Thursday Case of the Day What is the source of the artifact in the upper right-hand quadrant of the image? 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. Authors : Charles E. Willis, PhD 1, Ho-Ling Liu, PhD 2, and Mei-Yu Yeh, MS 3 1 U.T. M. D. Anderson Cancer Center, Houston, TX 2 Chang Gung University, Taoyuan, Taiwan 3 Chang Gung Memorial Hospital-Chia Yi, Taiwan Physics (anonymization) A.Ghost image B.Defective x-ray conversion layer C.Incorrect dead pixel map D.Incorrect gain and offset calibration E.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.

Both direct and indirect DR systems can exhibit “ghost” images, that is, evidence of previous exposures 1. 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-rays 2. 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: A. Ghost image 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”?

Indirect DR systems rely on a layer of material to convert x-rays into visible light 2. This layer is typically composed of Gd 2 O 2 S 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. 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: B. Defective x-ray conversion layer. Figure 2: Indirect DR detector configuration (courtesy J. A. Rowlands and Wei Zhao) conversion layer

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. Discussion: C. Incorrect dead pixel map. Figure 3: DR image without dead pixel correction

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-rays 3. 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 quadrants 4. 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: D. Incorrect gain and offset calibration Figure 4: Raw DR image above illustrates gain and offset differences.

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. Discussion: E. Hardware failure of TFT array 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: 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 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: Willis CE, Thompson SK and Shepard SJ. Artifacts and Misadventures in Digital Radiography. Applied Radiology 33(1):11-20, January 2004.