1. Computed Radiography (CR), & Digital Radiography (DR) Images
Artifacts on
Computed Radiography (CR),
& Digital Radiography (DR) Images
George David Associate Professor of Radiology
Medical College of Georgia

2. Quick Review of Technology

3. Computed Radiography (CR)
Re-usable metal imaging plates replace film & cassette
                                                                 

4. CR Exposure

5. Reading Imaging Plate
Reader scans plate with laser light using rotating mirror
Plate pulled through scanner by rollers
Light given off by plate measured by PM tube & recorded by computer

6. CR Operation after read-out, plate erased using a bright light
plate can be erased virtually without limit
Plate life defined not by erasure cycles but by physical wear

7. CR Latitude Very large Plate responds to decades of input exposure
Computer scale inputs exposure to viewable densities
               

8. CR Very Sensitive to Scatter

9. Digital Radiography (DR)
Digital bucky incorporated into equipment
Direct digital output
High Lattitude

10. Raw Data Image Unprocessed image as read from receptor
Not a readable diagnostic image
Requires computer processing before presentation as finished radiograph

11. Enhancing Raw Image (Image Segmentation)*
Identify collimated image border
Separate raw radiation from anatomy
Apply appropriate tone-scale to image
Create look-up table (LUT)
Maps pixel value to display gray shade
This process is specific to a particular body part and projection

12. Image Segmentation Define anatomic region
Establish location of collimated border
Define anatomic region
Produce look-up table based on histogram from anatomical area
Body part & projection specific

14. Gross Overexposure
Properly exposed
8X increase in mAs

15. Gross Underexposure
Properly exposed
mAs reduced by ~ 100

16. Artifacts in Digital Radiography

17. Artifact Categories Non-digital-related technical errors
Look-up table / image processing errors
Exposure artifacts
CR artifacts
DR artifacts
Interference artifacts
Display artifacts

18. Radiography Artifacts Digital or Not
Mis-positioning
Motion
Incorrect patient ID
Double exposures
Grid cut-off

19. Radiography No grid 102 cm SID .33R ESE Grid 122 cm SID 2.2 R ESE
No grid 102 cm SID
.33R ESE
Grid 122 cm SID
2.2 R ESE

20. References
“Digital Radiography Artifacts” from Veterinary Radiology & Ultrasound
Wm Tod Drost, David J. Reese, William J. Hornof
“Artifacts in Digital Radiography” from Veterinary Radiology & Ultrasound
David A. Jimenez, Lauara J. Armbrust, Robert T. O’Brien, David S. Biller
Radiographics

21. Look-up Table / Image Processing Errors

22. CR
Diagonal collimation caused algorithm to improperly recognize collimated area
Gonadal shields can fool algorithm
2 exposures on one plate can fool algorithm

23. CR
Chest incorrectly specified by technologist to CR reader as lumbar spine

24. Uberschwinger Artifact
Causes appearance of fine black line halo
Result of implementation of edge enhancement
Can be mistaken for evidence of loosening of orthpedic device
Can mimic pneumothorax

25. More Uberschwinger

27. Clipping Occurs because of applying LUT during pre-processing
Kodak recommends technologists do not window/level on qc workstation.
Raw data: bit image
LUT during pre-processing: Only bits saved
Information may be lost

28. Low Exposure Results in Image Noise
Increasing mAs

29. Exposure Artifact VERY high exposures can result in saturation
Maximum detector response reached
No response to increased dose
Uniformly dark
Cannot be windowed/leveled

30. CR Artifacts Fading Light leak Physical damage to imaging plates
Cracks, scuffs, scratches
Contamination
Dust / dirt
Dirt in reader
Highly sensitive to scatter radiation
Upside-down insertion into bucky

31. CR Artifact: Fading
CR latent image consists of excited electrons stuck in high energy state orbits
Over time if image not read then electrons fall back to ground energy state
Several days required before significant fading occurs

32. CR Artifact: Light Leak in Cassette
CR plates erased by exposure to bright light
Light leaks in cassette can cause partial premature erasure of plate

33. CR Dirt Artifact On imaging plate during reading In reader
Can get on plate
Can be blocked in light guide of reader
White line in direction of plate movement
CR readers require periodic preventive maintenance & cleaning

34. Dirt on CR plate. Right image repeated on screen-film

35. CR
What are white lines?

36. Cracks on image plate due to mechanical wear

37. CR

38. Transport problems in CR readerg M B m c d f l P T u b A / D C p h
Plate Travel

39. CR Plates are Very Susceptible to Scatter
Do not keep plates in room during other exposures
Do not store where likely to receive scatter
Erase plate before use if it has been sitting a long time

40. DR Artifacts Dead detector elements R/F interference
Software correction to a point…
R/F interference
Detector shielded to prevent this
Periodic pattern

41. DR Artifacts: Spatial Variations in Background Signal & Gain
Addressed by acquiring flood and building calibration mask
Similar to uniformity correction in nuclear medicine
Artifact results if attenuator in beam during calibration, then removed
Contrast
Tape

42. Tape artifact seen after tape removed

43. Interferance

44. CR Grid Interference
103 lines / inch grids have same frequency as CR laser scanner. This can cause “Moire” pattern artifact
Align grid lines perpendicular to scan orientation whenever possible
Reduces chances of artifacts caused by laser scanner.

45. Object Interferance
Moire effect because of interference between scan frequency of matrix and spiral

46. Display
Poor calibration
Dead pixels
Backlight issues

47. The End ?