Center frequency of the transducer/system/mode combination
(MHz)
=f
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.
61
iU22 User Manual
4535 614 45861
2
Safety
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