The results of a survey of worst-case in-water values for ISPTA, acoustic power and peak negative pressure, from a wide range of machines in clinical use, have been used with simple "fixed-path" tissue models to estimate worst-case exposure values at the fetus (in situ). The distributions of the estimated in situ values are peaked more toward their low value ends than is the case for distributions of the corresponding in-water values, with the ratio of maximum to median values being higher. Although the upper tails of the aerated ISPTA distributions are not heavily populated, they contain important instances of relatively high values, mainly for lower frequency probes. Spectral Doppler (SD) mode produces the highest in situ estimates for all three parameters, particularly for ISPTA values, which exceed 5000 mW cm-² in a third trimester minimum attenuation model, and 900 mW cm-² in a "typical" attenuation model. The maximum and median values of peak negative pressure do not show particularly large differences between modes for any of the attenuation models. Values up to -3 MPa are predicted for the third trimester minimum path model, and up to -1 MPa for the "typical" attenuation model. The maximum and median values of power are dependent on the mode, with the colour flow imaging (CFI) and SD modes producing the largest values (approximately 200 mW in the third trimester minimum path model).
In a previous publication, more accurate calculations of temperature elevations for a bone target third trimester model were calculated for some of the probes. Extrapolation of these results for different fixed attenuation models suggests that spectral Doppler exposure may produce temperature elevations of around 6°C in minimum attenuation models, and over 1°C in a more "typical" attenuation model. In CFI mode, a worst-case temperature elevation of approximately 1.8°C is estimated for the third trimester, and 1.3°C for the first and second trimesters. For B-mode, the corresponding figures are 1.4 and 1.1°C, respectively. For the case of a "typical" attenuation model, worst-case temperature elevations of approximately 0.3°C are predicted for both B-mode and colour flow imaging modes.
Whittingham TA. Estimated fetal cerebral ultrasound exposures from clinical examinations. Ultrasound in Med. & Biol 2001; 27; 877-882.
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