Subaqueous ultrasound: 45kHz and 1MHz machines compared

This study compared the tissue heating produced by 45kHz and 1MHz ultrasound machines used subaqueously in metal and in plastic basins. An intervention study with cross-over design was used. The setting was a laboratory. Two cross-sections of nonliving pig tissue were used with thermocouples positioned, in the tissues, at different distances along the ultrasound beam axis. Specimens were each exposed subaqueously to ultrasound produced by a 45kHz machine and a 1MHz machine. Both metal and plastic treatment basins were used (2 machines x 2 basin types). Thermocouples measured temperature variations for 30 minutes with each machine on and 10 minutes with it off. Temperature increases at different tissue depths varied with both the ultrasound machine and the type of basin used. The 45kHz frequency ultrasound machine produced a maximum temperature increase of 0.4 degrees C and an initial rate of heating of 0.05 degrees C per minute. The 1MHz machine consistently produced greater temperature increases. These were further increased by 2.65 degrees C (average) if a metal rather than plastic basin was used. The 45kHz machine, operated at maximum output, produced little heating of the irradiated tissue. By contrast, the 1MHz ultrasound unit produced appreciable heating to almost the full depth of the tissue irradiated. Use of a metal basin, rather than plastic, resulted in a markedly higher maximum temperature increase and a significantly greater initial rate of heating.