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 no attenuation. In the body, and in particular, in water tank measurements, nonlinear
propagation and saturation losses occur as pulser voltages increase.
Therefore, the display accuracy estimates are based on the variability range of transducers and
systems, inherent acoustic output modeling errors, and measurement variability. Display
accuracy estimates are not based on errors in, or caused by measuring according to, the AIUM
measurement standards, or the effects of nonlinear loss on the measured values.
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