Frequency dependence of threshold ultrasonic dosages for irreversible structural changes in mammalian brain

Abstract

Scribes the ultrasonic threshold dosages for irreversible changes in the mammalian brain with seeming indepen- dence of ultrasonic frequency has provoked several per- tinent queries. The apparent lack of dependence of the threshold region upon frequency raises questions of the resolving power of the experimental method to reveal such subtle dependencies and of the involvement of the various physical mechanisms. An analysis of thermal' processes, which assumes that the maximum tempera- ture developed in the structurally altered volume is de- termined by the tissue absorption coefficient and by the spatial distribution of the acoustic intensity, yields a very weak frequency dependence of the threshold curves in the dosage region beyond 1-sec exposures. 2 Further debates recognize that such threshold data are an essen- tim( element in the establishment of safe operating stan- dards for clinical and other applications. a.4 This paper reports results of a study undertaken to investigate in greater detail the structure of the threshold dosage re- gio n . This is accomplished largely by increasing the number of frequencies at which exposures are made and by obtaining more specimens at each frequency such that greater precision ensues in the identification of the threshold region.