SONOTRAX(2009) Series Ultrasonic Pocket Doppler User Manual
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tissue. Although no confirmed adverse mechanical
effects on patients or mammals caused by exposure
at intensities typical of present diagnostic ultrasound
instruments have ever been reported, the threshold
for cavitation is still undetermined. Generally
speaking, the higher the acoustic pressure, the greater
the potential for mechanical bioeffects; the lower the
acoustic frequency, the greater the potential for
mechanical bioeffects.
The AIUM and NEMA formulate mechanical index
(MI) in order to indicate the potential for mechanical
effects. The MI is defined as the ratio of the
peak-rarefactional acoustic pressure (should be
calculated by tissue acoustic attenuation coefficient
0.3dB/cm/MHz) to the acoustic frequency.
MI
=
P
r,
α
f
awf
×
CMI
CMI
= 1 (MPa / MHz )
A4.3.2 TI (Thermal Index)
Heating of tissues is caused by absorption of
ultrasound when the ultrasound energy is applied.
The temperature rise is determined by the acoustic
intensity, exposed area and thermophysical properties
of the tissue.
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