Safety
HD11 XE Getting Started
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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 non-linearly propagating
acoustic energy waveforms experience more distortion, saturation, and absorp-
tion in water than in tissue, where attenuation present all along the tissue path
will dampen the buildup of non-linear 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 deep-
est 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 situa-
tions from measurements made in water, and continued improvement and verifi-
cation 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 under-
estimation depends on each specific situation. For example, when the beam path
is longer than 3 cm and the propagation medium is predominantly fluid (condi-
tions 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, some-
times are used to estimate In Situ acoustical exposures when the beam path is
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