1. Chapter 7: Digital Applications and Image Enhancements

2. In the early days of digital imaging, computer memory was insufficient for capturing dynamic images digitally, but one use, DSA, became popular in the 1980’s, and is still used today. Types of Subtraction

3. Serial Angiography Zero FilmMask + Contrast Film Plain Film Subtraction = What kind of film? Subtraction Film What kind of film? Subtraction Print Film Mask Mode DSA

4. Temporal Modes of DSA A. Mask Mode B. Time Interval Difference

5. Reregistration or Pixel Shifting Misregistration

6. Time Interval Difference (TID) DSA Finished Products Etc. Utilizes Remasking 1. Temporal Subtraction Modes of DSA A. Mask Mode B. TID

7. 1. Temporal Subtraction Modes of DSA A. Mask Mode B. Time Interval Difference 2. Energy Subtraction

8. Photoelectric Absorbtion

9. Photoelectric Absorbtion K L

10. I 53 K Shell Binding Energy of 33 keV Probability of Absorbtion keV 33 keV Muscle Bone Iodine 32 keV 34 keV

11. 32 34 Energy Subtraction in Action Globules or Pixels Spaces between the Globules or pixels (where image data is lost) Colored globules/pixels represent contrast in vessel. Numbers represent digital values (shades of gray)

12. Energy Subtraction In Reality 70 to 90 kVp 60 to 130 kVp Via pulsed beams or flywheels Bottom Line: Because the kVp must be higher than optimal to penetrate the part, energy subtraction is not a magic bullet.

13. 1. Temporal Subtraction Modes of DSA A. Mask Mode B. Time Interval Difference 2. Energy Subtraction 3. Hybrid Subtraction

14. What is the difference? 1011 10111011 4 bits vs. 8 bits Contrast Resolution as a Function of the Processor

15. Windowing The human eye can distinguish 32 shades of gray. 2 5 It’s like contrast and brightness, but it’s not. 32

16. Windowing 2 5 But a digitized image may contain thousands of shades of gray, known as the dynamic range 2 10 1024 32

17. Bone = + 1000 CT Numbers (Hounsfield Units) Based on Water = 0 Air = - 1000 CT numbers (HU), express attenuation values relative to water

18. The range of displayed pixel values Window Width For example: A window width of 5

19. The range of displayed pixel values Window Width For example: A window width of 5 A narrow window is the digital equivalent of a short scale of contrast on a radiograph

20. The range of displayed pixel values Window Width For example: A window width of 500 A wide window is the digital equivalent of a long scale of contrast on a radiograph

21. The middle of the range of any given window width Window Center (or level) At a center of 2 For example: A window width of 5 0 7 -7-7 1 2 3 4 5 * 0 1 2

22. Window Center (or level) At a center of - 2 A window width of 5 0 7 -7-7 * *

23. This CT section through the abdomen was windowed with a width of 300 shades of gray The median number (the level or center) is 2 HU below water, -2

24. A soft tissue window set at a width of 110, at a level of 43 The same CT section of the head set at a width of 2010, at a level of 800 Note the fracture through the frontal sinus

25. Width 2290 Level 907 Bone windows demonstrating trauma to the left orbit, maxilla, and sinuses.

26. Two windows of the same section through the thorax Width 1269 Level 202 Width 1269 Level 2

27. DSA images use narrow windows to enhance the contrast difference between the vessel and the surrounding tissue

28. Double Windows A separate window width and center of the lungs is cut and pasted into a soft tissue windowed image of the thorax. Heart Diaphragm Lung tissue Bronchi Loss of image information at interface

29. Annotation

30. Gray Scale Inversion SPR CT Scout CT of Abdomen

31. Thumbnails, or token images are especially useful for simultaneous viewing of numerous examinations on a patient, on a PACS workstation

32. Region of Interest (ROI) The ROI cursor may be shaped and sized to select an area of an image

33. Using the ROI cursor to sample CT numbers This ROI has been sized to measure a density in the right kidney. The mean density of 15.9 indicates a fluid filled cyst.

34. Sampling the Voxel in Cross Sectional Images Cross sectional images have depth, which is selected prior to a scan. When a two dimensional section is viewed, the density of each pixel actually represents all the tissues in the volume of the section. This density represents the volume element, or the voxel. Three contiguous CT sections with parts of a frontal lobe mass in each one The CT numbers from these samples would represent an average of the mass and healthy tissue. The CT number from this sample would represent the true density of the mass.

35. Histograms In this example histograms were superimposed over six sections of a neck for the purpose of identifying areas in the field that need bolus material to equalize the exposure. Without a bolus, the carotid arteries would have been over-exposed on this DSA.

36. Histograms

37. Linear measurement through a lesion, plotted as a histogram

38. Linear measurements 6.3 cm hematoma from a ruptured aortic aneurysm Diameter of common iliac arteries

39. Bone densitometry performed on CT software using the SPR mode of CT

40. Bone densitometry

41. Repeated applications of a high pass filter demonstrates the effect of suppressing low spatial frequencies No filterHigh Ultra HighRidiculously High

42. No filter Low pass or smoothing filter is similar to a convolving filter Edge sharpening filter has an algorithm similar to a high pass filter

43. Selected by ROI Electronic Zoom

44. and bone window of IAC

45. Images zoomed to excess show pixelation, or tiling. This image looks much like early CT scans that used a small matrix size A small matrix size, and zoom, produce the same effect.

46. Magnification is often available in an ROI window that is moved around the screen, providing zoom on the fly. Mag off Mag on

47. Zoomed to the point where loss of spatial resolution is visible, though pixelation is not clearly evident.

48. An image acquired with a small field of view is preferable to a zoomed image Large FOV Small FOV

49. Colorization Nuclear Medicine study demonstrating myocardial ischemia Color enhancement demonstrates the profusion of blood in the heart muscle

50. Multiple Plane Reconstructions 3 sections of the head reconstructed from axial sections done in fine increments Pelvis reconstructed from 1 cm increments

51. 3D or Volumetric Imaging Test image demonstrates the use of shading to create the appearance of depth.

52. 3D or Volumetric Imaging After loading volumetric data, an image may be rotated on the screen, and displayed from any perspective

53. 3D or Volumetric Imaging 3D images help surgeons visualize the extent of injury. This trauma victim has numerous facial fractures. The extent of injury is easier to appreciate in 3D

54. 3D or Volumetric Imaging

55. CT scan of Joseph Merrick’s (the elephant man) skull.