The biological effects of ultrasound and its detection and measurement

Abstract

The aim of the dissertation is to try and relate the various effects of high frequency sound waves on biological materials to the parameters of the wave and, also, to indicate how these parameters can be measured. A brief survey of the early work done in this field is followed by a discussion of the various absorption mechanisms which are responsible for the observed biological effects. This consists of two sections. The first deals with the absorption of ultrasound in cellular suspensions and intact tissue that causes purely thermal effects. These effects are characteristic of waves of small amplitude The second section deals with higher amplitude waves which, although they cause heating effects, can also cause other more dramatic changes The mechanisms considered are the mechanical stresses exerted on cells and the altered permeabilities of cell membranes due to the acoustic streaming effects of the direct beam and microstreaming effects due to the presence of bubbles or inclusions of resonant size. Also, the mechanical effects and production of free radicals associated with collapse cavitation are described. Then, follows a discussion of the possible absorption mechanisms in intact tissue. The various applications of ultrasound in medicine are mentioned. These are for diagnostic and diathermic procedures and in the treatment of Parkinson's and Menieres diseases. A discussion of the various parameters that should be measured in order to try and explain the observed biological effects is then made and this is followed by a description of a number of techniques that are available for this purpose. The parameters described are the frequency; transducer output; intensity distribution; temperature rise; physical characteristics of the media and the occurrence of cavitation and measurement of its intensity. The conclusion sums up the rather unsatisfactory position of this field at the present time.