DIAGNOSTIC IMAGING
abdomen, thorax, or extremity, are all different
and application of the incorrect one to a region
of interest (for example, selecting thorax when
imaging the abdomen) will result in decreased
overall quality, and modification of opacity,
contrast, or detail. To mitigate this artefact,
manual processing can be performed at the
viewing station, or the image reprocessed with the
correct LUT lookup table at the workstation (i.e.
selecting thorax for a thoracic study). Repeating
the radiograph is usually unnecessary (Figure 7 a
and b).
Clipping
Raw CR files are usually obtained in 12 to 14 bits,
resulting in very large files that are cumbersome
to transfer. Thus, they generally are “clipped” to 10
or 12 bits by discarding some information, but can
result in complete darkening of areas of higher x-ray
exposure. This is different to “saturation” artefact,
which is as result of overexposure. Clipping occurs
within the normal or correct dynamic range of
the x-ray, and is as result of post-processing and
does not occur during the acquisition phase. This
artefact can be remedied at the workstation, but
once the images are sent to the viewing station,
cannot be corrected as the lost information
cannot be retrieved.
Density threshold
When objects of very high density, such as metallic
implants, are included in histogram analysis and
application of the LUT, the displayed grayscale is
widened to include these objects. The remaining
biologic tissues appear dark and have decreased
contrast between them because of this widened
greyscale and thus decreased contrast levels.
Application of a “density threshold” will allow
these structures to be excluded, by processing the
image such that anything above a certain density
w ill be excluded from the histogram, and appear
white. This allows biological tissues (the region of
actual interest on the radiograph) to be displayed
with the correct contrast levels and opacity.
Uberschwinger, “rebound” or “halo” artefact
This artefact occurs as a thin black line surrounding
objects of higher attenuation (such as metallic
implants), which runs evenly and parallel to
the object and are adjacent to areas of lower
attenuation (for example, a metallic screw placed
within bone for fracture repair) (Figure 8).
The radiolucent zone around the implant may be
mistaken for osteolysis, which would lead one to
the erroneous diagnosis of implant loosening or
infection. In contrast to osteolysis, uberschwinger
is of even thickness and surrounds all metallic
implants on the radiograph, versus more irregular
or focal lucencies of infection or loosening.
Conclusion
Identification of and pinpointing the source and type
of CR artefacts is important to prevent diagnostic
errors with the possibility of medicolegal implications.
At best, the decreased image quality is merely an
irritation to the veterinarian evaluating the image.
Several artefacts are due to operator error, and thus
proper training of the staff acquiring the images is
necessary, as well as careful use of the equipment and
regular maintenance.
Figure 8: Note the fine radiolucent zone surrounding the
surgical plate and each screw. The fact that it extends into
the interosseous space between the radius and ulna, and
is present dorsal to the plate, also helps to identify this as
an artefact rather than lysis.
References
1. Drost WT, Reese DJ, Hornof WJ. Digital radiography artifacts. Vet
Radiol Ultrasound. 2008;49:48–56.
2. LO WY, PUCHALSKI SM. Digital Image Processing. Vet Radiol Ultra-
sound [Internet]. 2008;49:S42–S47. Available from: http://doi.wiley.
com/10.1111/j.1740-8261.2007.00333.x
3. Jiménez DA, Armbrust LJ. Digital Radiographic Artifacts. Vet Clin
North Am - Small Anim Pract. 2009;39:689–709.
4. Shetty CM, Barthur A, Kambadakone A, Narayanan N, Kv R.
Computed radiography image artifacts revisited. Am J Roentgenol.
2011;196:37–47.
5. Jiménez D a., O’Brien RT, Wallace JD, Klocke E. Intraoperative con-
trast-enhanced ultrasonography of normal canine jejunum. Vet Radiol
Ultrasound. 2011;52:196–200.
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