7 Physical principles of CT

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