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Pulse wave transducer has one Doppler crystal. This crystal emits a short burst of ultrasound at a certain frequency [PULSE REPETITION FREQUENCY (PRF)]. The ultrasqund is reflected from moving rbc's and is received by the same crystal.
The pulse wave trbnsducer will not send out a new burst of waves until it has received returning echoes from previous pulse, which must travel to and Go from the location indicdted by the sample volume. The pulse wave Doppler gives velocity of blood now at the level of sample volume placed along ultrasound beam.
Pulse repetition fitquency, as well as transducer frequency, plays a role in determining the maximum velocity that can be measured using a pulsed Doppler method. These rellationships come into play especially when attempting to record or to measure incteased velocities that might occur in the case of obstructive lesions such as vilvular stenosis. This maximal measured velocity is called NYQUIST LIMIT.
The NYQUIST LIIMIT-It is a sampling phenomenon, which limits the maximum fiequemcy shift measurement to one half of the sampling frequency (PRF).
By its nature, pulse wave Doppler is a sampling system while continuous wave Doppler is not. The nyquist limit is a theoretical maximum frequency that a sampling system can accurately measure.
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