A plane layered model to estimate in situ ultrasound exposures.

Determination of in situ ultrasound exposures in experimental animals and patients is necessary to assess any potential hazard of ultrasound implied by reported biological effects in animals. A layered model was used to calculate total attenuation of ultrasound pulses as they passed through tissue layers to an organ site, thus enabling calculation of in situ intensities from free-field transducer emissions. The model assumes plane homogeneous layers of tissue of known thickness, attenuation and acoustic impedance. Calculations based on the model, compared with in situ measurements in a human cadaver specimen, were accurate to within approximately +/- 3 dB. We then calculated in situ clinical exposures using published obstetric B-scans. Total attenuation to the fetus averaged approximately 11 dB at 3.5 MHz.