Chapter 1 Safety 1 - 29
the measurement and calculation process. For example, the measured water tank values are
derated using a conservative, industry standard, attenuation coefficient of 0.3dB/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.
A steady state temperature rise is assumed in the industry standard TI models, and the
assumption is made that the ultrasound probe is held steady in one position long enough for a
steady state to be reached.
A number of factors are considered when estimating the accuracy of display values: hardware
variations, algorithm accuracy estimation and measurement variation. Variation among probes
and systems is a significant factor. Probe variability results from piezoelectric crystal efficiencies,
process-related impedance differences, and sensitive lens focusing parameter variations.
Differences in the system pulse voltage control and efficiencies are also a contributor to variability.
There are inherent uncertainties in the algorithms used for estimating acoustic output values over
the range of possible system operating conditions and pulse voltages. Inaccuracies in laboratory
measurements are related to differences in hydrophone calibration and performance, positioning,
alignment and digitization tolerances, as well as variation among test operators.
The conservative assumptions of the output estimation algorithms of linear propagation, at all
depths, through a 0.3dB/cm-MHz attenuated medium are not taken into account in calculation of
the accuracy estimate displayed. Neither linear propagation, nor uniform attenuation at the
0.3dB/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 particularly in water tank measurements, non-linear
propagation and saturation losses occur as pulse voltages increase.
The display accuracy estimates take into account the varying ranges of probes and systems,
inherent acoustic output modeling errors, and measurement variations. Display accuracy estimates
are not based on errors in, or caused by measuring according to, the AIUM measurement
standards. They are also independent of the effects of non-linear loss on the measured values.
▐Control Effect - Control Affecting the indices
As various system controls are adjusted, the TI and MI values may change. This will be most apparent
as the POWER control is adjusted; however, other system controls will affect the on-screen output
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