46 Chapter 5: Safety
Safety
Direct Controls
The selection of exam type limits acoustic output through default. The acoustic output parameters
that are set at default levels based on exam type are the mechanical index (MI), thermal index (TI),
and the spatial peak temporal average intensity (ISPTA). The system does not exceed an MI and TI
of 1.0 for all exam types. It does not exceed 720 mW/cm
2
for all exam types.
Indirect Controls
The controls that indirectly affect output are controls affecting imaging mode, freeze, and depth. The
imaging mode determines the nature of the ultrasound beam. Tissue attenuation is directly related
to transducer frequency. The higher the PRF (pulse repetition frequency), the more output pulses
occur over a period of time.
Receiver Controls
The receiver controls are the gain controls. Receiver controls do not affect output. They should be
used, if possible, to improve image quality before using controls that directly or indirectly affect
output.
Output Display
The system meets the AIUM output display standard for MI and TI (see last reference listed in
Related Guidance Documents below). The system and transducer combination do not exceed an MI
or TI of 1.0 in any operating mode. Therefore, the MI or TI output display is not required and is not
displayed on the system for these modes.
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
UD2-1998.
• 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, American Institute of Ultrasound in Medicine, 1998.
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