Radiation Protection in Digital Radiology

1. Radiation Protection in Digital Radiology Avoiding Artefacts in Computed Radiography L06

2. Educational Objectives • Explain how the CR image is created • Explain how errors in the process can produce sub-standard images • Artefacts in CR Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

3. A radiographic projection can be captured using photostimulable phosphor (PSP) plates • X-rays contribute energy to electrons by the photoelectric effect when it strikes the PSP plate • Electrons can give up energy (violet light)… • by emitting light immediately (fluorescence) • by emitting light slowly (phosphorescence) • Some electrons can retain (store) their energy • crystal defects can “trap” excited electrons • electrons can escape the traps when exposed to the proper wavelength (red) light using LASER source. Photo-stimulated luminescence) • electrons can also escape by thermal mechanisms (fading) PSP fluorescence Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

4. Computed Radiography (CR) or Photostimulable Phosphor (PSP) Radiography • Latent image of trapped electrons is formed when x-rays hit the imaging plate • Latent image is read out physically instead of chemical process • As the latent image is read out … • Stimulated light emitted with the help of LASER is directed to a Photomultiplier Tube (PMT) • The PMT signal is digitized using Analog-to-Digital Converter (ADC) • The digital image consists of an array of ADC Code Values • ADC Code values represent exposure information • Array locations represent spatial information Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

5. Photostimulable phosphor reader Rotating polygon mirror Analog-to-Digital Converter Photomultiplier tube ? Amplifier Light guide Laser fast scan slow scan Latent Image Imaging plate Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

6. Characteristics of PSP systems • Generally, but not exclusively, cassette-based systems • Moderate initial capital investment • Simple retrofit to existing radiographic equipment • Individual scanner can serve multiple acquisition devices • Workflow comparable to daylight loader film processing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

7. Before After Correction of non-uniformity improves contrast and sharpness Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

8. Imaging plate artefact • PSP Plates • Scratches in the plate. • Abdomen of a 3rd trimester pregnant woman. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

9. a b c • Chest (PA) obtained as chest Lat (flat-panel detector), 125 kV, 6.2 mAs, 0.54 mGy (patient entrance dose four times higher than necessary). AEC centre cell used. Saturated image at the lung area. • Same image displayed with inverted grey scale. • Same image. Isocontour 99% of pixel content. Some lung areas are saturated without any diagnostic information. The image was repeated. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

10. CR Imaging plates do not last forever Imaging plate artefact Agfa plate defect Same defect, different orientation Artefact remedy: The Imaging plate (IP) must be replaced when cracks occur in clinically useful areas Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

11. Imaging plate artefact • PSP Plates • Artifact due to humidity Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

12. Imaging plate artefact Phosphor plate deteriorated. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

13. Imaging plate artefact Phosphor plate misused. Impossible to clean. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

14. Plate reader artefact • Digitiser • Problems with the analog-digital conversion Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

15. Plate reader artefact • Digitiser • Reading: galvo Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

16. Dust on collection optics causes line in slow-scan dimension (Fuji CR) • Dirt on the light guide in the CR reader. The light guide collects light emitted from the imaging plate when it is scanned by the laser. • Artefact remedy: the light guide of the photomultiplier tube was cleaned by service personnel. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

17. Plate reader artefact • Digitiser • Problems with the storage of the image in the computer Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

18. Plate reader artefact • Digitiser • Processing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

19. Plate reader artefact Digitiser • Processing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

20. Plate reader artefact • Digitiser • Processing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

21. Plate reader artefact • Digitiser • Processing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

22. Plate reader artefact • Digitiser problem • Artefacts in the horizontal direction Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

23. Plate reader artefact • Digitiser problem. • Artefacts in the vertical direction. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

24. Plate reader artefact • Digitiser problem. • Saturation in the lower part of the image. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

25. Plate reader artefact Two exposures in the same plate. Humerus and lateral dorsal spine. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

26. Incorrect collimation can cause inappropriate image processing: Agfa CR Automatic processing Manual reprocessing Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

27. Incorrect collimation Bad collimation but with cropped image seems good Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

