= Doppler range ambiguity
{c}
\b[Doppler range ambiguity] means that a measured <Doppler ultrasound> signal does not uniquely identify the depth of the scatterers producing it. In <continuous-wave Doppler echocardiography>, simultaneous transmission and reception collect motion signals along the beam without a unique pulse travel time. A high-speed spectral component therefore does not, by itself, label an anatomical site.<Recommendations for Quantification of Doppler Echocardiography by Miguel A. Quiñones and others>[General Principles, printed p. 167, and CW Doppler, p. 169]{ref}
Repeated pulses can introduce a different range ambiguity. Let the pulse period be $T_r=1/f_r$, with <pulse repetition frequency> $f_r$. If reception is associated only with its delay $\tau$ within a pulse period, the possible round-trip travel times include $\tau+kT_r$. The corresponding depths are
$$
d_k=\frac c2(\tau+kT_r),\qquad k=0,1,2,\ldots,
$$
where $c$ is <speed of sound>. Identifying which transmitted pulse generated an echo requires additional information or a restriction on the possible depth.
For an original example, $f_r=5000$ Hz gives $T_r=200$ microseconds. With $c=1540$ m/s and observed within-period delay $\tau=100$ microseconds, possible depths are 7.7, 23.1 and 38.5 cm. Successive possibilities are separated by 15.4 cm. An anatomical restriction to depths below 15.4 cm would identify the first value; the echo timing alone would not.
Range ambiguity and <aliasing> of the <Doppler ultrasound> <frequency> are distinct. The first confuses position along the beam; the second confuses motion <frequencies> sampled at a given pulse rate. Increasing the pulse rate can improve the unaliased motion range while reducing unique depth coverage, producing the <pulsed Doppler depth-velocity tradeoff>. High-pulse-rate methods deliberately accept multiple possible <sampling (signal processing)> depths.<Recommendations for Quantification of Doppler Echocardiography by Miguel A. Quiñones and others>[General Principles, pp. 167–168, high-PRF modification]{ref} Anatomical imaging and acquisition geometry supply constraints; a plausible waveform is not itself a unique range label.
Back to article page
