Constant soft tissue distance model in pregnancy.

The thickness of various underlying tissue layers were measured at locations where the overall thickness and, therefore, the ultrasound attenuation prior to the fetus appeared minimized. Identification of a worst case, or a small group of potentially high exposure situations, is necessary for the optimal design and use of ultrasound equipment. Appropriate measurements in these cases allow calculation of estimated intensities in situ for comparison with future actual measurements. While other types of examinations, for example, intraoperative imaging with a saline or water standoff, provide even less overlying attenuation, obstetrical applications are more numerous and warrant attention. Earlier estimates of attenuation of tissues overlying the fetus centered on first trimester pregnancy, and were summarized by the NCRP (1983). Those measurements used phase-sensitive ultrasound receivers and yielded relatively high transducer-to-gestational sac pressure losses. Attenuation estimates in the first and last trimester were suggested by Carson (1988), based on an informal search of worst cases in a few clinical examinations. These and the NCRP estimates yielded a total minimum attenuation at 3.5 MHz of 3.9 dB in the first trimester and 1.6 dB in the third trimester, from overlying tissue of thicknesses of 2.6 and 1.0 cm, respectively. These estimates, being lower than earlier estimates and measurements, suggested a need for specific worst case measurements. The second trimester cases was studied in this current work because of the expected fetal sensitivity to thermal and other insults at this stage and because the overlying tissue was expected to be thin relative to the first trimester.