MEDS12001 Lecture Notes - Lecture 1: Medical Ultrasound, Hertz, Longwave
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PHYSICS WEEK 2
PULSED ULTRASOUND
• In continuous wave ultrasound, the cycles
repeat indefinitely.
• Pulsed ultrasound is used more in
sonography, and most of Doppler ultrasound
o An ultrasound pulse is a few cycles of
ultrasound
o Pulses are separated in time with gaps of
no ultrasound
PULSE-REPETITION FREQUENCY AND PERIOD
• Frequency is the number of cycles occurring
per second for a continuous wave
o The number of cycles per second that
would occur
o When qualified with pulse-repetition, it is
the number of pulses that occur in one
second (PRF)
• Diagnostic ultrasound involves a few thousand
pulses per second, so PRF is commonly
expressed in Kilohertz (KHz)
• Frequency and PRF are independently
controlled by the sonographic instrument
• The term period refers to the time for one
cycle to occur
o PRP refers to the time from beginning of
one pulse to the beginning of the next.
▪ Common units are milliseconds.
▪ The PRP is the reciprocal of PRF.
• PRF is controlled automatically by the
sonographic instruments to satisfy
requirements that are discussed later
o With Doppler techniques, the operator
controls the PRF
• PRF is important because it determines how
quickly images are generated
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Document Summary
1 | p a g e: e. g. with a prf of 5khz, 5000 pulses are produced each second, generating 5000 scan lines per second. Spl are millimetres: spl is an important quantity when considering image resolution, because shorter pulse lengths improve resolution. The instrument operator chooses the frequency wave that occur in 1 second. In pulsed ultrasound, gaps exist between pulses and therefore some of the cycles are missing: for example, the 5mhz frequency is given for continuous wave ultrasound and does not account for the gaps in the pulsed. In doppler ultrasound, the range is 0. 5% to 5% 1 sec in pulsed ultrasound depends on the df: for example, if the df is 0. 01 or 1%. Bandwidth: pulsed ultrasound contains a range of frequencies unlike the. It determines the number of scan lines produced per second and the number of images produced per second (the frame rate)
