This is not a direct result of physics, but physics has helped indirectly by providing an elegant solution for the production of ultrasonic waves (i.e., piezoelectric crystals) within the frequency and amplitude ranges required for medical imaging. Ultrasonic transducers used to initiate and to detect ultrasonic pulses, and the electronics that accompany them are relatively low cost compared with the equipment necessary for most other imaging techniques. The sound path’s orientation and the echo time-of-flight are used to map each echo intensity to a position in a reconstructed image of the medium.
The relative intensity of a returned echo is represented in terms of relative pixel brightness. Images are formed by sensitively detecting echoes of the pulse returned from interfaces or from scattering structures within the tissue. As the ultrasonic pulse passes through a medium, molecules within the body are reversibly perturbed from their equilibrium positions. Ultrasound imaging is safe and noninvasive. The higher the ultrasound frequency, the more limited is the accessible imaging depth. Attenuation of the wave increases with increasing frequency.
The wave progressively decreases in amplitude, or is attenuated, as it propagates through the medium because of energy lost through scattering from the principal propagation path and lost as heat. There is mechanical interaction between the wave and the body. In a viscoelastic medium, stress is related to strain amplitude and to the rate of strain variation. This relationship is described by Hooke’s law. In an element of an elastic medium, longitudinal stress (i.e., force per unit area) applied to the medium is proportional to the strain (i.e., change in the length divided by the initial length). Ultrasound requires an elastic or viscoelastic medium for propagation. Ultrasonic waves are elastic, mechanical waves.
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