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(3) Differences between displayed and "actual" Thermal and Mechanical effects
In operation, the system will display to the operator the Acoustic Output
Parameters Thermal Index, TI, or Mechanical Index, MI (or sometimes both
parameters simultaneously). These parameters were developed as general
indicators of risk from either thermal or mechanical action of the ultrasound wave.
They serve to indicate to the operator whether a particular setting of the system
increases or decreases the possibility of Thermal or Mechanical effect. More
specifically, they were designed to assist in the implementation of the ALARA
principle (refer to subsection 24.1.6 "Reminder"). As an operator changes a given
system control (refer to subsection 24.1.3 "Parameters affecting the MI/TI values"),
the potential effect of the change in output will be indicated. However, the Thermal
Index is not the same as temperature rise in the body, for several reasons. First of
all, in order to provide a single display index to the user, a number of simplifying
assumptions had to be made. The biggest assumption was the use of the
attenuating formula described above, which is much lower that the actual value for
most tissues within the body. Scanning through muscle or organ tissue, for
example, will produce much higher attenuation than 0.3 dB/cm/MHz. There were
also significant simplifications made for the thermal 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 (refer to subsection
24.2.2 "MI/TI display description"). 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.
24.1.8 References for MI/TI
For further information on measurement methods and MI/TI, refer to the followings:
(1) "Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment,
Revision 3" issued by AIUM/NEMA in 2004
(2) "Standard for real-time display of thermal and mechanical acoustic output indices
on diagnostic ultrasound equipment, Revision 2" issued by AIUM/NEMA in 2004
(3) "Bioeffects and Safety of Diagnostic Ultrasound" issued by AIUM in 1993
(4) "Medical Ultrasound Safety" issued by AIUM in 1994
(5) "Information for Manufacturers Seeking Marketing Clearance of Diagnostic
Ultrasound Systems and Transducers" issued by FDA in 2008
(6) World Federation for Ultrasound in Medicine and Biology (WFUMB), Conclusions
and Recommendations on Thermal and Non-thermal Mechanisms for Biological
Effects of Ultrasound. Report of the 1996 WFUMB Symposium on Safety of
Ultrasound in Medicine. Barnett, S.B. (ed). Ultrasound in Medicine and Biology,
Vol 24, suppl 1, 1998.
No. 2B771-004EN*M
24-8
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