This document discusses the physical principles of computed tomography (CT). It begins by describing limitations of conventional radiography like superimposition of tissues. CT aims to minimize superimposition and improve image contrast and ability to detect small tissue differences. CT acquires data through sequential slice-by-slice scanning using x-rays, with data processed to reconstruct cross-sectional images. These images map varying tissue densities on a grayscale and allow manipulation for optimal viewing of anatomy and pathology.

1. PHYSICAL PRINCIPLES OF
COMPUTED TOMOGRAPHY

2. RADIOGRAPHY LIMITATIONS
 SUPERIMPOSITION
 DIFFICULTY IN DISTINGUISHING
BETWEEN HOMOGENOUS OBJECTS OF
NON-UNIFORM THICKNESS.

3. SUPERIMPOSITION

4. TISUE DIFFERENCE SENSITIVITY
5%-10%

5. TOMOGRAPHY ( CONVENTIONAL)
 ELIMINATES TISSUE SUPERIMPOSITION
 INCREASES CONTRAST OF LOW
SUBJECT CONTRAST TISSUES

7. TOMOGRAPHY

8. TOMOGRAPHY

9. TOMOGRAPHY

11. CT ADVANTAGES

12. LIMITATIONS OF CT
 UNABLE TO DIFFERENTIATE BETWEEN
TISSUES WITH SLIGHT CONTRAST
DIFFERENCES < 1%.

14. GOALS OF CT
 MINIMAL SUPERIMPOSITION
 IMAGE CONTRAST IMPROVEMENT
 SMALL TISSUE DIFFERENCE
RECORDING

15. CT DATA AQUISITION

17. SLIP RINGS

19. SEQUENTIAL-SLICE BY SLICE
SCANNING

20. SCANNING

21. TRANSMISSION
RELATIVE TRANSMISSION=Io/I

24. Total # of trans. measurements=
# of views X # of rays in each
view

25. ATTENUATION

27. DATA AQUSITION GEOMETRIES
 CONTINUOUS
 STATIONARY

28. CONTINUOUS

29. STATIONARY

30. CT 120-140 KVP
 REDUCED DEPENDENCY ON
ATTENUATION COEFFICIENT
 REDUCED CONTRAST
 INCREASED PHOTON FLUX

31. ORIGINAL CLINICAL CT SCANS
COMPOSED OF
80 X 80 MATRIX
PIXELS
6400

32. X

33. Y

34. Z

35. ISOCENTER

36. SCAN FOV
SFOV
DETECTORS

37. SCAN FOV-SMALL
SFOV
DETECTORS

38. SFOV - HEAD

40. TOO SMALL OF SFOV – OUT OF
FIELD ARTIFACT

41. SCAN FOV-RESOLUTION
SFOV

42. RECONSTRUCTION
Ц CT#
RECONSTRUCTION

43. SCAN FOV-RESOLUTION
SFOV

44. DISPLAY FOV vs SCANNING FOV
 DFOV CAN BE EQUAL OR LESS OF SFOV
 SFOV – AREA OF MEASUREMENT
DURING SCAN
 DFOV - DISPLAYED IMAGE

45. PIXEL SIZE
PIXEL SIZE= FOV (mm)/ MATRIX SIZE

46. MOST SCANNERS PIXEL SIZE
1 TO 10mm

47. EXAMPLE:
 FOV= 40 CM= 40 X 10 MM=400 mm
 MATRIX= 512 X 512 = 5122
400/512 = 0.78 mm
0.8 mm

48. EACH PIXEL IN CT HAS RANGE OF
GRAY SHADES
 2 8 =
256 SHADES
 2 12 =
4096 SHADES = -100 TO 3095 SHADES
OF GRAY

49. PIXEL vs VOXEL
PIXEL
VOXEL

50. PIXEL SIZE DEPENDS ON:
 MATRIX SIZE
 FOV

51. VOXEL SIZE DEPENDS
 FOV
 MATRIX SIZE
 SLICE THICKNESS

52. IMAGE DISPLAY

53. GRAY SCALE DISPLAY MONITOR
RESOLUTION IS RELATED TO
THE SIZE OF THE PIXEL MATRIX
 64 X 64
 128 X 128
 256 X 256
 512 X 512
 1024 X 1024
 2048 X 2048 (HIGH PERFORMANCE MONITORS)

54. MATRIX

55. PIXEL MATRIX

56. IN CT DIGITAL
RECONSTRUCTED IMAGE IS
CONVERTED IMAGE IS
CONVERTED INTO A GRAY
SCALE IMAGE.

57. DAC
ANALOG
DIGITAL

58. The high performance video display of the
microcomputer is connected to the system via
an interface board. The video display is a form
of cathode ray tube sometimes referred to as a
raster display. The term raster describes the
technique of producing the picture or text which
is formed by a beam of electrons that repeatedly
scans across the screen to form a uniform
pattern of closely spaced, horizontal lines (the
raster), covering the entire screen. The screen
consists of a phosphor that converts the energy
of the electron beam into visible light. A picture
is formed by "turning on and off" the electron
beam at appropriate points in the scanning of
the screen surface.

59. CRT OPERATION

60. IMAGE FORMATION ON THE MONITOR

61. IMAGE FORMATION ON THE MONITOR

63. CT NUMBER

65. LINEAR ATTENUATION
COEFFICIENT ( cm-1
)
 BONE 0.528
 BLOOD 0.208
 G. MATTER 0.212
 W. MATTER 0.213
 CSF 0.207
 WATER 0.206
 FAT 0.185
 AIR 0.0004

66. CT # vs BRIGHTNESS LEVEL
+ 1000
-1000

67. CT # 1000

68. CT # - 500

69. CT # OF CYST 5

70. CT # OF LIPOMA ( FATTY TUMOR)
-100

74. W 120
L 40

75. W 80
L 40

76. DATA FLOW IN CT
REFERENCE DETECTOR
REFERENCE DETECTOR
ADC
PREPROCESSOR
COMPUTER
RAW DATA
CONVOLVED DATA
BACK
PROJECTORRECONSTRUCTED DATA
PROCESSORS
DISK TAPE
DAC CRT DISPLAY

77. CT ADVANTAGES AND DISADVANTAGES
 EXCELLENT LOW CONTRAST RESOLUTION
 WINDOWING- IMAGE MANIPULATION TAILORED TO
OBSERVER NEEDS
 SPIRAL CT-SINGLE BREATH HOLD STUDIES
 ( CTA, MPR, VIRTUAL REALITY CT, CT ENDOSCOPY)
 CT ASISST IN RADIATION THERAPY
 BONE SCAN PACKAGE
 XENON CT
 PERFUCION CT
 DIGITAL PROCESSING ABILITY