Measurement of the output of transvaginal probes and correlation with temperature variation in a phantom test
Piero Miloro, Ultrasound and Underwater acoustics, National Physical Laboratory
Background
The Thermal Index (TI) is commonly used as an indicator for the assessment of the thermal hazard. TI definition and the methods for its evaluation are reported in the IEC standard 62359. However, use of TI has been criticized for being too simplistic, not taking into account, for example, the spatial and temporal distribution of heat deposition and transducer self-heating. This latter aspect can be the dominant source of thermal hazard during endocavitary scans.
Method
32 different transvaginal probes from 5 manufacturers were tested in 17 hospitals. The surface temperature with the probe operating in air was measured using an infrared camera and the acoustic output power was measured using a Pyrometer. The results were correlated with temperature variations measured in a phantom, reported in a previous work.
Results
When B-mode was activated for gynaecology and obstetrics pre-sets, the average temperature rise at equilibrium for the probe in air was 5.5 °C (st. dev. 2.5 °C, max 12.5 °C). An average output power of 16.1 mW (st. dev. 7.1 mW, max 41 mW) was measured using the Pyrometer. When compared with the temperature after a sufficiently long exposure in the phantom experiments, the coefficients of determination R2 for linear regression were 0.70, 0.48 and 0.28 (at the surface and at 7 and 14 mm depth within the phantom respectively) for the infrared measurements, and 0.30, 0.30 and 0.36 using the acoustic power (see image below). R2 was always below 0.1 using the Thermal Index.
Conclusions
Both the probe temperature in air and the acoustic output power measurements were better predictors than the TI for the final temperature after a sufficiently long exposure, based on phantom measurements. Goodness of fit decreases with depth using infrared data and increases when output power is used.