Clarius Scanner HD3 Safety Topics
revision 9 58
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 scanners extend over a broad
range of values:
• A survey of 1990-equipment models yielded 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 B-Mode, M-mode, and
PW Doppler.
• Computed estimates of upper limits to temperature elevations during transabdominal
scans were obtained in a survey of 1988 and 1990 Doppler equipment. The vast majority
of models yielded upper limits less than 1°C and 4°C (1.8°F 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 scanners having I
spta
(derated) 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 through 4.3.2.6 in
“Bioeffects and Safety of Diagnostic Ultrasound” (AIUM Report, January 28, 1993).
Acoustic Measurement Precision & Uncertainty
All table entries have been obtained at the same operating conditions that give rise to the
maximum index value in the first column of the tables. Measurement precision and
uncertainty for power, pressure, intensity, and center frequency are listed in the following
tables.
Measurement precision on the following quantities is determined by making
repeated measurements and stating the standard deviation as a percentage.
ACOUSTIC MEASUREMENT PRECISION
Quantity Precision
(Percentage Standard Deviation)
Pr is the underated peak rarefactional pressure measured in megapascals (MPa) Pr: 5.4%
Wo is the ultrasonic power in milliwatts (mW) 6.2%
f
c
is the center frequency in megahertz (MHz) (NEMA UD-2 definition) <1%
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