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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.

Pulsed-wave Doppler echocardiography uses timed ultrasound transmissions and returns to measure motion within a selected sampling region of the heart or nearby vessels. An echo's round-trip delay provides depth information, so a time window can select a location. The resulting Doppler spectrum describes a finite sample volume rather than an exact point.^[ref]

For sound speed $c$ and round-trip delay $t$, the ideal depth is $d=ct/2$. Avoiding ambiguity between successive pulses in a simple single-depth system requires waiting at least $2d/c$ before the next pulse. If $f_r$ is the pulse repetition frequency, this givesThe Doppler signal is sampled once per pulse, so unaliased shifts obey $|\Delta f|<f_r/2$. For aligned flow and transmitted frequency $f_0$,This is the pulsed Doppler depth-velocity tradeoff in the stated idealization. With $c=1540m/s$, $f_0=2.5\mathrm{MHz}$ and $d=0.10\mathrm{m}$, the highest ideal pulse rate is $7700\mathrm{Hz}$ and the corresponding speed limit is about $1.19m/s$. Doubling the depth halves both limits. Practical timing margins can reduce them further.

Aliasing is an ambiguity of the sampled signal, not proof that blood reverses direction. For example, sinusoidal Doppler shifts of $5000\mathrm{Hz}$ and $1000\mathrm{Hz}$ have identical complex samples at a $4000\mathrm{Hz}$ pulse rate, because their frequencies differ by exactly one sampling rate. The example concerns the Doppler signal, not direct sampling of the megahertz acoustic carrier.

High-pulse-rate variants can trade unique range localization for a larger velocity range. Continuous-wave Doppler echocardiography instead transmits and receives continuously, accepting depth ambiguity. The choice of acquisition method therefore changes what information the measured spectrum contains.^[ref]