Computation of “worst-case” bioeffects indices and pulse-average intensity values based on acoustic output data submitted to the FDA

The authors recently have concluded an analysis of diagnostic ultrasound acoustic output data processed by the FDA during 1990 and 1991. As part of this analysis, “worstcase” models for transabdominal fetal exposures (assuming a “fixed-path” derating factor with an attenuation coefficient of 1 dB/MHz) have been used to calculate estimates of maximum temperature elevations and mechanical indices, and the results have been compared to corresponding calculations of the mechanical (MI), soft tissue thermal (TIS), and bone thermal (TIB) indices defined in the “Standard for Real-Time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment.” In this standard an attenuation coefficient of 0.3 dB/cm-MHz is used. In ≈75% of the approximately 100 cases examined, the TIB was within a factor of two of the corresponding worst-case value (i.e., worst-case value <2·TIB, and in over 95% of the cases the TIS and MI were within a factor of two of the corresponding worst-case values. In addition, maximum values of the derated spatial-peak, pulse-average intensity (I_SPPA.3) have been computed assuming that the FDA regulatory output limit was based on an MI of 1.9 rather than an I_SPPA.3 of 190 W/cm². Under this condition, and assuming that pulse shape would remain constant as pulse amplitude is increased, I_SPPA.3 values of up to 2300 W/cm² were calculated