Philips EPIQ 7 User Manual page 62

Safety Biological Safety
Since the ultrasonic path during an examination is likely to pass through varying lengths and
types of tissue, it is difficult to estimate the true in situ intensity. An attenuation factor of 0.3 is
used for general reporting purposes; therefore, the In Situ value which is commonly reported
uses the formula:
In Situ derated = Water [e ]
-0.069lf
Since this value is not the true in situ intensity, the term "derated" is used.
Mathematical derating of water based measurements using the 0.3 dB/cm-MHz coefficient,
may yield lower acoustic exposure values than would be measured in a homogenous
0.3 dB/cm-MHz tissue. This is true because nonlinearly propagating acoustic energy waveforms
experience more distortion, saturation, and absorption in water than in tissue, where
attenuation present all along the tissue path will dampen the buildup of nonlinear effects.
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 may have its largest derated intensity in
one of its shallowest focal zones.
Conclusions Regarding 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. Presently, available models may be limited in
their accuracy because of varying tissue paths during diagnostic ultrasound exposures and
uncertainties in acoustical 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 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
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