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compiter radiography and digital radiography
- 1. George David Associate Professorof Radiology Medical College of Georgia
- 2. Computed Radiography (CR)ComputedRadiography (CR) • Re-usable metal imaging plates replace film & cassette • Uses conventional bucky & x-ray equipment
- 3. CR Exposure &ReadoutCR Exposure & Readout
- 4.
- 5. Another View: CROperationAnother View: CR Operation
- 6. Computer Radiography (CR)ComputerRadiography (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. Reading Imaging PlateReadingImaging 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. Reading Imaging PlateReadingImaging 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. Laser & EmittedLight 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. 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. CR Phosphor LayerCRPhosphor 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. CR ResolutionCR Resolution •Small cassettes have better spatial resolution Smaller pixels More pixels / mm
- 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. 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. Film Screen vs.CR LatitudeFilm Screen vs. CR Latitude CR Latitude: .01 – 100 mR 100
- 16. CR Very Sensitiveto ScatterCR Very Sensitive to Scatter
- 17. Digital Radiography (DR)DigitalRadiography (DR) • Digital bucky • Incorporated into x-ray equipment
- 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. Direct vs. IndirectDirectvs. Indirect TFT = THIN-FILM TRANSISTOR ARRAY
- 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. “Indirect” DR“Indirect” DR •X-ray strike scintillator producing light • Photodiode array converts light to electrons Light
- 22. Indirect DRIndirect DR •Light spreads can limit spatial resolution • Can be controlled by “channeling” • Winning in the marketplace
- 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. SummarySummary • DR becomingindustry 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. Raw Data ImageRawData 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. Enhancing Raw Image (ImageSegmentation) 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. 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. 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. HistogramHistogram • Graph showing how muchof image is exposed at various levels
- 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. LUT can Simulate Appearanceof Film LUT can Simulate Appearance of Film
- 32. LUT SelectionLUT Selection •LUT calculated by algorithm depends on Body part projection • User can also alter LUT manually
- 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. Film/Screen Limited LatitudeFilm/ScreenLimited Latitude • Film use has little ambiguity about proper radiation exposure
- 35. Should I Worry?ShouldI Worry? In CR & DR, image density is no longer a reliable indicator of exposure factor control.
- 37. • Almost impossibleto under or overexpose CR / DR • Underexposures look noisy • Overexposures look GOOD!!! CR / DR LatitudeCR / DR Latitude DANGER Will Robinson!!!
- 38. Exposure CreepExposure Creep: Tendencyof 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
- 39. So how doI know if exposure is optimum by looking at my image? So how do I know if exposure is optimum by looking at my image?
- 41. Exposure IndexExposure Index •Each manufacturer provides feedback to technologist on exposure to digital receptor • Displayed on CR reader monitor • Displayed on workstations
- 42. 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
- 43. Exposure Indication Varies betweenManufacturers 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
- 44. 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!
- 45. Calculated Exposure Index Affectedby 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
- 46. Phototimed Phantom ImagePhototimedPhantom Image • 75 kVp • 88 mAs • 2460 EI
- 47. Let’s Approximately DoublemAsLet’s Approximately Double mAs • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 160 mAs • 2680 EI
- 48. Let’s Go CrazyLet’sGo Crazy • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 640 mAs • 3300 EI
- 49. How Low CanYou 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
- 50. Let’s Go CrazyLowLet’s Go Crazy Low • 75 kVp • 8 mAs • 1380 EI • 75 kVp • 1 mAs • 550 EI
- 51. CR ArtifactsCR Artifacts •Physical damage to imaging plates Cracks, scuffs, scratches Contamination Dust / dirt • Dirt in reader • Highly sensitive to scatter radiation
- 52. CR Grid InterferenceCRGrid 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.
- 53. DR ArtifactsDR Artifacts •Dead detector elements • Spatial variations in background signal & gain • Grid interference • Software can help correct for above
- 54. Shifting Gears: Fluoroscopy Issues ShiftingGears: Fluoroscopy Issues
- 55. Digital Video SourcesDigitalVideo 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
- 56. Digital Spot FilmDigitalSpot Film • Frame grabber digitizes image • Digital image saved by computer • Radiographic Technique used required to control quantum noise
- 57. Last Image HoldLastImage 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
- 58. 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
- 59. Fluoro Frame Averaging Tradeoff FluoroFrame Averaging Tradeoff • Advantage: Reduces quantum noise • Disadvantage Because history frames are averaged with current frame, any motion can result in lag
- 60. 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
- 61. The Future ofDigitalThe Future of Digital
- 62. DR Mobile UnitsDRMobile Units • See image immediately • Wireless transmission of images
- 63. 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.
- 64. DR & EnergySubtractionDR & Energy Subtraction • 2 images taken milliseconds apart at 2 different kVp’s • Combine / subtract images Soft Tissue Image Bone Image
- 65.