A comparison of AIUM/NEMA thermal indices with calculated temperature rises for a simple third-trimester pregnancy tissue model

Abstract

Temperature rises due to diagnostic ultrasound exposures have been calculated for a simple third-trimester pregnancy tissue model. This consisted of a layer of soft tissue representing the abdominal/uterine wall, a layer of liquid and a layer of fetal bone. The ultrasound field parameter used in the calculations was the temporal average of the square of the acoustic pressure ( p 2 TA ), measured in water but corrected for attenuation in the tissue model. The three-dimensional (3-D) distribution of p 2 TA was measured for five probes operating in B-mode, and four probes operating in pulsed Doppler and color flow imaging modes. The calculated temperature rises were compared to the AIUM/NEMA-defined thermal indices appropriate to third-trimester scanning. In B-mode, the ratio of calculated temperature rise to thermal index varied between 0.62 and 1.25, with calculated temperature rises as high as 1.4°C. In color-flow imaging mode, this ratio varied between 1.26 and 2.45 and, in pulsed Doppler mode, between 1.46 and 2.92, with calculated temperature rises as high as 1.8°C and 5.8°C, respectively. These results indicate that, for scanning situations where bone is insonated through an overlying low attenuation liquid layer, the thermal index may substantially underestimate the maximum temperature rise that could occur.