In vivo validation of 3D transperineal ultrasound estimates of prostate motion during radiotherapy
Piero Miloro1, Suzanne Beattie-Jones2,3, Lee Wing Fai3, Christoph Lees4, 1Ultrasound and Underwater acoustics National Physical Laboratory, 2Queen Charlotte’s and Chelsea Hospital, 3Imperial College Healthcare NHS Trust, 4Imperial College London
Background
Transvaginal examinations, particularly of the first trimester fetus, are safety-critical due to the close proximity of sensitive tissues to the transducer that can produce heat during use. However, knowledge of how heat is generated and transferred to tissues remains limited, and the monitoring of thermal hazards mostly relies on a derived parameter, the Thermal Index (TI).
Method
We carried out a registered Service Evaluation on 24 patients and measured the temperature rise during transvaginal scans, using fine wire thermocouples (75 µm spatial resolution), secured to the surface of the transducer. Two scanners were used (GE Voluson E8 with a RIC 6D transducer and Samsung WS80 with a V5-9 probe). With the thermocouple in place, no imaging artefacts were detected. Where possible, examinations were recorded to DVD. Generated images were anonymised and analysed using custom-developed software, which was able to extract the values of the TI, and the active imaging mode and status. This information was correlated with the recorded temperature.
Results
The average duration of the scans was 7 minutes 15 seconds (minimum 3, maximum 18 minutes). The majority of the scans last around 4 minutes. The average peak temperature was 36.3°C, minimum 34.1°C, maximum 38°C, with most of the scans showing a value around 37.5°C, with no significant difference shown between the machines. When the video was available, it was possible to correlate the temperature curves to the active mode and TI.
Conclusions
The measured temperature at the probe surface never approached values reported as hazardous in the international standards. For an average body temperature of 36.5°C, the most frequent temperature rise was around 1°C. Results of duration and maximum temperature are in line with previous studies. Doppler modes were often correlated with a faster increases in temperature and higher temperatures being achieved.