Tuesday Case of the Day
Physics
Authors: Mark P. Supanich, PhD; Phil Rauch, MS; Susan M. Lang, MS;
Michael Flynn, PhD
Department of Radiology, Henry Ford Health System Detroit, MI
HISTORY: X-ray technicians suspect ghosting as the cause of
artifacts overlaying anatomy on 2nd and 3rd images of a whole leg
stitching series on a digital indirect detector system. However,
Images have normal exposure index values of 400, 500 and 500 ,
which seems to obviate ghosting as the cause of the artifact.
What is the best solution to mitigate the artifact?
A. There is no artifact, patient is wearing compression garment(s)
which are constricting the tissue in this image
B. Reduce exposure to eliminate image lag
C. Collimate to exclude areas exterior to the hips and long bones.
D. Increase time between exposures to allow for complete erasure
of detector residual signal
Figure 1: Three image whole leg series with 8 second delays between
acquisitions on an a-Si indirect detector.
FINDINGS: Physicists compared each image of the 3-image series and
found that the outline of the patient’s anatomy in the first image reappeared at
the same detector locations in subsequent images. For the second and third
images, the detector signal was greater in the regions to the right of the outline.
85 kVp
195 mAs
66 kVp
6.5 mAs
66 kVp
3.4 mAs
Figure 2: Images (cropped) which
compose the stitched image in
Figure 1.
FINDINGS: The image of a ruler from another patient with the same artifact
demonstrated that the artifact is due to increased detector signal rather than
signal retention. In Figure 3 a previous exposure of an unattenuated region with a
lead ruler overlay resulted in signal changes in subsequent images. The
phenomenon shown is known as “bright burn” or “after-glow” and results in
increased signal in the highly exposed region.
The bright region on the
number zero has less
signal because this is
where the detector was
masked by the tick mark
on the ruler in the
previous image and thus
the bright burn was not
present. The rest of the
number zero has
increased signal due to
bright burn.
The right portion of the
base of the numbers 2
and 4 have more signal
than that rest of the
number because of the
bright burn phenomenon.
Figure 3: Region of detector receiving high exposure demonstrates bright burn in the subsequent image
Diagnosis:
Correct answer is C: Collimate the exposures to
exclude areas exterior to the hips and long bones.
This approach will protect the unattenuated area of
the detector from the high exposure and eliminate
the manifestation of increased signal in subsequent
images.
DISCUSSION: The artifact noted
by the technicians is due to extreme over-exposure of the
unattenuated areas of the first image, resulting in increased signal in the same detector regions of the
subsequent images. The fundamental cause of the artifact is known as bright burn, which occurs due to
light yield enhancement in the CsI(Tl) scintillator. There are two additional mechanisms that will produce
a subsequent increase in signal in regions subjected to high exposure. These are (1) photodiode
residual signal, and (2) photodiode gain effect. Photodiode residual signal is usually corrected via the
use of reset lights, so it is unlikely that this was a significant factor. However, both bright burn from the
CsI(Tl) and the photodiode gain effect likely contributed to the production of the artifact.
While the resultant Exposure Index (EI) values were indeed normal, the actual exposure technique
required for the first image was quite high, causing the areas of the detector with no overlying tissue to
be exposed to a x-ray fluence. EI is derived from pixel values in the segmented, anatomical portion of
the image, and as this region contained normal pixel values, a normal EI was reported. It is important to
keep in mind that a normal EI does not provide any information about areas outside of the segmented
area. In this case, the detector area which was over-exposed was outside the segmented area and
contributed no information to the EI.
Answer A is incorrect because although the patient was wearing a constricting garment around the
waist in the first image, there was no such garment on the anatomy in the second or third images.
Answer B is not a good solution because reducing the exposure to the level required to eliminate the
artifact would result in an inadequate image requiring re-exposure.
Answer D is incorrect because bright burn has been known to manifest for hours or longer.
The manufacturer has suggested the utilization of their “stitching filters”, which are a form of
equalization filters designed for use in whole spine imaging. The filters are placed lateral to the neck to
mediate high exposures outside the skin line. However, the filters did not match the anatomy of the
patient in the current whole leg study.
A final alternative might be to acquire the images in reverse order, saving the highest exposure image
for last. Unfortunately, the system did not allow acquisition from the bottom up.
References/Bibliography:
• Wieczorek, H and Overdick, M. Afterglow and Hysteresis in CsI:Tl. Proc. 5th
International Conf on Inorganic Scintillators and their Applications 2000; 385-390.
•Yorkston, J. Recent Developments in Digital Radiography Detectors. Nuclear
Instruments and Methods in Physics Research A 2007; 974-985.
•Overdick, M et al. Temporal Artefacts in Flat Dynamic X-Ray Detectors. Proc of SPIE
Vol 2001; 4320:47-58.
•Chotas, HG et al. Principles of Digital Radiography with Large-Area, Electronically
Readable Detectors: A Review of the Basics. Radiology 1999; 210:595-99.
•McDermott, LN et al. Dose Response and Ghosting of an Amorphous Silicon Electronic
Portal Imaging Device. Med Phys 2004; 31:285-95.
•Bloomquist, AK et al. Lag and Ghosting in a Clinical Flat-Panel Selenium Digital
Mammography System. Med Phys 2006; 33:2998-3005.