2 – 26 HS40 Service Manual
A 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
a steady state to be reached.
A number of factors are considered when estimating the accuracy of display values:
Hardware deviation, algorithm accuracy, and measurement deviation. Deviation among
probes and systems in particular is an important factor. Probe deviation 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 the
calculation of the accuracy estimate displayed. Neither linear propagation nor uniform
attenuation at the 0.3dB/cm-MHz rate occurs in underwater 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
underwater 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 the measurement variability. Display
accuracy estimates are measured according to AIUM measurement standards but not based
on errors caused during the measurement or inherent errors. They are also independent of
the effects of non-linear loss on the measured values.
2.5.1.6. Control Effects – Controls Affecting the Indices
As various system controls are adjusted, the TI and MI values may change. This will be most
apparent as the Power control is adjusted; however, other system controls will also affect the
on-screen output values.
■ Power
Power controls the system acoustic output. Two real-time output values are on the screen: a
TI and a MI. They change as the system responds to Power adjustments.
In combined modes, such as simultaneous Color, 2D mode, and Pulsed Doppler, the total TI
is the sum of TIs of the individual modes. Each mode is a vital contributor to this total; the
displayed MI will be from the mode with the largest peak pressure.
2.5.1.7. 2D Mode Controls
■ 2D Mode Size
Narrowing the sector angle may increase the frame rate. This will increase the TI. Pulse
voltage may be automatically adjusted down with software controls to keep the TI below the
system maximum. A decrease in pulse voltage will decrease MI.
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