Fetal depth and ultrasound path lengths through overlying tissues.

Measurements of minimum thicknesses, in a four-layer, overlying tissue model, on 22 pregnancies between 15 and 20 weeks gestation, yielded a global minimum and a mean of the minimum total thickness per patient of 1.7 and 2.9 cm, respectively, and a minimum and mean subcutaneous fat thickness of 0.7 and 1.4 cm. Conservative calculations of the minimum attenuation per patient, at 3.5 MHz indicated that less than 2.5% of 15 to 20 week pregnancies should fall below the lower 95% prediction line of: Attenuation (dB) = 0.10 X Maternal Weight (kg) - 3.0. The smallest calculated attenuation for any of the 21 subjects was 0.8 dB MHz-1 indicating just under a factor of two protection at 3.5 MHz of proximal fetal tissues compared with ultrasound intensities measured in water. This value is lower than those generally used in the past. The knowledge of distributions of transducer-to-fetal distances and thicknesses of overlying tissues is also important for improvement of image quality. Measurement of minimum depth of the anterior fetal thorax in 57 examinations of 25 to 40 week fetuses yielded minimum and mean values of 2.5 and 4.1 cm, respectively.