PROPAGATION OF ULTRASOUND THROUGH TISSUES

Propagation of Ultrasound through Tissues

Propagation of Ultrasound through Tissues

Introduction

Ultrasound is sound possessing a frequency above 20 kHz. It consists of a propagated mechanical vibration of the molecules of a material. Sound waves are classified as longitudinal or transverse, depending on whether the vibration of each particle is parallel or transverse to the direction of propagation. Parameters of an ultrasound wave include frequency, pressure, wavelength, velocity, power, and intensity. The last two are measures of the “strength” of the wave. Ultrasound is attenuated as it travels through the body due to beam divergence, absorption, and deflection ofacoustic energy out of the beam. Deflection includes 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 difference in acoustic impedance between tissues on either side ofthe reflecting surface. Although ultrasound is nonionizing, at sufficiently high intensities, it can produce temperature elevation, mechanical effects, cavitation, and chemical effects. In the past 25 years, there have been no confirmed reports of any adverse effects; nevertheless, health care professionals must remain vigilant.

Nature Of Ultrasound

Ultrasound is sound possessing a frequency above 20 kHz, the upper limit for human hearing. Ultrasound consists of a mechanical vibration of the particles or mole. cules of a material. Although each particle moves small distances from its rest position, the vibrational energy is propagated as a wave traveling from particle to particle through the material. Sound waves are classified as being longitudinal or transverse, depending on whether the vibration of each particle is parallel or trans. verse to the direction ofpropagation (Fig 1). Although all materials can support the propagation of longitudinal sound waves, only solids can support transverse sound waves. From ...