Acoustic Output
23-2
23.3 ALARA Principle (As Low As
Reasonably Achievable)
It is required to practice ALARA when using ultrasound energy. Practicing ALARA
ensures that the total energy level is controlled below a low enough level at which
bioeffects are not generated while diagnostic information is being accumulated. The total
energy is controlled by output intensity and total radiation time. The output intensity
necessary for examinations differs depending on the patient and the clinical case.
Not all examinations can be performed with an extremely low level of acoustic energy.
Controlling the acoustic level at an extremely low level leads to low-quality images or
insufficient Doppler signals, adversely affecting the reliability of the diagnosis. However,
increasing the acoustic power more than necessary does not always contribute to an
increase in quality of information required for diagnosis, rather increasing the risk of
generating bioeffects.
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”.
23.4 MI/TI Explanation
23.4.1 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
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