FujiFilm SONOSITE X-PORTE, User Manual

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10-12 Acoustic output
The maximum derated and the maximum water values do not always occur at the same operating
conditions; therefore, the reported maximum water and derated values may not be related by the
In Situ
(derated) formula. For example: a multi-zone array transducer that has maximum water value intensities in
its deepest zone, but also has the smallest derating factor in that zone. The same transducer may have its
largest derated intensity in one of its shallowest focal zones.
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 ISPTA
values greater than 500 mW/cm2. 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).