SonoSite 180PLUS Guide D'utilisation page 14

Factors that Contribute to Display Uncertainty
The net uncertainty of the displayed indices is derived by combining the quantified uncertainty from
three sources; measurement uncertainty, system and transducer variability, and engineering
assumptions and approximations made when calculating the display values.
Measurement errors of the acoustic parameters when taking the reference data are the major source
of error that contributes to the display uncertainty. The measurement error is described in the section
on Acoustic Measurement Precision and Uncertainty included in the system user guide.
The displayed TI values are based on calculations that use a set of acoustic output measurements that
were made using a single reference ultrasound system with a single reference transducer that is
representative of the population of transducers of that type. The reference system and transducer are
chosen from a sample population of systems and transducers taken from early production units, and
they are selected based on having an acoustic output that is representative of the nominal expected
acoustic output for all transducer/system combinations that might occur. Of course every transducer/
system combination has its own unique characteristic acoustic output, and will not match the
nominal output on which the display estimates are based. This variability between systems and
transducers introduces an error into displayed value. By doing acoustic output sampling testing
during production, the amount of error introduced by the variability is bounded. The sampling
testing ensures that the acoustic output of transducers and systems being manufactured stays within
a specified range of the nominal acoustic output.
Another source of error arises from the assumptions and approximations that are made when
deriving the estimates for the display indices. Chief among these assumptions is that the acoustic
output, and thus the derived display indices, are linearly correlated with the transmit drive voltage of
the transducer. Generally, this assumption is very good, but it is not exact, and thus some error in the
display can be attributed to the assumption of voltage linearity.
Related Guidance Documents
• Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems
and Transducers, FDA, 1997.
• Medical Ultrasound Safety, American Institute of Ultrasound in Medicine (AIUM), 1994. (A copy is
included with each system.)
• Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment, NEMA UD2-2004.
• Acoustic Output Measurement and Labeling Standard for Diagnostic Ultrasound Equipment,
American Institute of Ultrasound in Medicine, 1993.
• Standard for Real-Time Display of Thermal and Mechanical Acoustic Output Indices on Diagnostic
Ultrasound Equipment, NEMA UD3-2004.
• Guidance on the interpretation of TI and MI to be used to inform the operator, Annex HH, BS EN
60601-2-37 reprinted at P05699.
8 Chapter 2: Safety