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Biological Safety
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
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 Output Tables
Acoustic output tables are in Acoustic Output Tables, on your User Information CD.
Acoustic Measurement Precision and 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.
EPIQ 7 User Manual 4535 617 25341
Safety
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