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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 hermal 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.
1.2.3 Acoustic Output Statement
1.2.3.1 The Influencing Factors of Acoustic Uncertainty
When estimating accuracy of displayed numerical values,many factors are considered:
●
The probe changeability
●
The system changeability
●
Changeability and accuracy of measurement
●
Possible operating conditions and testing numbers needed to obtain displayed result accuracy
of the
diagnostic system
●
Whether the display accuracy depends on specific system combination, mode combination,
probe component
and launch mode combination, or all of above
●
Algorithm accuracy of the system software used to calculate the MI/TI
●
Approximation engineering method used in real time computation
1.2.3.2 Differences between Actual and Displayed MI and TI
For many assumptions used in the process of measurement and calculation, actually they are
conservative.
For most organizations path, high estimate is made in the measurement and calculation
process of tissue
exposure intensity. For example, using attenuation coefficient 0.3dB cm
MHz1
much lower than the actual
human tissue attenuation coefficient, choosing conservative values
of tissue characteristic. Therefore, displayed
MI and TI values should be relative information for
reference, they serve to indicate to the operator whether a
particular setting of the system increases
or decreases the possibility of Thermal or Mechanical effect, used to
help the operator be careful to
use ultrasonic diagnostic system and follow the ALARA principle, these values
can not be equal to
actual values.
1.2.3.3 Uncertainty of Measurement
Sound pressure is the most basic data of sound field measurement, and other sound field parameters
can be
deduced from sound pressure, so when analysing measurement uncertainty, only take sound
pressure for
analysis and uncertainty of other parameters can be deduced from the sound pressure.