brochure developed by the American Institute of Ultrasound in Medicine (AIUM) and
supplied with ultrasound systems.
Direct Controls
Per 60601-2-37, the ultrasound system control acoustic output to not exceed a mechanical
index (MI) level of 1.9, and I
SPTA.3
of 720 mW/cm
2
or a thermal index (TI) value of 6.0.
The systems’ factory-set defaults, which are user adjustable, are such that when changing
from one preset to another, one transducer to another, or a non-fetal to a fetal application,
the
system selects default MI & I
SPTA.3
values that are significantly lower than 60601-2-37,
Medical
Electrical Equipment Part 2-37: Particular Requirements for the Safety of Ultrasonic
Medical Diagnostic and Monitoring Equipment and the United States Food an
d Drug
Administration’s Guidance Document, “Information for Manufacturers Seeking Marketing
Clearance of Diagnostic Ultrasound Systems and Transducers” limits.
The thermal index that is displayed (TIS, TIB, TIC) is based on the selected application.
The system acoustic output display and controls follow the "Standard for Real-Time Display
of Thermal and Mechanical Acoustic Output Indices on Diagnostic Ultrasound Equipment,
Revision 2” (NEMA UD3).
Indirect Controls
The controls that indirectly affect output are controls affecting the imaging mode,
optimization, probe selection, and depth. The imaging mode determines the nature of the
ultrasound beam. Tissue attenuation is directly related to transducer frequency. The higher
the pulse repetition frequency (PRF), the more output pulses occur over a period of time.
Receiver Controls
The receiver controls (such as Gain, Dynamic Range, etc.) do not affect acoustic output. Use
the receiver controls to improve the image quality before using controls that directly or
indirectly affect output.
General Controls
Transducer: Each transducer has different physical characteristic and acoustic response to
transmit voltages. Differences include size and shape, center frequency, piezo-electric
efficiency, and focus properties of the lens. Changing from one transducer model to another
will change the acoustic output and therefore the displayed mechanical index and thermal
index values.
Mode: Changing from one mode to another will affect the acoustic output. Different modes
use different transmit waveforms, focusing, duty cycles, and pulsing sequences. Doppler
modes tend to have higher duty cycles, and therefore higher TI values, than B and M modes.
Modes using the tightest focusing will tend to have higher MI values.
Defaults: Default settings exist and are accessed whenever a transducer and mode
combination is selected. Although the acoustic output associated with these defaults may
differ from combination to combination, the default acoustic output is lower than 60601-2-37,
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