DIAGNOSTIC IMAGING
of points of higher attenuation results which has
the appearance of straight or curved lines on the
resultant image (Figure 6).
This artefact typically occurs with stationery grids
with a low grid line rate/frequency, or malfunction
of an oscillating grid. Several actions can be taken
to remedy this artefact, depending on what grid is
used, but includes: using an oscillating grid (Potter-
Bucky), using a grid with high ratio, ensuring
correct perpendicular alignment of the grid to the
sampling direction, or increasing exposure times
(if an oscillating grid is already used).
advantages. Some of this is done automatically
by the workstation. A histogram, or a graphic
representation of all of the opacities on the image,
is generated and analysed by means of a lookup
table (LUT). LUTs are designed to display optimal
grayscale, contrast, and detail, and are based
on specific diagnostic regions. The LUT for an
7a
Workstation artefacts
Faulty transfer
After image acquisition in CR, the data from the IP
plate needs to be transferred to the workstation.
Any disruption in this process can result in
misplacement of distortion of body parts or
regions of interest on the resultant image. Parallel
streaks, elongation, or replacement of portions of
the image with areas that are completely black
or white can occur. Parts of the radiograph may
also be incorrectly localised on the image, or
duplicated or superimpose over each other, or
pixels may be missing. Stable data transfer, a stable
connection and reliable power source is required
to prevent this artefact.
Border detection
In CR, there is no direct communication
between the x-ray tube and the cassette, and
thus the workstation does not “know” how
much collimation has occurred or information
about the image size. Thus, the workstation is
automated to detect the collimated margin and
crop the image accordingly, however if this is
done incorrectly may lead to omission of parts
of the image or inclusion of the area outside the
primary x-ray field. This has a significant effect on
post-processing, as the regions excluded from
the image are not processed. This artefact usually
occurs at the margin of highly attenuating objects/
structures, with an off-centred object of interest,
and when the IP is rotated even slightly relative to
the collimated field.
Clinically, it is most commonly seen when
trying to obtain multiple images on the same
cassette, which is typically not a problem in film/
screen radiography. If this artefact is present,
then processing on the workstation needs to be
performed, either by removing border delineation
or by reprocessing the image with deactivated
border detection.
Diagnostic specifier
“Post-processing”, or the ability to alter the
image after obtaining it, is one of CR’s major
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Issue 02 | MAY 2018 | 20
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Figure 7: Selection of the incorrect lookup table (LUT). The
two images were obtained using identical exposure factors
and are of the same patient, but with different LUTs selected.
The image on the top (a) was obtained using an “abdomen”
LUT, while the image on the bottom (b) was obtained with the
correct “thorax” LUT. Note that the “abdomen” LUT results in
loss of the peripheral soft tissues in the image, as well as loss
of the pulmonary detail, especially noticeable in the caudal
lung lobes, and overall gives a higher contrast image which is
undesirable in the thorax.