User's Guide

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5.2 Principles of Using Acoustic Power
5.2.1 Biological Safety
Diagnostic ultrasound is recognized as being safe, but the risk of biological effects exists when using
it in high exposure levels and long exposure times. Thus ultrasound should be used in a prudent
manner to provide medical benefit to the patient.
5.2.2 Mechanical and Thermal Indices
The ultrasound system displays two parts: thermal Index (TI) and Mechanical Index (MI). The MI/ TI
value of the machine is real time displayed at the upper right corner, regarding how to change TI
display type, please choose: Preset → [System Preset] → [TI].
■ Meaning of MI/TI
Mechanical bioeffects are threshold phenomena that occur when a certain level of output is
exceeded. The threshold level varies with tissue type. The potential mechanical bioeffects varies
with peak pressure and ultrasound frequency. The higher the MI value, the greater the likelihood
of mechanical bioeffects occurring. There is no specific MI value theat means that a mechanical
effect is actually occurring. The MI should be used as guide for implementing the ALARA
principle.
The TI value informs the operator about the conditions that might lead to an increase in
temperature at surface of the body, within the body tissue, or at the point of focus of the
ultrasound beam on bone. That is, the TI value informs the operator about the potential
temperature rise in body tissue. It is an estimate of temperature increase in body tissue with
specific properties. The actual amount of an temperature rise is influenced by factor such as
tissue type, vascularity, mode of operation and others. The TI value should be used as a guide
for implementing the ALARA principle. Depending on the examination and type of tissue
involved, TI could be one of three types.
Soft Tissue Thermal Index (TIS) is used when imaging soft tissue only, it provides an estimate
of potential temperature rise in soft tissue.
● Bone Thermal Index (TIB) is used when bone is near the focus of the image as in the third
cropester OB examination, it provides an estimate of potential temperature rise in the bone or
adjacent soft tissue.
● Cranial Bone Thermal Index (TIC) is used when bone is near the skin surface as in transcranial
examination, it provides an estimate of potential temperature rise in the bone or
adjacent soft tissue.
■ Precision of MI/TI
TI and MI values are displayed in real time on the screen. The operator should observe these
index values during examinations and ensure that exposure time and output values are
maintained at minimum amounts needed for effective diagnosis. The MI and TI precision is 0.1.
5.2.3 Acoustic Output Statement
5.2.3.1 The Influencing Factors of Acoustic Uncertainty
When estimating accuracy of displayed numerical values,many factors are considered:
●The probe changeability