Thursday Case of the Day
Physics
Authors: Charles E. Willis, PhD1, Ho-Ling Liu, PhD2, and Mei-Yu Yeh, MS3
1U.T.
M. D. Anderson Cancer Center, Houston, TX
2Chang Gung University, Taoyuan, Taiwan
3Chang Gung Memorial Hospital-Chia Yi, Taiwan
History: An AP view of the pelvis was acquired using a flat-panel digital
radiography (DR) system. The problem was not corrected by
flat-field calibration.
(anonymization)
What is the source of the artifact in the
upper right-hand quadrant of the image?
A.
B.
C.
D.
E.
Ghost image
Defective x-ray conversion layer
Incorrect dead pixel map
Incorrect gain and offset calibration
Hardware failure of TFT array
Diagnosis:
E. Hardware failure of TFT array
Discussion:
Flat-panel DR detectors are subject to a number of failure modes.
Each selection represents a possible cause of artifacts in DR images.
Artifacts arise from different stages of formation of the digital image.
Isolation of the root cause requires consideration of each failure mode
and the expected effects on the image.
Discussion:
A. Ghost image
Both direct and indirect DR systems can exhibit “ghost”
images, that is, evidence of previous exposures1. There is
a distinction between “ghosting”, which is a change in the
sensitivity of the x-ray converter after exposure to x-rays,
and “lag”, which is an effective increase in dark current in
the absence of x-rays2.
Figure 1: DR image of sensors in the x-ray beam on the left;
no sensors in the x-ray beam for image on the right. Is this
an example of “ghosting” or “lag”?
In the upper right quadrant of the AP view of the pelvis,
anatomic features are properly aligned with features in
other quadrants. The artifactual non-uniformity in the
upper quadrant does not have the radiographic
appearance of other human anatomy or an inantimate
object from a previous exposure. Therefore, it is
unlikely to be a result of ghosting or lag.
Discussion: B. Defective x-ray conversion layer.
conversion layer
Indirect DR systems rely on a layer of material to
convert x-rays into visible light2. This layer is typically
composed of Gd2O2S or CsI. The x-ray conversion
layer is either bonded to an array of photodiodes (each
element of which is connected to an element of a TFT
array) or optically coupled to a CCD array. Defects and
degradation in the conversion layer, and defects in the
bonding or optical coupling can manifest as artifacts in
the digital image.
Figure 2: Indirect DR detector configuration
(courtesy J. A. Rowlands and Wei Zhao)
In the upper right quadrant of the AP view of the
pelvis, anatomic features do not appear less sharp
compared with similar features in other quadrants.
Close inspection shows horizontal lines that are
more suggestive of electronic rather than optical
origin. Therefore, it is unlikely to be a result of a
defective x-ray conversion layer.
Discussion:
C. Incorrect dead pixel map
.
A digital image is comprised of an array of discrete
picture elements, or pixels. The pixels correspond to
discrete detector elements, or “dels”. For any DR
detector, a subset of these dels are non-functional, or
“dead”. Every DR system has a software method for
locating dead pixels and correcting the digital image
to compensate for their presence.
There is nothing in the upper right quadrant of the AP
view of the pelvis that resembles the shotgun-like
pattern of random dead pixels. Therefore, it is unlikely
that the non-uniformity is caused by an incorrect dead
pixel map.
Figure 3: DR image without dead pixel correction
Discussion:
D. Incorrect gain and offset calibration
DR detector elements are inherently different with
respect to signal gain and signal offset. All DR
systems have software to measure and modify gain
and offset of each individual detector element in
order to equalize their response to a uniform field of
x-rays3.
Figure 4: Raw DR image above illustrates gain and offset differences.
The particular DR system in this case uses
four detector elements tiled together. The image on the
left is the same type of DR detector with slight differences
in gain among the quadrants4. Flat-field calibration is
easily able to correct such non-uniformities. Therefore, it
is unlikely that the artifact was caused by incorrect gain
and offset correction.
Discussion:
E. Hardware failure of TFT array
In this indirect DR system, charge from each
photodiode element is collected and read out by
an element of a thin film transistor (TFT) array. In
our case, the gross non-uniformity could not be
eliminated by flat-field calibration. The entire DR
detector had to be replaced.
Figure 5: The non-uniformity above was eliminated by flat-field calibration.
References/Bibliography:
1. Siewerdsen, J. H., Jaffray, D. A. A ghost story: Spatio-temporal response
characteristics of an indirect-detection flat-panel imager. Medical Physics. 26:
1624-1641.1999.
2. Yorkston J. Flat-panel DR detectors for radiography and fluoroscopy. In:
Specifications, Performance Evaluations, and Quality Assurance of
Radiographic and Fluoroscopic Systems in the Digital Era. Goldman LW and
Yester MV eds. Madison, WI: Medical Physics Publishing.177-228. 2004.
3. Chotas HG, Dobbins JT III, and Ravin CE. Principles of Digital Radiography with
Large-Area Electronically Readable Detectors: A Review of the Basics.
Radiology. 210:595-599. 1999.
4. Willis CE, Thompson SK and Shepard SJ. Artifacts and Misadventures in Digital
Radiography. Applied Radiology 33(1):11-20, January 2004.