PHYSICAL PRINCIPLES OF
COMPUTED TOMOGRAPHY
RADIOGRAPHY LIMITATIONS


SUPERIMPOSITION
DIFFICULTY IN DISTINGUISHING
BETWEEN HOMOGENOUS OBJECTS OF
NON-UNIFORM THICKNESS.
SUPERIMPOSITION
TISUE DIFFERENCE SENSITIVITY
5%-10%
TOMOGRAPHY ( CONVENTIONAL)


ELIMINATES TISSUE SUPERIMPOSITION
INCREASES CONTRAST OF LOW
SUBJECT CONTRAST TISSUES
TOMOGRAPHY
TOMOGRAPHY
TOMOGRAPHY
CT ADVANTAGES
LIMITATIONS OF CT

UNABLE TO DIFFERENTIATE BETWEEN
TISSUES WITH SLIGHT CONTRAST
DIFFERENCES < 1%.
GOALS OF CT



MINIMAL SUPERIMPOSITION
IMAGE CONTRAST IMPROVEMENT
SMALL TISSUE DIFFERENCE
RECORDING
CT DATA AQUISITION
SLIP RINGS
SEQUENTIAL-SLICE BY SLICE
SCANNING
SCANNING
TRANSMISSION
RELATIVE TRANSMISSION=Io/I
Total # of trans. measurements=
# of views X # of rays in each
view
ATTENUATION
DATA AQUSITION GEOMETRIES


CONTINUOUS
STATIONARY
CONTINUOUS
STATIONARY
CT 120-140 KVP



REDUCED DEPENDENCY ON
ATTENUATION COEFFICIENT
REDUCED CONTRAST
INCREASED PHOTON FLUX
ORIGINAL CLINICAL CT SCANS
COMPOSED OF
80 X 80 MATRIX
PIXELS
6400
X
Y
Z
ISOCENTER
SCAN FOV
SFOV
DETECTORS
SCAN FOV-SMALL
SFOV
DETECTORS
SFOV - HEAD
TOO SMALL OF SFOV – OUT OF
FIELD ARTIFACT
SCAN FOV-RESOLUTION
SFOV
RECONSTRUCTION
Ц
RECONSTRUCTION
CT#
SCAN FOV-RESOLUTION
SFOV
DISPLAY FOV vs SCANNING FOV



DFOV CAN BE EQUAL OR LESS OF SFOV
SFOV – AREA OF MEASUREMENT
DURING SCAN
DFOV - DISPLAYED IMAGE
PIXEL SIZE
PIXEL SIZE=
FOV (mm)/ MATRIX SIZE
MOST SCANNERS PIXEL SIZE
1 TO 10mm
EXAMPLE:


FOV= 40 CM= 40 X 10 MM=400 mm
MATRIX= 512 X 512 = 5122
400/512 = 0.78 mm
0.8 mm
EACH PIXEL IN CT HAS RANGE OF
GRAY SHADES


2 8 = 256 SHADES
2 12 = 4096 SHADES = -100 TO 3095
SHADES OF GRAY
PIXEL vs VOXEL
PIXEL
VOXEL
PIXEL SIZE DEPENDS ON:


MATRIX SIZE
FOV
VOXEL SIZE DEPENDS



FOV
MATRIX SIZE
SLICE THICKNESS
IMAGE DISPLAY
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)
MATRIX
PIXEL MATRIX
IN CT DIGITAL
RECONSTRUCTED IMAGE IS
CONVERTED IMAGE IS
CONVERTED INTO A GRAY
SCALE IMAGE.
DIGITAL
DAC
ANALOG
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.
CRT OPERATION
IMAGE FORMATION ON THE MONITOR
IMAGE FORMATION ON THE MONITOR
CT NUMBER
LINEAR ATTENUATION
COEFFICIENT ( cm-1)








BONE
BLOOD
G. MATTER
W. MATTER
CSF
WATER
FAT
AIR
0.528
0.208
0.212
0.213
0.207
0.206
0.185
0.0004
CT # vs BRIGHTNESS LEVEL
+ 1000
-1000
CT #
1000
CT #
- 500
CT # OF CYST
5
CT # OF LIPOMA ( FATTY TUMOR)
-100
W 120
L 40
W 80
L 40
DATA FLOW IN CT
REFERENCE DETECTOR
REFERENCE DETECTOR
PREPROCESSOR
ADC
COMPUTER
RAW DATA
PROCESSORS
RECONSTRUCTED DATA
DISK
TAPE
BACK
PROJECTOR
CONVOLVED DATA
DAC
CRT DISPLAY
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