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.
MI
=
P
r,
awf
f
C
MI
C
MI
= 1 (MPa / MHz )
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.
