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diagnostic ultrasound exposures and uncertainties in the acoustic properties of soft
tissues. No single tissue model is adequate for predicting exposures in all situations
from measurements made in water, and continued improvement and verification of
these models is necessary for making exposure assessments for specific exam
types.
A homogeneous tissue model with attenuation coefficient of 0.3 dB/cm MHz
throughout the beam path is commonly used when estimating exposure levels. The
model is conservative in that it overestimates the
In Situ acoustic exposure when the
path between the transducer and site of interest is composed entirely of soft tissue.
When the path contains significant amounts of fluid, as in many first and second-
trimester pregnancies scanned transabdominally, this model may underestimate the
In Situ acoustic exposure. The amount of underestimation depends upon each
specific situation.
Fixed-path tissue models, in which soft tissue thickness is held constant, sometimes
are used to estimate
In Situ acoustic exposures when the beam path is longer than 3
cm and consists largely of fluid. When this model is used to estimate maximum
exposure to the fetus during transabdominal scans, a value of 1 dB/cm MHz may be
used during all trimesters.
Existing tissue models that are based on linear propagation may underestimate
acoustic exposures when significant saturation due to non-linear distortion of beams
in water is present during the output measurement.
The maximum acoustic output levels of diagnostic ultrasound devices extend over a
broad range of values:
l
A survey of 1990-equipment models yielded MI values between 0.1 and 1.0 at
their highest output settings. Maximum MI values of approximately 2.0 are
known to occur for currently available equipment. Maximum MI values are
similar for real-time 2D and M Mode imaging.
l
Computed estimates of upper limits to temperature elevations during
transabdominal scans were obtained in a survey of 1988 and 1990 pulsed
Doppler equipment. The vast majority of models yielded upper limits less than
1° and 4°C (1.8° and 7.2°F) for exposures of first-trimester fetal tissue and
second-trimester fetal bone, respectively. The largest values obtained were
approximately 1.5°C (2.7°F) for first-trimester fetal tissue and 7°C (12.6°F) for
386 Appendices