Pulse repetition frequency
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Pulse repetition frequency (PRF) indicates the number of ultrasound pulses emitted by the transducer over a designated period of time. It is typically measured as pulses per second or hertz (Hz). In medical ultrasound the typically used range of PRF varies between 1 and 10 kHz 1. PRF is defined by using the formula below 3:
PRF = 1/period between pulses (or pulse repetition period, PRP)
Pulse repetition period (PRP) consists of pulse duration and receiving time. Pulse duration is typically less than 1 μs (less than 1% of PRP) to generate a pulse with a wide bandwidth (or greater receiving time), thus enhancing the spatial resolution of the image 4.
PRF is limited by the maximum depth (R) to be sampled and the acoustic velocity of the medium (c) 3.
PRFmax= acoustic velocity/ (maximum depth to be sampled (R) x 2)(c/2R)
Rearranging the equation, depth of view (R) = 0.5 x (c/PRF)
Each line within an ultrasound frame (or image) needs to be produced sequentially. Therefore, the frame rate (number of frames per second) is determined by the following formula 3:
Frame rate = PRF/lines per frame
The frame rate affects the temporal resolution of ultrasound 3.
A number of artifacts are directly influenced by the pulse repetition frequency, e.g. increasing it diminishes the aliasing artifact commonly encountered during color and spectral Doppler imaging, while decreasing it facilitates e.g. the display of the useful twinkling artifact occurring behind stones and calcifications 2.
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