George David Associate Professor of Radiology Medical College of Georgia

1. Dose Issues in Pediatric Radiology 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. CR Latitude • Much greater latitude than screen/film • Plate responds to many decades of input exposure • under / overexposures unlikely • Computer scales image to provide viewable densities • Unlike film, receptor separate from viewer

6. Film Screen vs. CR Latitude CR Latitude: .01 – 100 mR 100

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

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

9. 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 applied to image prior to presenting it as finished radiograph

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

11. 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

12. 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

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

14. Film/Screen Limited Latitude • Film use has little ambiguity about proper radiation exposure

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

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

18. CR / DR Latitude More DANGER • If adult technique used on peds patients, images look GOOD! • If grid removed for ped patient & grid technique used, image looks GOOD • NO ONE COMPLAINS

19. Exposure Creep:Tendency of radiographs toward higher-then-necessary exposures • High doses have no detrimental effect on image quality • Desire to see less noise on radiographs • Increased exposure latitude

20. So how do I judge the exposure if I can’t tell by looking at the image?

21. Exposure Index

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

23. Exposure Index • Measure of radiation received by receptor below anatomy • Not a direct measure of patient exposure • If exposure index higher than necessary, patient overexposed

24. Displayed 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

25. 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

26. Doubling Exposure Index is Logrithmic • EI = 2000 +1000 * log(exposure)

27. Initial Set-up for Exposure Index • Kodak recommendation for exposure index: 1800 – 2200 • Manual technique: • Technologist should strive to keep exposure index consistent • Phototiming: • Set-up by service & physics according to manufacturer’s instructions

28. Imaging is NOT a Beauty Contest • CR/DR exposure should be selected to provide “Maximum tolerable noise” • Keeps dose as low as possible (ALARA) • Noise tolerance depends on study & objective. • Technologist requires noise feedback from radiologists

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

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

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

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

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

34. Fluoroscopy Doses • Beam time • Geometry • Application of features

35. Last Image Hold • Computer displays last fluoro image before radiation shut off. • Image noisier than for digital spot • Image made at fluoroscopic technique / intensity • Less radiation than digital spot • Allows operator to review static processes while beam off • ideal for teaching • ideal for orthopedic applications such as hip pinning

36. Fluoro Frame Averaging • Computer averages current with user-selectable number of previous frames • Averages current frame & history • Conventional fluoro only displays current frame

37. Fluoro Frame Averaging Tradeoff • Advantage: • Reduces quantum noise • Disadvantage • Because history frames are averaged with current frame, any motion can result in lag

38. Fluoro Frame Rates • Many systems allow option of using lower frame rates • 15, 7.5, 3.75 fps rather than 30 • computer displays last frame until next one • reduces flicker • Most implementations lower patient & scatter exposure • Exposure proportional to frame rate • dynamic studies may be jumpy

39. Patient CT Patient Dose • Tube rotates around patient during study • Dose distribution different than radiography

40. USA Today 6/19/2001 “Each year, about 1.6 million children in the USA get CT scans to the head and abdomen — and about 1,500 of those will die later in life of radiation-induced cancer, according to research out today.”

41. Biggest Question about CT Doses? Appropriateness of Study

42. Pediatric Color Coding • Broselow-Luten Pediatric System • Based on child’s size & weight • Used in ER’s for • Resuscitation & support apparatus • Medications • IV fluids

43. Pediatric Color Coding In CT • Slightly modified Broselow-Luten Pediatric System • Color code is used to determine complex CT protocols • Contrast options • Scanner protocols • Multi-slice has even greater variety of options • Detector configuration • Gantry rotation speed • Table speed • Color coding significantly reduces protocol errors

45. CT Dose depends on X-Ray Beam Image Quality & Processing Selections • noise • detector efficiency • matrix resolution • reconstruction algorithm • kVp • mA • time • pitch • filtration

46. CT Contrast Resolution Depends on NoiseNoise is function of mA

47. CT Dose Measurement • Lucite phantom • 5 holes • One center • Four 90o apart in periphery • Chamber placed in one hole • Lucite plugs placed in remaining 4 holes • Slice centered on phantom Chamber Plugs

48. Measuring CT Dose IndexCTDI • “Pencil” ion chamber used • Pencil oriented in “Z” direction Dose Phantom Chamber Hole for Chamber Active Chamber Area (exposed area of chamber) Z Slice Z

49. CTDI100 • Measurement made with 100 mm chamber • Includes scatter tails Dose Phantom Chamber 100 mm Slice Z

50. CTDIW • Weighted average of center & peripheral measurements • Represents “average” dose in scan plane CTDIW = 1/3 CTDI100-center + 2/3 CTDI100-periphery CTDIC CTDiP