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24. Using MI/TI
24.1 Using MI/TI (Outside the USA and Canada)
24.1.1 Basic knowledge of MI/TI
(1) Concerns with bioeffects
Studies have revealed that ultrasound with extremely high intensity is harmful to
body tissues. This rapid advance has generated concerns about the potential risk
of bioeffects when new applications or diagnostic technologies become available.
(2) Mechanical effects and thermal effects
Studies have revealed that two different kinds of ultrasound characteristics affect
human bodies: one is the mechanical effects due to cavitation generated when
the negative pressure of ultrasound exceeds a certain limit, and the other is the
effects due to thermal energy generated when tissues absorb ultrasound. The
levels of these two kinds of effects are represented by indices: the MI (Mechanical
Index) and the TI (Thermal Index), respectively.
(3) MI/TI values
The smaller the MI/TI values, the less the bioeffects.
(4) Controlling the ultrasound output
In 1976, the FDA in the USA established limits to the acoustic output level for
diagnostic ultrasound equipment (TRACK1), along with the rapid advance in
diagnostic ultrasound technology, and the versatile proliferation of its applications,
in order to ensure the safety of patients. Thereinafter, manufacturers were
required to control the acoustic output level below the limits, when releasing new
diagnostic ultrasound systems on the market.
NOTE:
I
( = 0.3dB/cm-MHz) (IEC 60601-2-37).
Application
Peripheral blood vessels
Cardiovascular systems
Fetal and other
Eyes
3 may be represented as I
spta.
Maximum acoustic power specified by FDA (TRACK1)
No. 2B771-004EN*M
24-1
( = 0.3dB/cm/MHz) or I
zpta,
Ispta.3
Isppa.3
2
(mW/cm
)
(W/cm
720
190
430
190
94
190
17
28
spta,
MI
2
)
1.9
1.9
1.9
0.23
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