppler <Nyquist frequency> of only $2\,\mathrm{kHz}$. Continuous-wave acquisition avoids that particular pulse-rate restriction. Its receiving electronics and digital sampling still have finite bandwidth; continuous transmission does not imply unlimited measurable frequency.
The price is <Doppler range ambiguity>. If a beam intersects two moving regions, a peak in the spectrum does not by itself label which depth produced it. The anatomical image, direction, timing and waveform context supply additional evidence. The outer spectral envelope describes the fastest detected motion along the beam, rather than a direct spatial average over one known vessel opening.<Recommendations for Quantification of Doppler Echocardiography by Miguel A. Quiñones and others>[CW Doppler, p.169, outer envelope and anatomical localization]{ref}
This matters when estimating <stroke volume>. Multiplying a continuous-wave peak-velocity integral by an arbitrarily selected area can combine measurements from different sites or different parts of a velocity distribution. <Flow-profile mismatch in Doppler stroke-volume estimation> explains the separate requirement that the chosen velocity represent the area used in the volume calculation.
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