ESTIMATION OF INSITU ULTRASOUND EXPOSURE DURING OBSTETRIC EXAMINATIONS

Layered tissue models are developed to estimate in situ ultrasound int ensity during common obstetric examinations by incorporation of measur ed overlying tissue thicknesses with data on the attenuation propertie s 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 m odels based on attenuation values of 1.2 dB/MHz and 0.6 dB/MHz, respec tively, are representative of worst-case exposure conditions. For seco nd and third trimesters, a fixed attenuation value of 0.8 dB/MHz is re presentative of worst-case exposure conditions. A fixed attenuation va lue of 1.0 dB/MHz is suggested for common Doppler examinations. The st udy suggests that the FDA-derating factor of 0.3 dB/cm . MHz may not g ive a conservative estimate of in situ intensity for certain obstetric examinations.