# Pulse-wave Doppler ultrasound

1. Oct 27, 2015

### BobP

Hi.
I understand the principle behind Pulsed doppler ultrasound. If the echo from a transmit pulse is sampled (gated) during a narrow fixed time period, the echo will correspond to a particular depth. What I do not understand is how we can extract phase information by repeating the process of transmit-echo-transmit-echo over and over again.

Here is why (I am going to use an analogy to explain my problem).
If we have a door that is moving away from us then if we transmit-recieve-transmit-recieve... then each time we transmit our signal the door will be slightly further away so it will take longer to receive the reflection. Hence we get a phase shift.

If however we have a stack of thousands of doors flowing along a line and we measure the reflection from a distance, d, from our transmitter (as we do with pulsed ultrasound) we will be receiving the reflection from a different door each time (we are not following a single door) hence there should be no phase shift as the reflection will occur at the same time, each time.

Is my problem clear?
Thanks for your help

2. Oct 31, 2015

### tech99

If a Doppler radar uses frequency to find velocity there is no ambiguity. If the radar measures phase, it can find distance, but there will be an ambiguity if the velocity creates a phase change of more than one cycle for each pulse. So it is for stationary or slow moving targets.