A homogeneous tissue model with an 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 the site of interest is composed
entirely of soft tissue, because the attenuation coefficient of soft tissue is generally
higher than 0.3 dB/cm-MHz. 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 acoustical exposure. The amount of
underestimation depends on each specific situation. For example, when the beam
path is longer than 3 cm and the propagation medium is predominantly fluid
(conditions that may exist during transabdominal OB scans), a more accurate
value for the derating term is 0.1 dB/cm-MHz.
Fixed-path tissue models, in which soft tissue thickness is held constant,
sometimes are used to estimate in situ acoustical 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/MHz may be used during all trimesters.
The maximum acoustic output levels of diagnostic ultrasound devices extend
over a broad range of values:
• A survey of 1990-equipment models yielded mechanical index (MI) values
between 0.1 and 1 at their highest output settings. Maximum MI values of
approximately 2 are known to occur for currently available equipment.
Maximum MI values are similar for real-time 2D, M-mode, PW Doppler,
and Color flow imaging.
• Computed estimates of upper limits to temperature elevations during
transabdominal scans were obtained in a survey of 1988 and 1990 PW
Doppler equipment. The vast majority of models yielded upper limits less
than 1°C and 4°C for exposures of first-trimester fetal tissue and
second-trimester fetal bone, respectively. The largest values obtained were
approximately 1.5°C for first-trimester fetal tissue and 7°C for
second-trimester fetal bone. Estimated maximum temperature elevations
given here are for a “fixed-path” tissue model and are for devices having
Ispta (derated) values greater than 500 mW/cm
2
. The temperature elevations
for fetal bone and tissue were computed based on calculation procedures
iU22 User Manual
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Safety
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