 Pulsed-wave Doppler echocardiography

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]