Gammex 404GS LE, Руководство пользователя

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17
System Linearity
Ultrasound systems use special processing circuits to translate the amplitude of echoes
into brightness levels on the video monitor. These circuits use mathematical functions
that often produce an S-shaped curve when graphed. As shown in the figure below,
each echo level produces a corresponding brightness level on the monitor.
An S-shaped curve is used to translate echo levels into brightness levels on the video
display. Notice how each echo level, –X, 0 and +X dB produce the corresponding bright-
ness values B–, Bo and B+.
As long as the shape of the curve remains constant, the contrast, or difference in
brightness between different echo levels, will remain constant. If the shape of the
S-curve changes, the relative image brightness for each echo level will also change.
For example, image post-processing techniques help the user identify subtle tissue
variations by modifying the shape of the S-curve to emphasize certain ranges of echo
levels. Degradation in the system hardware can also affect the shape of the curve and
produce unexpected variations in the contrast between echo levels. The distortion in the
information displayed to the user may affect the interpretation of the ultrasound image.
This is why it is important to do quality measurement with as little post-processing as
possible. An example of these distortions is shown on the following page.