Samsung ACCUVIX XG Uživatelský manuál stáhnout pdf (Strana 46)

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Operation Manual

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

coefficient 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 significant factor. Probe variability results from piezoelectric crystal efficiencies,

process-related impedance differences, and sensitive lens focusing parameter variations.

Differences in the system pulse voltage control and efficiencies 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 differences 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,

different tissues and organs have dissimilar attenuation characteristics. In water, there is almost no

attenuation. In the body, and particularly in water tank measurements, non-linear propagation and

saturation losses occur as pulse voltages increase.

The display accuracy estimates take into account the variability ranges of probes and systems,

inherent acoustic output modeling errors, and measurement variability. Display accuracy estimates

are not based on errors in, or caused by measuring according to, the AIUM measurement standards.

They are also independent of the effects of non-linear loss on the measured values.

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Samsung ACCUVIX XG Uživatelský manuál Strana 46