Appendix F Acoustic Output Reference
Instructions for Use F - 3
Display Accuracy and Measurement Uncertainty
When a transducer is capable of exceeding a mechanical index (MI) or thermal index (TI) of
1.0, the ultrasound imaging system displays indices starting from 0.4 in increments of 0.1 for all
displayed values.
A number of factors contribute to the estimation error for the displayed index. Variations among
the following factors contribute to the index display error: transducers and systems,
assumptions and approximations for real-time index calculations, and measurement
uncertainty. The MI and TI display values should be interpreted as relative information and only
used to assist the operators to implement the ALARA (As Low As Reasonably Achievable)
principle for patient examinations; the acoustic output should be the lowest level required to
satisfactorily perform the examination.
Measurement uncertainty for acoustic pressure, acoustic intensity, power, and center frequency
is within 12%, 25%, 10% and 5% respectively assessed by considering both the systematic and
random uncertainties. The total estimated display accuracy is ±25% for MI and ±50% for TI.
Definitions for these parameters can be found in the IEC 62359, "Ultrasonics – Field
Characterization – Test Methods for the Determination of Thermal and Mechanical Indices
Related to Medical Diagnostic Ultrasonic Fields."
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