Comparison of heating of nonliving soft tissue produced by 45 kHz and 1 MHz frequency ultrasound machines.

Abstract

Ultrasound of 1 MHz frequency is used routinely for patient treatment because it can penetrate to and heat deeply located tissue. Benefits are claimed for newly introduced 45 kHz ultrasound machines, yet little evidence is available to confirm these benefits. This paper compares claims supporting the therapeutic use of 45 kHz ultrasound with those supporting the traditional MHz frequencies. The results of an empirical study comparing the heating effects are also presented. Thermocouples placed at different tissue depths in nonliving pig tissue measured temperature variations over a 40-minute period. Direct heating produced by the 45 kHz ultrasound machine was very superficial. By contrast, the 1 MHz ultrasound machine produced direct heating at all tissue depths. The results confirmed that although 45 kHz ultrasound machines may be useful for heating superficial tissues, they have limited or no use in heating deeper tissues. For treatment of deeply located tissue, 1 MHz ultrasound should continue to be used.