Herrick, J. F.

Many therapeutic procedures have been introduced into the art and science of medical practice empirically by a method of trial and error. Such a method has marked limitations and may produce unfavorable consequences. The facilities available in experimental medicine permit a thorough and extensive investigation of biological effects of many therapeutic agents or procedures before any clinical application is made. Optimal doses of the therapeutic agent can be approximated and a comparative study of various technics designed for clinical application can be made. Unfortunately ultrasound was used clinically by certain enthusiastic therapeutists before a thorough study of its biologic effects was completed. These therapeutists regarded the major biologic effect of ultrasonic energy to be mechanical. They were aware of a heating effect, but considered that it played a minor role. Experimental measurements have shown clearly that the major effect of ultrasound on living tissues is thermal when an intensity suitable for therapy is employed. The heating of bone by ultrasound is spectacular. No other heating agent used in therapy can raise the temperature of bone to such high levels so quickly. The application of ultrasound for diagnostic purposes and also for instrumentation will be described.

Experimental investigations indicate that ultrasound may prove useful for therapy, for diagnosis, and for biologic measurement. Ultrasound, although it may be dangerous in unskilled hands, is an excellent agent for the structural heating of living tissues. The term “structural heating” is used to denote heating at abrupt interfaces.The living organism presents numerous abrupt interfaces. Ultrasound promises to become a reliable diagnostic tool. Either the transmitted or the reflected ultrasonic energy can be used in this application. Tumors, gallstones, kidney stones, stones in the salivary glands, foreign bodies, and abnormal collections of fluid have been located accurately. Preliminary observations on the use of ultrasound for certain biologic measurements are encouraging. Instrumentation for biologic measurement includes ultrasonic flowmeters, devices for measuring thickness of some tissue structures, and equipment for measuring the viscosity of blood.

Experiments have been carried out to investigate ultrasound as a possible therapeutic agent. These show that selective heating occurs in nerves and bones treated in situ. Effects of direct heat on nerves are strikingly similar to the effects of ultrasound. Blocking of nerves by sound or heat with present techniques seems impractical because the margin of safety is too narrow.

As early as 1932 Freundlich, Sollner, and Rogowski proposed the use of ultrasonics in medicine for diathermy. They were able to demonstrate that bone marrow could be heated. The results of their study indicated clearly that further experiments needed to be done to extend these preliminary observations. We have completed a study of the temperatures produced by ultrasonics in bone marrow, bone, and adjacent tissues in the extremities of the dog. The frequency used in these studies was 800 kilocycles per second. The maximum output of the ultrasonic generator was approximately 15 watts. High temperatures can be produced in bone marrow and bone with relatively low outputs of ultrasonic energy. The adjacent tissues are heated only moderately. Ultrasonics must be used with caution in the proximity of bones because the temperatures produced in bone and bone marrow may rise to dangerous levels within a very few minutes.