Doppler range ambiguity

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.^[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 $\tau +k{T}_r$. The corresponding depths arewhere $c$ 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 depths.^[ref] Anatomical imaging and acquisition geometry supply constraints; a plausible waveform is not itself a unique range label.