A calibration study of therapeutic ultrasound units.

BACKGROUND AND PURPOSE Physiological effects of therapeutic ultrasound (US) are dependent on the intensity and duration of application. The purpose of this study was to test US machines used in clinical settings for proper calibration of time and power output. METHODS Measurements of power output and timer accuracy were obtained from 83 US units in clinical use. The machines were tested at 4 intensity settings (0.5, 1.0, 1.5, and 2.0 W/cm2) using a continuous waveform and a 1-MHz frequency. The measured intensities were converted to percentages of error and compared with the +/-20% standard. RESULTS Of the machines tested, 32 (39%) were outside the calibration standard for at least one output setting. Of these machines, 15 (18%) were above the +20% standard, and 17 (21%) were below the -20% standard for at least one output setting. Of the 32 machines outside the standard, 26 (31%) were outside the standard for 2 or more settings, and 3 (4%) produced no output at any of the settings. Of the mechanical timers tested, 7 (28%) were outside of the +/-10% standard for timer accuracy at the 5-minute interval, and 6 (24%) were outside of the standard at the 10-minute interval. All digital timers tested were within the standard. DISCUSSION AND CONCLUSION More than one third of machines tested in this study were outside the standard for power output, and approximately one fourth of the mechanical timers were outside the standard. Therefore, further improvements in the accuracy of US machine calibration are needed.