Download - 7 Physical principles of CT
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PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY
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RADIOGRAPHY LIMITATIONS
SUPERIMPOSITION DIFFICULTY IN DISTINGUISHING
BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS.
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SUPERIMPOSITION
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TISUE DIFFERENCE SENSITIVITY 5%-10%
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TOMOGRAPHY ( CONVENTIONAL)
ELIMINATES TISSUE SUPERIMPOSITION INCREASES CONTRAST OF LOW
SUBJECT CONTRAST TISSUES
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TOMOGRAPHY
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TOMOGRAPHY
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TOMOGRAPHY
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CT ADVANTAGES
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LIMITATIONS OF CT
UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH SLIGHT CONTRAST DIFFERENCES < 1%.
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GOALS OF CT
MINIMAL SUPERIMPOSITION IMAGE CONTRAST IMPROVEMENT SMALL TISSUE DIFFERENCE
RECORDING
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CT DATA AQUISITION
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SLIP RINGS
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SEQUENTIAL-SLICE BY SLICE SCANNING
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SCANNING
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TRANSMISSION
RELATIVE TRANSMISSION=Io/I
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Total # of trans. measurements=
# of views X # of rays in each view
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ATTENUATION
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DATA AQUSITION GEOMETRIES
CONTINUOUS STATIONARY
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CONTINUOUS
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STATIONARY
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CT 120-140 KVP
REDUCED DEPENDENCY ON ATTENUATION COEFFICIENT
REDUCED CONTRAST INCREASED PHOTON FLUX
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ORIGINAL CLINICAL CT SCANS COMPOSED OF
80 X 80 MATRIX
PIXELS
6400
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X
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Y
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Z
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ISOCENTER
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SCAN FOV
SFOV
DETECTORS
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SCAN FOV-SMALL
SFOV
DETECTORS
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SFOV - HEAD
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TOO SMALL OF SFOV – OUT OF FIELD ARTIFACT
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SCAN FOV-RESOLUTION
SFOV
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RECONSTRUCTION
Ц CT#RECONSTRUCTION
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SCAN FOV-RESOLUTION
SFOV
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DISPLAY FOV vs SCANNING FOV
DFOV CAN BE EQUAL OR LESS OF SFOV SFOV – AREA OF MEASUREMENT
DURING SCAN DFOV - DISPLAYED IMAGE
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PIXEL SIZE
PIXEL SIZE= FOV (mm)/ MATRIX SIZE
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MOST SCANNERS PIXEL SIZE
1 TO 10mm
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EXAMPLE:
FOV= 40 CM= 40 X 10 MM=400 mm MATRIX= 512 X 512 = 5122
400/512 = 0.78 mm0.8 mm
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EACH PIXEL IN CT HAS RANGE OF GRAY SHADES
2 8 = 256 SHADES 2 12 = 4096 SHADES = -100 TO 3095
SHADES OF GRAY
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PIXEL vs VOXEL
PIXELVOXEL
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PIXEL SIZE DEPENDS ON:
MATRIX SIZE FOV
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VOXEL SIZE DEPENDS
FOV MATRIX SIZE SLICE THICKNESS
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IMAGE DISPLAY
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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)
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MATRIX
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PIXEL MATRIX
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IN CT DIGITAL RECONSTRUCTED IMAGE IS CONVERTED IMAGE IS CONVERTED INTO A GRAY SCALE IMAGE.
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DAC
ANALOG
DIGITAL
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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.
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CRT OPERATION
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IMAGE FORMATION ON THE MONITOR
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IMAGE FORMATION ON THE MONITOR
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CT NUMBER
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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
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CT # vs BRIGHTNESS LEVEL
+ 1000
-1000
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CT # 1000
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CT # - 500
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CT # OF CYST5
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CT # OF LIPOMA ( FATTY TUMOR)
-100
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W 120L 40
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W 80L 40
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DATA FLOW IN CTREFERENCE DETECTOR
REFERENCE DETECTOR
ADCPREPROCESSOR
COMPUTER
RAW DATA
CONVOLVED DATABACK
PROJECTORRECONSTRUCTED DATA
PROCESSORS
DISK TAPE DAC CRT DISPLAY
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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