7 physical principles of ct
TRANSCRIPT
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 CT#RECONSTRUCTION
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 mm0.8 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
PIXELVOXEL
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.
DAC
ANALOG
DIGITAL
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 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
CT # vs BRIGHTNESS LEVEL
+ 1000
-1000
CT # 1000
CT # - 500
CT # OF CYST5
CT # OF LIPOMA ( FATTY TUMOR)
-100
W 120L 40
W 80L 40
DATA FLOW IN CTREFERENCE DETECTOR
REFERENCE DETECTOR
ADCPREPROCESSOR
COMPUTER
RAW DATA
CONVOLVED DATABACK
PROJECTORRECONSTRUCTED DATA
PROCESSORS
DISK TAPE 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