Mindray DC-T6, Operator'S Manual

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17-2 Acoustic Output
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 ”.
17.4 MI/TI Explanation
17.4.1 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.
MI =
P
r,
α
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).
z
TIS: Thermal index related to soft tissues, such as abdominal and cardiac
applications.
z
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.
z
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.