Samsung SONOACE R5 Manual do Utilizador descarregar pdf (Página 41)

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Chapter 1 Safety

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Mechanical and Thermal indices Display Precision and Accuracy

The Mechanical and Thermal Indices on the system are precise to 0.1 units.

The MI and TI display accuracy estimates for the system are given in the Acoustic Output Tables

manual. These accuracy estimates are based on the variability range of probes and systems, inherent

acoustic output modeling errors and measurement variability, as described below.

The displayed values should be interpreted as relative information to help the system operator

achieve the ALARA principle through prudent use of the system. The values should not be interpreted

as actual physical values investigated tissue or organs. The initial data that is used to support the

output display is derived from laboratory measurements based on the AIUM measurement standard.

The measurements are then put into algorithms for calculating the displayed output values.

Many of the assumptions used in the process of measurement and calculation are conservative in

nature. Over-estimation of actual in situ exposure, for the vast majority of tissue paths, is built into

the measurement and calculation process. For example:

The measured water tank values are de-rated using a conservative, industry standard, attenuation

coeicient of 0.3dB/cm-MHz.

Conservative values for tissue characteristics were selected for use in the TI models. Conservative

values for tissue or bone absorption rates, blood perfusion rates, blood heat capacity, and tissue

thermal conductivity were selected.

Steady state temperature rise is assumed in the industry standard TI models, and the assumption is

made that the ultrasound probe is held steady in one position long enough for steady state to be

reached.

A number of factors are considered when estimating the accuracy of display values: hardware

variations, algorithm accuracy estimation and measurement variability. Variability among probes

and systems is a signicant factor. Probe variability results from piezoelectric crystal eciencies,

process-related impedance dierences, and sensitive lens focusing parameter variations.

Dierences in the system pulse voltage control and eciencies are also a contributor to variability.

There are inherent uncertainties in the algorithms used for estimating acoustic output values over

the range of possible system operating conditions and pulse voltages. Inaccuracies in laboratory

measurements are related to dierences in hydrophone calibration and performance, positioning,

alignment and digitization tolerances, and variability among test operators.

The conservative assumptions of the output estimation algorithms of linear propagation, at all

depths, through a 0.3dB/cm-MHz attenuated medium are not taken into account in calculation of

the accuracy estimate displayed. Neither linear propagation, nor uniform attenuation at the 0.3dB/

cm-MHz rate, occur in water tank measurements or in most tissue paths in the body. In the body,

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Samsung SONOACE R5 Manual do Utilizador Página 41