FUNDAMENTAL PHYSICS OF ULTRASOUND AND ITS PROPAGATION IN TISSUE

Ultrasound is sound possessing a frequency above 20 kHz. it consists o f a propagated mechanical vibration of the molecules of a material. So und waves are classified as longitudinal or transverse, depending on w hether the vibration of each particle is parallel or transverse to the direction of propagation. Parameters of an ultrasound wave include fr equency, pressure, wavelength, velocity, power, and intensity. The las t two are measures of the ''strength'' of the wave. Ultrasound is atte nuated as it travels through the body due to beam divergence, absorpti on, and deflection of acoustic energy out of the beam. Deflection incl udes the processes of reflection, refraction, and scattering. An echo is a reflected wave, and its magnitude depends on (a) the orientation of the reflecting surface with respect to the sound beam and (b) the d ifference in acoustic impedance between tissues on either side of the reflecting surface. Although ultrasound is nonionizing, at sufficientl y high intensities, it can produce temperature elevation, mechanical e ffects, cavitation, and chemical effects. In the past 25 years, there have been no confirmed reports of any adverse effects; nevertheless, h ealth care professionals must remain vigilant.