User guide

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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:
• 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.
• 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
second-trimester fetal bone. Estimated maximum temperature elevations given here are for a
“fixed path” tissue model and are for devices having I
SPTA
values greater than 500 mW/cm
2
.
The temperature elevations for fetal bone and tissue were computed based on calculation
procedures given in Sections 4.3.2.1-4.3.2.6 in “Bioeffects and Safety of Diagnostic Ultrasound”
(AIUM, 1993).
Acoustic output tables
Table 6 through Table 39 indicate the acoustic output for the system and transducer combinations
with a TI or MI equal to or greater than one. These tables are organized by transducer model and
imaging mode. For a definition of terms used in the tables, see “Terms used in the acoustic
output tables” on page 163.