Tuesday Case of the Day
Physics
Authors: Laurence Parr, MS, DABR1 and Charles E. Willis, PhD, DABR2
1Department
2U.T.
of Radiology, Naval Medical Center Portsmouth, VA
M. D. Anderson Cancer Center, Houston, TX
History: Radiolucent (dark) artifacts were noted by radiologists on images
acquired using several mobile DR x-ray units. In addition, embedded
accelerometers indicated the detectors may have been dropped.
Normal window
Pediatric KUB
70 kVp 5 mAs
Narrow window
Adult Chest
110 kVp 6 mAs
Figure 2. Uniform exposure images showed
numerous similarly sized bright and dark
parallelogram-shaped defects.
Figure 1. Examples of the artifact (red arrows)
are representative of those seen on all images.
The likely cause of the artifact is:
A.Detector damage
B.Objects in the imaging path unrelated to
the detector
C.Out-of-date correction map
D.Incorrectly obtained correction map
Findings:
The detector was recalibrated according to the manufacturer’s
directions.
1. After recalibration, a uniform exposure image acquired without moving the detector showed no
artifacts (Fig. 3).
Figure 3: Uniform Exposure – calibration orientation
Handle Edge
Handle Edge
2. A second uniform exposure image acquired after rotating the detector 180º showed numerous
increased (dark) and decreased (light) signal artifacts (Fig. 4).
Figure 4: Uniform Exposure – rotated180º
Findings:
(continued)
4. The correction map image (Fig. 5) showed focal regions of increased correction values that
correlated with the increased signal artifacts (dark) on the rotated image (Fig. 6).
5.
The decreased signal artifacts (light) did not correlate with the correction map.
Figure 5: Correction Map – rotated 180º
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6. Radial symmetry through the center of the 180º rotated image was noted between the dark and
light artifacts (Fig. 6).
Figure 6: Uniform Exposure – rotated180º
Diagnosis:
B. Objects in the imaging path unrelated to the detector
Discussion:
Reappearance of the artifacts after calibration when the detector was
rotated indicates the source of the artifact is not fixed in position relative
to the detector. If the source of the artifact had been fixed in position
relative to the detector, recalibration of the gain map would compensate
and the artifact would not reappear when the detector was rotated.
This dark artifact is over-correction
at the time of imaging in an area of
the detector where low exposure
existed at the time of calibration.
Physical damage to the detector would have resulted in
artifacts that maintained a fixed orientation with respect to
the detector. Therefore, choice A. Detector damage is
incorrect.
Handle Edge
Recalibration only corrected the problem so long as the
detector orientation was maintained. Therefore, choice C.
Out-of-date correction map is incorrect.
This light artifact is an area of low
exposure on the detector at the time of
imaging where uniform exposure existed
at the time of calibration.
Calibration produced a gain correction map that was
appropriate for the conditions of exposure, as
demonstrated by the initial disappearance of the artifacts.
Therefore, choice D. Incorrectly obtained correction map
is incorrect.
Figure 7: Light artifacts are focal spot images from the inverse pinhole
effect. Dark artifacts are over-compensation in the gain correction map
where focal spot images were present during calibration.
Several small lead filings were found
on the collimator exit window of the
x-ray generator. After the filings were
removed and the detector was
recalibrated, the artifacts no longer
appeared in any detector orientation.
Discussion:
The artifacts are, in fact, images of the focal spot of the x-ray
generator, by means of the inverse pinhole effect.
The artifacts have the
typical “double-banana”
shape of a focal spot
image.
Figure 8: Slit camera image of the focal spot (Jones 2008).
A small amount of high Z material in the x-ray beam path,
such as the Pb filings in the collimator exit window, can
produce the image of the focal spot in the image receptor
plane. This artifact was first reported in diagnostic
radiology by Poznanski (1969).
Figure 9: Xeroradiograph at 60 kVp and 40 mAs of an inverse pinhole
array. The inverse pinholes are tungsten from 75 -300 micron in diameter
arrayed in ten rows from left to right on a sheet of lucite (Cowart 1976). Note
the distortion of the focal spot projection as lateral distance increases from
the central ray of the x-ray beam.
References/Bibliography:
Cowart, RW. An investigation of the inverse pinhole camera. Thesis. University of
Texas Health Science Center at Houston. Graduate School of
Biomedical Sciences. Houston, TX. June 1976. 111 pages.
Jones, AK. Personal communication. 2008.
Poznanski, AK. Focal spot artefacts on breast radiographs. Radiology 92: 644.
1969.