15-2 Acoustic Output
15.4 MI/TI Explanation
15.4.1 Basic Knowledge of MI and TI
Mechanical Bioeffect and Thermal Bioeffect
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.
C
MI
= 1 (MPa /
)
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.
TIB: Thermal index for applications, such as fetal (second and third trimester) or neonatal
cephalic (through the fontanel), in which the ultrasound beam passes through soft tissue and a
focal region is in the immediate vicinity of bone.
TIC: Thermal index for applications, such as pediatric and adult cranial applications, in which
the ultrasound beam passes through bone near the beam entrance into the body.
Although the output power is automatically controlled for the selected applications, high TI
values should be kept to a minimum or avoided in obstetric applications. WFUMB (World
Federation for Ultrasound in Medicine and Biology) guidelines: state that temperature increase
of 4 degree C for 5 min or more should be considered as potentially hazardous to embryonic
and fetal tissue.
The smaller the MI/TI values, the lower the bioeffects.
To Next Page
Loading...
