User guide
Chapter 8: Acoustic Output 125
Acoustic Output
The displayed MI and 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.
Transducer surface temperature rise
Table 4 and Table 5 list the measured surface temperature rise from ambient (23°C ± 3°C) of
transducers used on the ultrasound system. The temperatures were measured in accordance with
EN 60601-2-37 section 42 with controls and settings positioned to give maximum temperatures.