Introduction
Knowledge of basic ultrasound physics is necessary when embarking on the journey of learning point of care ultrasound.
Piezoelectric Effect
Jacques and Pierre Curie first described the piezoelectric effect in 1880. The piezoelectric effect is the ability of crystals to produce electrical currents in response to mechanical deformation. The reverse can also occur. An electrical current applied across a crystal will cause it to undergo mechanical deformation.
The piezoelectric effect forms the cornerstone of ultrasound wave generation. Application of an electrical current through materials such as quartz causes vibration of the quartz and generation of ultrasound waves.
Reflected ultrasound signals are picked up by the quartz crystals, and an electrical current is generated. Interpretation and processing of this electrical signal leads to ultrasound image display.
Sound Waves
Sound is a series of pressure waves that propagates through a medium. The waves consist of areas of compression and rarefaction. The terms of amplitude, wavelength and frequency are used the describe sound waves.
Fig 1.00. Characteristics of sound wave. Licensed under Creative Commons Attribution 3.0 Unported
Amplitude
Amplitude describes the measure of the height of the wave. The amplitude of a sound wave is the intensity (loudness) or the amount of maximum displacement of vibrating particles of the medium from their mean position. C and R describe the areas of particle compression and rarefaction.
Wavelength
Wavelength refers to the distance between one peak and another peak or between 2 throughs. This corresponds to one cycle of the sound wave. The time period is defined as the the time taken for a sound wave to complete one cycle.
Frequency
Frequency refers to the number of cycles completed per second, and is measured in Hertz (Hz).
The relationship between speed of soundwave (v), frequency (f) and wavelength (lambda) is given by the following equation:
speed=frequency x wavelength
The normal range of human hearing ranges from 20Hz to 20kHz. Ultrasound waves are sound waves above the limit of human hearing (> 20kHz). Ultrasound frequencies used in diagnostic imaging typically range from 2 to 20MHz (MegaHertz).
Fig 1.01. Image shows the frequency ranges of infrasound, acoustic sound and ultrasound. Licensed under Creative Commons Attribution-Share Alike 2.5 Generic license.