Chapter 4 - Acoustic Properties of Tissue at Ultrasonic Frequencies

Abstract

This chapter discusses the properties of tissue at ultrasonic frequencies, although there is also a brief mention of transmission properties at audio frequencies. The mechanical response of tissues to the ultrasonic range of frequencies, that is, those over 20 kHz, with emphasis on the frequency range 1–10 MHz, can be studied to add to available knowledge because of its wide use in clinical medicine. Shear waves propagate in hard tissues at velocities around 30–50% lower than the equivalent longitudinal wave velocity. Thereafter, the scattering of sound from tissue is anisotropic. The total attenuation in tissue results from the combined losses as a result of absorption and scattering and can be expressed by the equation: α = α a + α s , where α a is the amplitude absorption coefficient and α s is the amplitude scattering coefficient. The intensity attenuation coefficient, μ, is 2α. Subsequently, the intensity coefficients μ a and μ s are related by μ = μa + μs = 2α, where μ a , and μ s are also called absorption and scattering cross section per unit volume respectively. The measurement of attenuation has commonly been achieved using a substitution method, recording the reduction in signal strength at a receiver following the introduction of a tissue sample of known thickness into an acoustic path. Factors affecting attenuation are essential, the same as those affecting acoustic velocity, such as temperature, frequency, anisotropy, tissue composition, and tissue fixation among others.