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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 (see subsection 20.7). As an operator changes a given system control
(see subsection 20.6), 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 (see subsection 20.2).
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.
20.9 References for MI/TI
For further information on measurement methods and MI/TI, refer to the followings:
(1) "510 (K) Guide For Measuring and Reporting Acoustic Output of Diagnostic
Ultrasound Medical Devices" issued by FDA in 1993
(2) "Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment,
Revision 3" issued by AIUM/NEMA in 2004
(3) "Standard for real-time display of thermal and mechanical acoustic output indices
on diagnostic ultrasound equipment, Revision 2" issued by AIUM/NEMA in 2004
(4) "Bioeffects and Safety of Diagnostic Ultrasound" issued by AIUM in 1993
(5) "Medical Ultrasound Safety" issued by AIUM in 1994
(6) "Information for Manufacturers Seeking Marketing Clearance of Diagnostic
Ultrasound Systems and Transducers" issued by FDA in 1997
(7) 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. 2B730-973EN*D
20-8
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