SonoSite iLook, User Manual

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48
Chapter 5: Safety
Sa
fe
ty
Tissue Models and Equipment Survey
Tissue models are necessary to estimate attenuation and acoustic exposure levels In Situ from
measurements of acoustic output made in water. Currently, available models may be limited in their
accuracy because of varying tissue paths during 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:
• 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).