1. Image Display

2. But first a review

3. Remember the 3 main steps 1. Data Acquisition 2. Image Reconstruction 3. Image Display

4. Review

6. Image formation review A ray is the pathway of a portion of the x-ray beam from one specific focal-spot position to a specific detector position. As the ray passes through the body, it measures the total x-ray attenuation (or penetration) along it's path. This is the data recorded by the detector. A view is made-up of many individual rays Ref. Sprawls.com

7. Image formation review A view is made-up of many individual rays As the x-ray beam is scanned around the body, forming many views, the data recorded by the detectors are stored in computer memory for later image reconstruction. Information from the view is then projected onto a matrix Ref. Sprawls.com

8. Beam Attenuation Structures in a CT image are represented by varying shades of gray The creation of these shades of gray is based on basic radiation principles The degree to which an x-ray beam is reduced by an object is referred to as attenuation

9. Beam Attenuation (cont’d) Photons that pass through objects unimpeded are represented by a black area on the image When photons are completely absorbed by an object, the corresponding area on the image is white Areas of intermediate attenuation are represented by various shades of gray

10. Beam Attenuation (cont’d) The number of photons that interact depends on the object’s –Thickness –Density –Atomic number

12. A CT SCANNER IS BASICALLY A DENSITY MEASURING DEVICE !

13. FROM GE

14. DETERMINE THE X-RAY ATTENUATION OF ALL THE TISSUES IN EACH MATRIX ELEMENT. DETERMINE THE LINEAR ATTENUATION COEFFICIENT FOR EACH MATRIX ELEMENT,  X.

15. CT NUMBERS  x -  water  water 1000 x In HOUNSFIELD UNITS (H.U.)

16. CT Number – Hounsfield Unit (HU) Each Voxel represents a numerical value – aka CT Number, which is the average of all the attenuation values within the voxel. This scale assigns water a HU value of 0. Air of -1000. Each number represents a shade of grey with +1000 as white and -1000 as black.

17. Review Air in the lungs is considered a.High attenuation b.Low attenuation c.Intermediate attenuation

18. Display Monitors CT images are usually displayed on a black- and-white or color monitor Display devices can be either –A cathode-ray tube (CRT) –Some form of flat panel, such as TFT LCD Monitors consist of –The display device –Circuitry to generate an image from electronic signals –An enclosure

19. Display Monitors (cont’d) CRT monitors are heavier, bulkier, hotter, and less durable than the newer LCD monitors LCD monitors produce higher luminance and high spatial resolution Digital-to-analog converters change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor

20. Monitors AWS – Acquisition workstation – typically 1 or 2 MP color monitor. RWS/PDM – Review workstation/Primary Diagnostic Monitor. 2, 3 or 5 MP monitor. DICOM GSDF calibration, Higher maximum luminance and higher monochrome bit depth -> better image contrast.

21. Storage format DICOM - Digital Imaging and Communications in Medicine Standard format for image storage. Uncompressed images. Lots of other info in header (technical factors, scanner info, technologist id!) Stored in PACS – Picture archiving and communications system (computer network and servers)

22. Window Settings Adjusting the window width and window level will change the way an image is viewed on the monitor

23. Window Settings (cont’d) Many studies require each image to be viewed at two or more window settings “Ideal” window settings are somewhat subjective Imaging departments typically have an established setting for each type of examination However, because many factors have an effect, technologists must adjust settings in individual situations

24. Window Settings (cont’d) Images cannot be displayed with a different shade of gray for each HU because –There are more 2,000 Hounsfield values; most monitors can only display 256 shades of gray –Different Window & Level settings can bring out contrast in different areas of the image.

25. Gray Scale The gray scale is used to display CT images This system assigns a certain number of HU to each shade of gray –By convention, the gray scale assigns higher HU values lighter shades of gray

26. Window Width The window width determines the number of HUs represented on a specific image –Values higher than the selected range will be white –Values lower than the selected range will be black Increasing the window width assigns more HU to each shade of gray

27. Window Width To illustrate, assume –we have just 10 shades of gray available –300 is selected as the window width 300 density values will be represented on the image as a shade of gray All others will be either black or white

28. Window Width/Window Level If the window width is set at 300, which 300 Hounsfield values, from the more than 2,000 possible, will be displayed? Window width selects the quantity of HU Window level selects the range of HU

29. Window Level Selects the center CT value of the window width Window level is also called window center Window level determines which HUs are displayed as shades of gray

30. Window Level (cont’d) Continuing with the previous example, assume –0 is selected as the level The HUs that are represented as a shade of gray on this image will range from –150 to 150

31. Review The selected window width is 400 The selected window level is 50 Which HUs are displayed on the image as shades of gray? a.–200 to 200 b.–150 to 450 c.–150 to 250 d.350 to 450

32. Answer a.–150 to 250

33. General Rule for Setting Window Level The window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

34. Setting Window Width In general, wide window widths (500–2,000) are best for imaging tissue types that vary greatly, when the goal is to see all of the various tissues on one image, such as lung Wider window widths encompass greater anatomic diversity, but subtle density discrimination is lost Because wider window width settings decrease image contrast, they suppress the display of noise on an image

35. Review A window width of 90 is most likely used to display an image of the a.Lung b.Femur c.Brain d.Abdomen

36. Answer c. Brain

37. Region of Interest (ROI) An ROI is most often circular, but may be elliptic, square, or rectangular, or may be custom drawn by the operator Defining the size, shape, and location of the ROI is the first step in many display and measurement functions

38. HU Measurement and Standard Deviation HU measurements may be affected by volume averaging or image noise A cursor (+) displays a measurement of the HU of the pixel that the cursor covers Conversely, an ROI provides an averaged measurement of all of the pixels within the ROI –When an ROI is used, the standard deviation is also displayed

39. Other Image Display Options Distance measurements Image annotation Reference image Image magnification Multiple image display Histogram Multiplanar and three-dimensional reformatting

40. WINDOW WIDTH - 5000+500 WINDOW LEVEL

41. SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 50 H.U. : WIDTH = 350 H.U.

42. SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 166 H.U. : WIDTH = 350 H.U.

43. SIEMENS VOLUME ZOOM 4 SLICE LEVEL = -106 H.U. : WIDTH = 350 H.U.

44. SIEMENS VOLUME ZOOM 4 SLICE LEVEL = 50 H.U. : WIDTH = 2000 H.U.

45. GE LIGHTSPEED 16 SLICE LEVEL = 40 H.U. & WIDTH = 400 H.U.

46. GE LIGHTSPEED 16 SLICE LEVEL = -99 H.U. & WIDTH = 400 H.U.

47. GE LIGHTSPEED 16 SLICE LEVEL = 200 H.U. & WIDTH = 400 H.U.

48. GE LIGHTSPEED 16 SLICE LEVEL = 40 H.U. & WIDTH = 57 H.U.

49. GE LIGHTSPEED 16 SLICE LEVEL = 350 H.U. & WIDTH = 2000 H.U.