Estimation of in situ ultrasound exposure during obstetric examinations.

Layered tissue models are developed to estimate in situ ultrasound intensity during common obstetric examinations by incorporation of measured overlying tissue thicknesses with data on the attenuation properties of tissues. Results are compared with attenuation models recommended by the United States Food and Drug Administration (FDA) and National Council on Radiation Protection and Measurements (NCRP). For abdominal and transvaginal scanning in the first trimester, fixed attenuation models based on attenuation values of 1.2 dB/MHz and 0.6 dB/MHz, respectively, are representative of worst-case exposure conditions. For second and third trimesters, a fixed attenuation value of 0.8 dB/MHz is representative of worst-case exposure conditions. A fixed attenuation value of 1.0 dB/MHz is suggested for common Doppler examinations. The study suggests that the FDA-derating factor of 0.3 dB/cm.MHz may not give a conservative estimate of in situ intensity for certain obstetric examinations.