Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Computed Radiography (CR), Digital Radiography (DR ...

8,746 views

Published on

Computed Radiography (CR), Digital Radiography (DR ...

  1. 1. George David Associate Professor of Radiology Medical College of Georgia
  2. 2. Computed Radiography (CR)Computed Radiography (CR) • Re-usable metal imaging plates replace film & cassette • Uses conventional bucky & x-ray equipment
  3. 3. CR Exposure & ReadoutCR Exposure & Readout
  4. 4. CR ReadoutCR Readout
  5. 5. Another View: CR OperationAnother View: CR Operation
  6. 6. Computer Radiography (CR)Computer Radiography (CR) • plate is photostimulable phosphor • radiation traps electrons in high energy states • higher states form latent image Higher Energy Electron State Lower Energy Electron State - - - - - - - - - - - - - - - - - - - - - - - - - - - X-Ray Photon - Photon pumps electron to higher energy state
  7. 7. Reading Imaging PlateReading Imaging Plate • reader scans plate with laser • laser releases electrons trapped in high energy states • electrons fall to low energy states • electrons give up energy as visible light • light intensity is measure of incident radiation Laser Beam Higher Energy Elect ron St at e Lower Energy Elect ron St at e - - - - - - - - - - - - - - - - - - - - - - - - - - - - Lower Energy Electron State
  8. 8. Reading Imaging PlateReading Imaging Plate • Reader scans plate with laser light using rotating mirror • Film pulled through scanner by rollers • Light given off by plate measured by PM tube & recorded by computer
  9. 9. Laser & Emitted Light are Different ColorsLaser & Emitted Light are Different Colors • Phosphor stimulated by laser light • Intensity of emitted light indicates amount of radiation incident on phosphor at each location • Only color of light emitted by phosphor measured by PMT
  10. 10. CR OperationCR 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
  11. 11. CR Phosphor LayerCR Phosphor Layer • Phosphor balanced for x-ray absorption characteristics light output laser light scatter screen thickness • Above variables affect electronic noise image resolution properties speed of imaging system • Overcoat protects plate from physical damage
  12. 12. CR ResolutionCR Resolution • Small cassettes have better spatial resolution Smaller pixels More pixels / mm
  13. 13. CR ThroughputCR Throughput • Generally slower than film processing • CR reader must finish reading one plate before starting to read the next • Film processors can run films back to back
  14. 14. CR LatitudeCR Latitude • Much greater latitude than screen/film • Plate responds to many decades of input exposure under / overexposures unlikely • Computer scale inputs exposure to viewable densities Unlike film, receptor separate from viewer
  15. 15. Film Screen vs. CR LatitudeFilm Screen vs. CR Latitude CR Latitude: .01 – 100 mR 100
  16. 16. CR Very Sensitive to ScatterCR Very Sensitive to Scatter
  17. 17. Digital Radiography (DR)Digital Radiography (DR) • Digital bucky • Incorporated into x-ray equipment
  18. 18. Digital Radiography (DR) Digital Radiography (DR) • Receptor provides direct digital output • No processor / reader required Images available in < 15 seconds Much less work for technologist
  19. 19. Direct vs. IndirectDirect vs. Indirect TFT = THIN-FILM TRANSISTOR ARRAY
  20. 20. “Direct” DR“Direct” DR • X-ray energy converted directly to electrical signal • X-rays interact with semiconductor material Amorphous selenium • X-rays converted directly into electrical charge No intermediate steps
  21. 21. “Indirect” DR“Indirect” DR • X-ray strike scintillator producing light • Photodiode array converts light to electrons Light
  22. 22. Indirect DRIndirect DR • Light spreads can limit spatial resolution • Can be controlled by “channeling” • Winning in the marketplace
  23. 23. Digital Radiography (DR) Digital Radiography (DR) • Potentially lower patient dose than CR • High latitude as for CR • Digital bucky fragile First DR portables coming to market
  24. 24. SummarySummary • DR becoming industry leader in radiographic imaging • DR images displayed & stored in about 8 seconds • DR has faster throughput Up to 2-4 times faster than traditional screen-film-darkroom technology
  25. 25. Raw Data ImageRaw Data Image • Unprocessed image as read from receptor CR » Intensity data from PMT’s as a result of scanning plate with laser DR » Raw Data read directly from TFT array • Not a readable diagnostic image • Requires computer post- processing Specific software algorithms must be applied to image prior to presenting it as finished radiograph
  26. 26. Enhancing Raw Image (Image Segmentation) Enhancing Raw Image (Image Segmentation) 1. Identify collimated image border 2. Separate raw radiation from anatomy 3. Apply appropriate tone- scale to image  Done with look-up table (LUTLUT) This process is specific to a particular body part and projection *
  27. 27. Look Up Table (LUT)Look Up Table (LUT) • Converts a raw data pixel value to a processed pixel value • “Original” raw data pixel value indicates amount of radiation falling on pixel
  28. 28. Image SegmentationImage Segmentation • Computer must establish location of collimated border of image • Computer then defines anatomic region • Finished image produced by tone scaling Requires histogram analysis of anatomic region
  29. 29. HistogramHistogram • Graph showing how much of image is exposed at various levels
  30. 30. Tone Scaling Post-Processing Tone Scaling Post-Processing • Body part & projection-specific algorithms determine average exposure Must correctly identify anatomical region • LUT computed to display image with proper Density Contrast
  31. 31. LUT can Simulate Appearance of Film LUT can Simulate Appearance of Film
