User's Manual

Clarius Ultrasound Scanner - HD3 Scanners Safety Topics
revision 1 55
Because 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)]
Because 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 scanner 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 & 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 scanner 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 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/cm MHz may be used during all trimesters.