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Users must take responsibility for the safety of patients and utilize ultrasound deliberately.
Deliberate use of ultrasound means that output power of ultrasound must be selected
based on ALARA.
Additional information regarding the concept of ALARA and the possible bioeffects of
Ultrasound is available in a document from the AIUM (American Institute of Ultrasound
Medicine) title "Medical Ultrasound Safety".
14.4 MI/TI Explanation
14.4.1 Basic Knowledge of MI and TI
The relationship of various ultrasound output parameters (frequency, acoustic pressure
and intensity, etc.) to bioeffects is not fully understood presently. It is recognized that two
fundamental mechanisms may induce bioeffects. One is a thermal bioeffect with tissue
absorption of ultrasound, and another one is a mechanical bioeffect based on cavitations.
Thermal Index (TI) gives the relative index of temperature increase by thermal bioeffect,
and Mechanical Index (MI) gives the relative index of mechanical bioeffect. TI and MI
indices reflect instantaneous output conditions, so they DO NOT consider the cumulative
effects of the total examination time. TI and MI models contain practical simplifications to
complex bioeffects interaction. Then the operator should be aware that the actual worst
case temperature rise may be up to several times higher than the displayed TI value.
MI (Mechanical Index):
The mechanical bioeffects are the result of compression and decompression of
insonated tissues with the formation of micro bubbles that may be referred to as
cavitations.
MI is an index that shows the possibility of the cavitations generation based on
acoustic pressure, and the value in which the peak-rarefactional acoustic pressure is
divided by the square root of the frequency. Therefore MI value becomes smaller
when the frequency is higher or the peak-rarefactional acoustic pressure is lower, it
becomes difficult to generate the cavitations.
For the frequency 1 MHz and the peak-rarefactional acoustic pressure 1 MPa, MI
becomes 1.It is possible to think MI to be one threshold of the cavitations generation.
Especially, it is important to keep MI value to be low when both gases and the soft tissues
exist together, for such as lung exposure in cardiac scanning and bowel gas in abdominal
scanning.
TI (Thermal Index):
TI is determined by the ratio of the total acoustic power to the acoustic power
required to raise the tissue temperature by 1 degree C. In addition, because the
temperature rises is greatly different according to tissue structures, TI is divided three
kinds: TIS (Soft-tissue Thermal Index), TIB (Bone Thermal Index) and TIC (Cranial-
bone Thermal Index).
TIS: Thermal index related to soft tissues, such as abdominal and cardiac
applications.
14-2 Acoustic Output
MI =
P
r, α
× C
f
awf
MI
= 1 (MPa / MHz )
C
MI
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