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other qualified ultrasound users, using good judgment and insight, determine the exposure
that is “as low as reasonably achievable.” There are no set rules to determine the correct
exposure for every situation.
The qualified ultrasound user determines the most appropriate way to keep exposure low
and bioeffects to a minimum, while obtaining a diagnostic examination.
Users must be responsible for the safety of patients, to the use of ultrasonic, the output
power of ultrasonic to choose according to the principle of ALARA.
Other information about the ALARA principle and the potential biological effects of
ultrasound, can see the Research Institute of medical ultrasound (AIUM) published the
"medical ultrasound safety" file.
9.4
MI/TI description
9.4.1
Basic knowledge of MI and TI
MI: An indication of the likelihood of mechanical bioeffects occurring: the higher the MI, the
greater the likelihood of mechanical bioeffects.
TI: The ratio of total acoustic power to the acoustic power required to raise tissue
temperature by 1°C under defined assumptions.
⚫
MI (Mechanical index):
The mechanical effect is caused by the acoustic wave within the organization micro
bubbles, increasing vibration and the results diabrosis, this reaction is called cavitation.
MI said the possibility of pressure cavitation effect, MI value is obtained by the square root
of the peak negative pressure divided by the frequency. Therefore, the higher the frequency
or peak negative pressure is low, the MI value is smaller, so the possibility of generating
cavitation effect is smaller.
MI
=
P
r,
awf
f
C
MI
C
MI
= 1 (MPa
/
MHz
)
When the frequency is 1 MHz and the peak pressure is 1 MPa, the MI value is 1 MI can be
considered as a threshold for cavitation. When gas and soft tissue exist at the same time
to set the MI value to the low value.
⚫
TI (Thermal index):
TI is determined by the total acoustic power and the tissue temperature rise of 1 degrees
Celsius required sound power ratio. In addition, the organizational structure of different
temperature is different, so the TI is divided into three categories: TIS (soft tissue thermal
index (TIB), and TIC (bone thermal index) skull thermal index).
