Medical Acoustics

Abstract

Medical acoustics can be subdivided into diagnostics and therapy. Diagnostics are further separated into auditory and ultrasonic methods, and both employ low amplitudes. Therapy (excluding medical advice) uses ultrasound for heating, cooking, permeablizing, activating and fracturing tissues and structures within the body, usually at much higher amplitudes than in diagnostics. Because ultrasound is a wave, linear wave physics are generally applicable, but recently nonlinear effects have become more important, even in low-intensity diagnostic applications. This document is designed to provide the nonmedical acoustic scientist or engineer with some insights into acoustic practices in medicine. Auscultation with a stethoscope is the most basic use of acoustics in medicine and is dependent on the fields of incompressible (circulation) and compressible (respiration) fluid mechanics and frictional mechanics. Detailed discussions of tribology, laminar and turbulent hemodynamics, subsonic and supersonic compressional flow, and surfactants and inflation dynamics are beyond the scope of this document. However, some of the basic concepts of auscultation are presented as a starting point for the study of natural body sounds. Ultrasonic engineers have dedicated over half a century of effort to the development of ultrasound beam patterns and beam scanning methods, stretching the current technical and economic limits of analog and digital electronics and signal processing at each stage. The depth of these efforts cannot be covered in these few pages. However, the basic progression of progress in the fields of transducers and signal processing will be covered. The study of the interaction of ultrasound with living tissues is complicated by complex anatomic structures, the high density of scatterers, and the constantly changing nature of the tissues with ongoing life processes including cardiac pulsations, the formation of edema and intrinsic noise sources. A great deal of work remains to be done on the ultrasonic characterization of tissues. Finally, the effect of ultrasound on tissues, both inadvertent and therapeutic will be discussed. Much of the medical acoustic literature published since 1987 is searchable online, so this document has included key words that will be helpful in performing a search. However, much of the important basic work was done before 1987. In an attempt to help the reader to access that literature, Denis White and associates have compiled a complete bibliography of the medical ultrasound literature prior to 1987. Under Further Reading in this chapter, the reader will find a link to a complete compilation of 99 citations from Ultrasound in Medicine and Biology which list the thousands of articles on medical acoustics written prior to 1987. The academically based authors develop, use and commercialize diagnostic ultrasonic Doppler systems for the benefit of patients with cardiovascular diseases. To translate ultrasonic and acoustic innovation into widespread clinical application requires as much knowledge about the economics of medicine, the training and practices of medical personnel, and the pathology and prevalence of diseases as about the diffraction patterns of ultrasound beams and signal-to-noise ratio of an echo. Although a discussion of these factors is beyond the scope of this chapter, a few comments will help to provide perspective on the likely future contribution of medical acoustics to improved public health.