George David Associate Professor of Radiology Medical College of Georgia - PowerPoint PPT Presentation

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George David Associate Professor of Radiology Medical College of Georgia
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George David Associate Professor of Radiology Medical College of Georgia

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  1. Computed Radiography (CR), Digital Radiography (DR), & Digital Spots George DavidAssociate Professor of Radiology Medical College of Georgia

  2. Computed Radiography (CR) • Re-usable metal imaging plates replace film & cassette • Uses conventional bucky & x-ray equipment

  3. CR Exposure & Readout

  4. CR Readout

  5. Another View: CR Operation

  6. - - - - - - - - - - - - - - - - - - - - - - - - - - - - Computer Radiography (CR) • plate is photostimulable phosphor • radiation traps electrons in high energy states • higher statesform latent image H i g h e r E n e r g y E l e c t r o n S t a t e P h o t o n p u m p s e l e c t r o n t o h i g h e r e n e r g y s t a t e X - R a y P h o t o n L o w e r E n e r g y E l e c t r o n S t a t e

  7. Reading Imaging Plate • reader scans plate with laser • laser releaseselectrons trapped inhigh energystates • electrons fall to lowenergy states • electrons give upenergy as visible light • light intensity ismeasure of incident radiation Lower Energy Electron State

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

  9. Laser & 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 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 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 Resolution • Small cassettes have better spatial resolution • Smaller pixels • More pixels / mm

  13. CR 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 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 Latitude CR Latitude: .01 – 100 mR 100

  16. CR Very Sensitive to Scatter

  17. Digital Radiography (DR) • Digital bucky • Incorporated into x-ray equipment

  18. 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. Indirect TFT = THIN-FILM TRANSISTOR ARRAY

  20. “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. Light “Indirect” DR • X-ray strike scintillator producing light • Photodiode array converts light to electrons

  22. Indirect DR • Light spreads can limit spatial resolution • Can be controlled by “channeling” • Winning in the marketplace

  23. Digital Radiography (DR) • Potentially lower patient dose than CR • High latitude as for CR • Digital bucky fragile • First DR portables comingto market

  24. Summary • 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. Raw 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. Enhancing Raw Image (Image Segmentation) * • Identify collimated image border • Separate raw radiation from anatomy • Apply appropriate tone-scale to image • Done with look-up table (LUT) This process is specific to a particular body part and projection

  27. 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 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. Histogram • Graph showing how much of image is exposed at various levels

  30. Tone ScalingPost-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 Appearance of Film

  32. LUT Selection • LUT calculated by algorithm depends on • Body part • projection • User can also alter LUT manually

  33. LUT 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 Latitude • Film use has little ambiguity about proper radiation exposure

  35. Should I Worry? • In CR & DR, image density is no longer a reliable indicator of exposure factor control.

  36. CR / DR Latitude DANGER Will Robinson!!! • Almost impossible to under or overexpose CR / DR • Underexposures look noisy • Overexposures look GOOD!!!

  37. Exposure 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. So how do I know if exposure is optimum by looking at my image?

  39. Exposure Index

  40. Exposure Index • Each manufacturer provides feedback to technologist on exposure to digital receptor • Displayed on CR reader monitor • Displayed on workstations

  41. Exposure 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

  42. Exposure Indication Varies between Manufacturers 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

  43. Exposure 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 providingtolerable noise • This is not a beauty contest!

  44. 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

  45. Phototimed Phantom Image • 75 kVp • 88 mAs • 2460 EI

  46. Let’s Approximately Double mAs • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 160 mAs • 2680 EI

  47. Let’s Go Crazy • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 640 mAs • 3300 EI

  48. How Low Can You Go? Cut mAs in Half! • 75 kVp • 88 mAs • 2460 EI • 75 kVp • 40 mAs • 2060 EI

  49. Let’s Go Crazy Low • 75 kVp • 8 mAs • 1380 EI • 75 kVp • 1 mAs • 550 EI