28. Failure to detect radiation field (exposure recognition failure) : Fuji CR • Causes: • nonparallel collimation • multiple fields • poor centering • Implants • violation of collimation rules • Result: inappropriate histogram analysis => incorrect rescaling • Artefact remedy: Use proper collimation and appropriate positioning Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

29. Over-exposure should cause a strong signal, less noise, less mottle 8 mAs 24 mAs But extra scatter => Contrast loss Barry Burns, UNC Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

30. Over-exposure should cause a strong signal, less noise, less mottle S = 68 S = 252 ~ 4 X Overexposure Barry Burns, UNC Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

31. What’s wrong with this CR image? Galvanometer bearing failure Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

32. Artifact observed after radiographer frees jam (Agfa CR) Also observed on multiple subsequent images Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

33. Faulty ground on scanner control board Barry Burns, UNC Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

34. CR and film-screen from dose efficiency point of view • Typically CR systems are initially adjusted to work with the same level of dose as the screen film systems. • Later, with the optimisation programmes, it is possible to work with lower doses. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

35. CR and film-screen. Latitude advantage. • The wide dynamic range of the CR, allows more information to be shown on the images (modifying the window and level for a good visualisation). • In addition, if same mistake occurs in the selection of the radiographic technique, this wide dynamic range avoids in most of the cases, the need to repeat the exposure. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

36. a b • Relative exposure index 1.15, image too noisy. • Relative exposure index 1.87, image with sufficient quality Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

37. CR lumbar spine adult 42 y; Dose Level (Agfa): 0.85 (too low) image too noisy Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

38. CR lumbar spine adult 42 y; Dose Level (Agfa): 1.41 (normal) image good for diagnosis Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

39. Dose level (Agfa): 2.36 (higher than necessary) Good image quality Patient 10 months old. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

40. Dose level (Agfa): 2.95 (too high) Saturated image Patient 10 months old. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

41. DL= 2.95 DL = 2.36 The first image was not valid for diagnosis (saturated due to the very high dose) and repeated. The first image required 4 times the dose used for the second one, and 18 times more dose than necessary. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

42. Deletion of image files at the viewing or workstation of apparently non diagnostic images • Sometimes, radiographers can assume that some images are unuseful for diagnosis and they could proceed to delete the files. • This could be a bad practice because they avoid the possible use of some of the information contained in the bad image and in addition, they could prevent the appropriate analysis of retakes. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

43. Practical advice • Poor monitor setup (e.g. low illumination, contrast or spatial resolution) could lead to unnecessary repeated exposures, due to apparent non diagnostic image. • All users of the monitor should fully understand how to use correct settings. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

44. Postprocessing • ICRP 93 advice: • Image quality can be compromised by inappropriate levels of data compression and/or postprocessing techniques. This is dependent on the modality. • Data compression and postprocessing requirements should be defined by the modality and the medical imaging task. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

45. Influence of postprocessing in image quality Figure from ICRP 93 (courtesy of R. Loose). Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

46. Effect of the postprocessing • The standard postprocessing parameters offered in some CR workstations includes the noise reduction and the edge enhancement. • The basic principle for Agfa postprocessing tool called “MUSICA” (Multi scale Image Contrast Enhancement): • contrast enhancement irrespective of feature size. • difference with respect to spatial frequency band filtering. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

47. Edge enhancement Noise reduction Standard image Examples of different postprocessing using Agfa CR software (MUSICA) Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

48. Edge enhancement Noise reduction Standard image Examples of different postprocessing using Agfa CR software (MUSICA) Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

49. Some key points: • Use of different post processing (sometimes could avoid repetitions). • In this case using different post-processing means that the lung tissue and mediastinum can be visualised. Figure from ICRP 93 Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography

50. Practical advice • Many problems with, eg, the digitiser, printer, local hard disk, faults in electrical power supply, network problems can influence patient dose. • All of these elements occasionally can lead to problems with image quality resulting in a repeated exposure. • Correct maintenance of all these elements is a key aspect of the quality assurance programmes. Radiation Protection in Digital Radiology L06 Avoiding Artefacts in Computed Radiography