PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

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

6
(No Transcript)
7
TOMOGRAPHY
8
TOMOGRAPHY
9
TOMOGRAPHY
10
(No Transcript)
11
CT ADVANTAGES
12
LIMITATIONS OF CT

• UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH
  SLIGHT CONTRAST DIFFERENCES lt 1.

13
(No Transcript)
14
GOALS OF CT

• MINIMAL SUPERIMPOSITION
• IMAGE CONTRAST IMPROVEMENT
• SMALL TISSUE DIFFERENCE RECORDING

15
CT DATA AQUISITION
16
(No Transcript)
17
SLIP RINGS
18
(No Transcript)
19
SEQUENTIAL-SLICE BY SLICE SCANNING
20
SCANNING
21
TRANSMISSION

• RELATIVE TRANSMISSIONIo/I

22
(No Transcript)
23
(No Transcript)
24
Total of trans. measurements of views X
of rays in each view
25
ATTENUATION
26
(No Transcript)
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
39
(No Transcript)
40
TOO SMALL OF SFOV OUT OF FIELD ARTIFACT
41
SCAN FOV-RESOLUTION
SFOV




42
RECONSTRUCTION

?



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 MM400 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
ANALOG
DAC
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
62
(No Transcript)
63
CT NUMBER
64
(No Transcript)
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
71
(No Transcript)
72
(No Transcript)
73
(No Transcript)
74
W 120
L 40
75
W 80
L 40
76
DATA FLOW IN CT
REFERENCE DETECTOR
REFERENCE DETECTOR
ADC
PREPROCESSOR
COMPUTER
RAW DATA
PROCESSORS
CONVOLVED DATA
BACK PROJECTOR
RECONSTRUCTED DATA
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