The possibility of hazard in medical and industrial applications of ultrasound

Over the past ten years ultrasound has become well established as the basis of a varied and expanding group of medical diagnostic techniques. It is also used therapeutically in a wide variety of conditions and has found further extensive application in industrial processing. The basis for its therapeutic use is that ultrasound is known to be capable, under certain conditions, of modifying or even destroying living tissue: it therefore seems important to have a clear understanding of the factors that determine whether any potentially harmful forms of biological change may arise in the course of any of the applications to which ultrasound is being put at present, or for which it may be contemplated in the future. This article reviews not only the somewhat fragmentary evidence at present available that has specific bearing on the possible hazards of current ultrasonic applications, but also the equally relevant but more fundamental question of the nature of the biophysical mechanisms of action of ultrasound. The term ultrasound is used to describe mechanical vibrations at frequencies above the human limit of audibility (about 16 kHz; 1 Hz = l cycle persecond). Ultrasound can be generated and detected at frequencies up to at least 2,000 MHz, but in considering current medical and industrial practice an upper limit of 10 MHz may be taken. Within this range (16 kHz–10 MHz) sound exhibits important differences, in a number of characteristics, from those found in the audible range.