7 physical principles of CT

TOMOGRAPHY CONVENTIONAL ELIMINATES TISSUE SUPERIMPOSITION  INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES... GOALS OF CT MINIMAL SUPERIMPOSITION  IMAGE CONTRAST IMPROVEMENT  S

PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

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

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

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

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

 MATRIX SIZE

 SLICE THICKNESS

IMAGE DISPLAY

GRAY SCALE DISPLAY MONITOR RESOLUTION IS RELATED TO THE SIZE OF THE PIXEL MATRIX

MATRIX

PIXEL MATRIX

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

CT NUMBER

LINEAR ATTENUATION COEFFICIENT ( cm -1 )

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

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