Biological Safety Safety
Lumify Ultrasound System 49
Philips Healthcare 4535 618 58571_A/795 * MAY 2016
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 applications.
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/cm‑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
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