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APPENDIX 2 ULTRASOUND INTENSITY AND SAFETY
1.1 ULTRASOUND IN MEDICINE
The use of diagnostic ultrasound has proved to be a valuable tool in medical practice.
The ultrasound output of the device is controlled internally and cannot be changed by
the user during the inspection process. Although no confirmed bioeffects on patients
caused by exposure from present diagnostic ultrasound equipment have ever been
reported, the potential exists that such bioeffects may be identified in the future.
Therefore, the ultrasound should be used prudently. High levels of acoustic output and
long exposure time should be avoided while acquiring necessary clinical information.
1.2 EXPLANATION OF MI/TI
1.2.1 MI (MECHANICAL INDEX)
When ultrasound waves penetrate and contact tissues, cavitation effects may occur,
causing local instantaneous high temperatures. The occurrence of this effect depends
on various factors and has a threshold phenomenon. Currently, there are no reports of
harmful mechanical effects from human use of ultrasound diagnostic equipment, and
the threshold for cavitation effects is unclear. As the peak sound pressure of ultrasound
waves increases, the occurrence rate of mechanical effects increases, but decreases
with increasing frequency. The American Institute of Ultrasound in Medicine and the
National Electrical Manufacturers Association have established the Mechanical Index (MI)
to characterize the probability of ultrasound mechanical effects.
1.2.2 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.
According to different thermophysical properties of the tissue, TI is divided into three kinds:
TIS, TIB and TIC.
TIS (Soft Tissue Thermal Index): It provides an estimate of potential temperature rise in soft
or similar tissues.
TIB (Bone Thermal Index): It provides an estimate of potential temperature rise when the
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