  32. 32. LUT SelectionLUT Selection • LUT calculated by algorithm depends on Body part projection • User can also alter LUT manually
  33. 33. LUT SelectionLUT Selection • Monitors on CR reader or DR console compared to reading workstations have lower resolution poorer quality Recommended that LUT not be manually modified
  34. 34. Film/Screen Limited LatitudeFilm/Screen Limited Latitude • Film use has little ambiguity about proper radiation exposure
  35. 35. Should I Worry?Should I Worry? In CR & DR, image density is no longer a reliable indicator of exposure factor control.
  36. 36. • Almost impossible to under or overexpose CR / DR • Underexposures look noisy • Overexposures look GOOD!!! CR / DR LatitudeCR / DR Latitude DANGER Will Robinson!!!
  37. 37. Exposure CreepExposure Creep: Tendency of radiographs toward higher-then-necessary exposures Exposure CreepExposure Creep: Tendency of radiographs toward higher-then-necessary exposures • No detrimental effect on image quality • Desire to see less noise on radiographs • Increased exposure latitude • No one complains
  38. 38. So how do I know if exposure is optimum by looking at my image? So how do I know if exposure is optimum by looking at my image?
  39. 39. Exposure IndexExposure Index • Each manufacturer provides feedback to technologist on exposure to digital receptor • Displayed on CR reader monitor • Displayed on workstations
  40. 40. Exposure IndexExposure Index • Measure of radiation received by receptor below anatomy • Not a direct measure of patient exposure • If exposure index higher than recommended range, patient overexposed
  41. 41. Exposure Indication Varies between Manufacturers Exposure Indication Varies between Manufacturers Receptor Exposure Kodak EI Fuji S Number 0.5 1700 400 1 2000 200 2 2300 100 4 2600 50 Fuji “S” number goes down as exposure goes up! S is half when exposure doubled Kodak Logarithmic scale EI goes up 300 when exposure doubled
  42. 42. Exposure IndexExposure Index • Technologist should strive to keep exposure index consistent • Kodak recommendation for exposure index 1800 – 2200 • George’s recommendation “Maximum tolerable noise” As low as possible while providing tolerable noise This is not a beauty contest!
  43. 43. Calculated Exposure Index Affected by Calculated Exposure Index Affected by • X-Ray technique selection • Improper centering of image on cassette • Improper selection of study or projection • Placing two or more views on same cassette Can cause image to appear dark
  44. 44. Phototimed Phantom ImagePhototimed Phantom Image • 75 kVp • 88 mAs • 2460 EI
  45. 45. Let’s Approximately Double mAsLet’s Approximately Double mAs • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 160 mAs • 2680 EI
  46. 46. Let’s Go CrazyLet’s Go Crazy • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 640 mAs • 3300 EI
  47. 47. How Low Can You Go? Cut mAs in Half!How Low Can You Go? Cut mAs in Half! • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 40 mAs • 2060 EI
  48. 48. Let’s Go Crazy LowLet’s Go Crazy Low • 75 kVp • 8 mAs • 1380 EI • 75 kVp • 1 mAs • 550 EI
  49. 49. CR ArtifactsCR Artifacts • Physical damage to imaging plates Cracks, scuffs, scratches Contamination Dust / dirt • Dirt in reader • Highly sensitive to scatter radiation
  50. 50. CR Grid InterferenceCR 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.
  51. 51. DR ArtifactsDR Artifacts • Dead detector elements • Spatial variations in background signal & gain • Grid interference • Software can help correct for above
  52. 52. Shifting Gears: Fluoroscopy Issues Shifting Gears: Fluoroscopy Issues
  53. 53. Digital Video SourcesDigital Video Sources • DR type image receptor • Conventional Image Intensifier with Video Signal Digitized (“Frame Grabber”) I m a g e T u b e X-Ray Input Image Tube TV Amplfier Analog to Digital Convert er Digital Memory (Computer) Lens System
  54. 54. Digital Spot FilmDigital Spot Film • Frame grabber digitizes image • Digital image saved by computer • Radiographic Technique used  required to control quantum noise
  55. 55. Last Image HoldLast Image Hold • Computer displays last fluoro image before radiation shut off. • Image noisier than for digital spot Image made at fluoroscopic technique / intensity • Allows operator to review static processes without keeping beam on ideal for teaching environments ideal for orthopedic applications such as hip pinning • Less radiation than digital spot
  56. 56. Fluoro Frame Averaging Fluoro Frame Averaging • Conventional fluoro only displays current frame • Frame averaging allows computer to average current with user- selectable number of previous frames Averages current frame & history
  57. 57. Fluoro Frame Averaging Tradeoff Fluoro Frame Averaging Tradeoff • Advantage: Reduces quantum noise • Disadvantage Because history frames are averaged with current frame, any motion can result in lag
  58. 58. Other Fluoro Features Other Fluoro Features • Real-time Edge Enhancement / Image Filtering • Option of using lower frame rates (15, 7.5, 3.75 fps rather than 30) computer displays last frame until next one » reduces flicker Lowers patient and scatter exposure » Exposure proportional to frame rate dynamic studies may be jumpy
  59. 59. The Future of DigitalThe Future of Digital
  60. 60. DR Mobile UnitsDR Mobile Units • See image immediately • Wireless transmission of images
  61. 61. Other PossibilitiesOther Possibilities • Tomosynthesis Multi-slice linear tomography from one exposure series • Histogram Equalization Use computer to provide approximately equal density to various areas of image.
  62. 62. DR & Energy SubtractionDR & Energy Subtraction • 2 images taken milliseconds apart at 2 different kVp’s • Combine / subtract images Soft Tissue Image Bone Image
  63. 63. The EndThe End ?

×