Development of a Thermal Test Object for the Measurement of Ultrasound Intracavity Transducer Self-Heating

Abstract

The elevated surface temperature of diagnostic ultrasound transducers imposes an important limitation to their safe use in clinical situations. Moreover, particular care should be taken if transvaginal transducers are to be used during routine scans in the first few weeks of pregnancy as the transducer surface can be very close to embryonic/fetal tissues. Published results have shown that the heating of tissue due to transducer self-heating can equal and often exceed the acoustic heating contribution. In this article, we report the development of a portable self contained thermal test object (TTO) capable of assessing the self-heating of intracavity diagnostic ultrasound transducers. The thermal conductivity and volumetric heat capacity of the tissue mimicking material (TMM) used in the TTO were measured, yielding values of (0.56 ± 0.01) W m −1 K −1 and (3.5 ± 0.8) MJ m −3 K −1. The speed of sound of the TMM was measured as 1540 m s −1 and the attenuation over a frequency range of 2 to 10 MHz was found to be (0.50 ± 0.01) dB cm −1 MHz −1. These results are in excellent agreement with the International Electrotechnical Commission (IEC 60601–2-37) requirements and the previously published properties of biological soft tissue. The temperature stability and uniformity, and suitability of the TTO for the measurement of transducer self-heating were tested and found to be satisfactory. The TTO reached a stable temperature of 37°C in 3 h and the spatial variation in temperature was less than ± 0.2°C. Lastly, transducer self-heating measurements from a transvaginal transducer exceeded the IEC temperature limit of 43°C in less than 5 min and the temperature reached after 30 min was 47.3°C. (E-mail: a.killingback@sgul.ac.uk)