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

Temperature rises due to diagnostic ultrasound exposures have been calculat ed for a simple third-trimester pregnancy tissue model. This consisted of a layer of soft tissue representing the abdominal/uterine wall, a layer of l iquid and a layer of fetal bone. The ultrasound field parameter used in the calculations was the temporal average of the square of the acoustic pressu re (p(TA)(2)), measured in water but corrected for attenuation in the tissu e model. The three-dimensional (3-D) distribution of p(TA)(2) was measured for five probes operating in B-mode, and four probes operating in pulsed Do ppler and color flow imaging modes. The calculated temperature rises were c ompared to the AIUM/NEMA-defined thermal indices appropriate to third-trime ster scanning. In B-mode, the ratio of calculated temperature rise to therm al index varied between 0.62 and 1.25, with calculated temperature rises as high as 1.4 degrees C, In color-flow imaging mode, this ratio varied betwe en 1.26 and 2.45 and, in pulsed Doppler mode, between 1.46 and 2.92, with c alculated temperature rises as high as 1.8 degrees C and 5.8 degrees C, res pectively. 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 co uld occur, (C) 1999 World Federation for Ultrasound in Medicine & Biology.