Some Factors Influencing the Temperature Distribution in Thighs Exposed to Ultrasound

Abstract: Justus F. Lehmann M.D. Seattle and Ernest W. Johnson M.D. Columbus Ohio Summary and Conclusions No selective rise of temperature was found in the bone of a pig thigh specimen exposed to ultrasound. The absence of the selective rise of temperature in the bone was surprising because it occurred in spite of a greater attenuation of ultrasonic energy in bone than in the overlying soft tissues and in spite of the comparatively low specific heat of the bone. This unexpected result could be explained partially an the basis that a large share of the ultrasonic energy was reflected at the surface of the bone and thus decreased the amount of energy penetrating the bone and increased at the same time the amount of ultrasonic energy converted into heat in the overlying soft tissue structures. This finding also was perhaps partially explained as a result of the comparatively high thermal conductivity of bone. A suggestive selective rise of temperature in subcutaneous fat was partially explained an the Basis of a higher absorption of ultrasonic energy in fat than in the underlying muscle. However this finding seems to be specific for our type of specimen and generalization is not warranted since lower absorption values for subcutaneous fat were reported in the literature. A slight selective rise of temperature in subcutaneous fat might also be explained by the fact that the specific heat of subcutaneous fat is slightly lower than that of the underlying muscle and that its thermal conductivity is approximately half that of muscle. Finally the highest peak of the temperature distribution was found in the superficial layers of the tissues exposed to ultrasound. The location of this peak of temperature was also influenced by the original temperature gradient within the specimen. When this gradient resembled that found in the thighs of human volunteers the peak of the temperature distribution after ultrasonic exposure was found in the deeper layers of the tissues. This experiment suggests that the temperature peak can be transferred into deeper portions of the treated part if the applicator surface or the surrounding coupling medium is cooled during ultrasonic application. ___MH

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