27.08.2014
When ultrasound is used for therapeutic purposes, much higher acoustic output power is generally needed than for diagnostic applications. Therefore, necessarily high electrical input power often causes a temporal change in the characteristics of the ultrasonic transducer as the transducer is heated. Currently, a reference setup with selected devices and a feedforward control software has been accomplished at PTB which has proven to significantly improve the temporal constancy of ultrasonic fields.
Stable operating conditions are essential for the reliable and reproducible characterization of ultrasound sources. For many therapeutic ultrasound applications, this is a difficult task, since the required high electric input power may heat the ultrasound transducer during operation and thus change its electrical properties.
A particular scenario in which this leads to problems, is secondary calibrations (e. g. of acoustic power meters or hydrophones), in which measurements obtained by the device that shall be calibrated are compared with measurements obtained by a calibrated device, which of course requires the same conditions for both measurements.
A second example is bioeffect studies in which biological effects in tissue are correlated with acoustic field values. For this purpose, usually the acoustic value of interest (e. g. the negative peak pressure or the temporal average intensity) is measured in a first step and, in a second step, the biological effect after sonicating the tissue under the same conditions is evaluated.
Currently, it is being investigated at PTB whether and how the stability of therapeutic ultrasound fields can be improved. For this purpose, initially devices that are as stable as possible for signal generation, as well as two therapeutic ultrasound transducers (a transducer for high intensity therapeutic ultrasound applications (HITU) and a transducer for physiotherapeutic treatments), were selected. In Figure 1, the portable designed setup is shown. Furthermore, a feedforward control algorithm was implemented, which continuously monitors the input voltage to the transducer, using computer-controlled stabilization.
Figure 1: Therapeutic Ultrasound Reference Sources setup including devices for signal creation and voltage measurement (in the rack)), a laptop for the feedforward control software and one of the two selected ultrasound transducers (in the water vessel in the background).
With the use of this setup, it was then checked whether the stabilization of the input voltage also results in more stable acoustic field values. In the diagrams shown in Figure 2 it is clearly visible that the stabilized input voltage (red curve in the upper diagram; black curve: unstabilized input voltage for comparison) also leads to a much more stable sound pressure amplitude (lower diagram).
Figure 2: Upper diagram: relative deviation of the stabilized input voltage (red curve) in comparison with the unstabilized input voltage (black).
Lower diagram: corresponding relative deviation of the ultrasound pressure amplitude.
Furthermore, bimonthly ultrasonic field measurements and power measurements were performed to check the long-term stability of the setup over a period of one year. The scattering of the values measured over one year was 5 % (field measurements) and 2% (power measurements), which is reasonably low.
Contact persons:
Julian Haller, Dept. 1.6, WG 1.62, e-mail: julian.haller@ptb.de
Volker Wilkens, Dept. 1. 6, WG 1. 62, e-mail: volker.wilkens@ptb.de