23
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The system changeability
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Changeability and accuracy of measurement
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Possible operating conditions and testing numbers needed to obtain displayed result accuracy of the
diagnostic system
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Whether the display accuracy depends on specific system combination, mode combination , probe
component and launch mode combination, or all of above
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Algorithm accuracy of the system software used to calculate the MI/TI
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Approximation engineering method used in real time computation
5.2.3.2 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
-1
MHz
-1
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.
5.2.3.3 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.
Measurement uncertainty mainly include repeated measurement uncertainty and the system
uncertainty, the system uncertainty is an order of magnitude higher than repeated measurement
uncertainty, so the main analysis is the system uncertainty.
Mainly decided by the following
factors:
1. The sensitivity of hydrophone:According to hydrophone calibration report provided by ONDA
company, the maximum allowable error of sound pressure for hydrophone is plus or minus 12%;
2. Scope: according to agilent DSO6502A specifications, its effect on the sound pressure is plus or
minus 2%;
3. Temperature: effect of the thermocouple on sound pressure error is plus or minus 4%;
Above all uncertainty components are not related, synthetic standard uncertainty of sound pressure
is :plus or minus 13%.
5.2.4 Operator Control Property
There are three types of operation control related to the generation of mechanical/thermal effect:
direct control and indirect control, receiver control. Qualified operator should try to cut down the
acoustic output in the premise of effective diagnostic images.
■ Direct control The direct control of the acoustic output of this system is adjusting voltage size. But
its maximum acoustic output shouldn't be more than displayed acoustic output limit in any modes.
■ Indirect control
The controls that indirectly affect output are many imaging parameters. These are operating modes,
frequency, focal point number/position, image depth and pulse repetition frequency (PRF)(By
adjusting the [Scale] of the toolbar).
The operating mode determines whether the ultrasound beam is scanning or non-scanning. Thermal
effect is closely connected to M Mode, PW Doppler and Color Mode.
Acoustic attenuation of tissue is directly related to transducer frequency.
The focal point number and position is related to active aperture of transducer and beam width.
For the pulse repetition frequency( PRF)(By adjusting the [Scale] of the toolbar), the higher the PRF,
