Ultrasound in Medicine (AIUM) measurement standard. The measurements are
then put into algorithms for calculating the displayed output values.
Many of the assumptions used in the process of measurement and calculation
are conservative in nature. Overestimation of actual in situ intensity exposure,
for the vast majority of tissue paths, is built into the measurement and calculation
process. For example:
• The measured water tank values are derated using a conservative, industry
standard, attenuation coefficient of 0.3 dB/cm-MHz.
• Conservative values for tissue characteristics were selected for use in the
TI models. Conservative values for tissue or bone absorption rates, blood
perfusion rates, blood heat capacity, and tissue thermal conductivity were
selected.
• Steady State temperature rise is assumed in the industry standard TI models,
and the assumption is made that the ultrasound transducer is held steady
in one position long enough for steady state to be reached.
A number of factors are considered when estimating the accuracy of the displayed
values: hardware variations, estimation algorithm accuracy, and measurement
variability. Variability among transducers and systems is a significant factor.
Transducer variability results from piezoelectric crystal efficiencies,
process-related impedance differences, and sensitive lens-focusing parameter
variations. Differences in system pulser voltage control and efficiencies is also a
contributor to variability. There are inherent uncertainties in the algorithms used
to estimate acoustic output values over the range of possible system operating
conditions and pulser voltages. Inaccuracies in laboratory measurements are
related to, among others, differences in hydrophone calibration and performance,
positioning, alignment, and digitization tolerances, and variability among test
operators.
The conservative assumptions of the output estimation algorithms of linear
propagation, at all depths, through a 0.3 dB/cm-MHz attenuative medium is not
considered in the accuracy estimate for the display. Neither linear propagation,
nor uniform attenuation at the 0.3 dB/cm-MHz rate, occur in water tank
measurements or in most tissue paths in the body. In the body, different tissues
and organs have dissimilar attenuation characteristics. In water, there is almost
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iE33 User Manual
4535 616 23901
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Safety
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