14-6 Acoustic Output
properties of tissue. Therefore, scanning through highly perfused tissue, such as the
heart or vasculature, will produce significantly less thermal effect than that suggested by
the Thermal Index.
Similarly, the Mechanical Index was derived to indicate the relative possibility of
mechanical (cavitation) effects. The MI is based on the derated peak-rarefactional
pressure and the center frequency of the ultrasound wave. The actual peak-rarefactional
pressure is affected by the actual attenuation caused by tissue in the path between the
transducer and the focal point. Again, all solid tissues within the body have higher
attenuation than the proscribed 0.3 dB/cm/MHz value, and therefore, the actual peak-
rarefactional pressure will be lower. Further, the actual peak-rarefactional pressure will
change depending upon the region of the body being scanned.
For these reasons, the TI and MI displays should only be used to assist the operator in
implementing ALARA at the time of the patient examination.
14.8 Measurement
Uncertainty
Ispta
28.5%
Isppa
28.5%
Center frequency (fc)
2%
Total power (W)
28.5%
(5.1% for Scan-mode and Combined-mode)
Peak-rarefactional pressure
14.7%
14.9 References for Acoustic Power and
Safety
1. “Bioeffects and Safety of Diagnostic Ultrasound” issued by AIUM in 1993
2. “Medical Ultrasound Safety” issued by AIUM in 1994
3. "Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment,
Revision 3" issued by AIUM/NEMA in 2004
4. "Standard for real-time display of thermal and mechanical acoustic output indices on
diagnostic ultrasound equipment, Revision 2" issued by AIUM/NEMA in 2004
5. "Information for Manufacturers Seeking Marketing Clearance of Diagnostic
Ultrasound Systems and Transducers" issued by FDA in 2008.
6. “Medical electrical equipment – Part 2-37: Particular requirements for the safety of
ultrasonic medical diagnostic and monitoring equipment” issued by IEC in 2005.
