Biological Safety Safety
Lumify Ultrasound System 43
Philips Healthcare 4535 618 58571_A/795 * MAY 2016
The MI and TI display accuracy estimates for the system are given in Acoustic Output Tables, on
your User Information CD. Those accuracy estimates are based on the variability range of
transducers and systems, inherent acoustic output modeling errors, and measurement
variability, as discussed in this section.
The displayed values should be interpreted as relative information to help the system operator
achieve the ALARA principle through prudent use of the system. The values should not be
interpreted as actual physical values in interrogated tissue or organs. The initial data that is
used to support the output display is derived from laboratory measurements based on the
American Institute of 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.
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