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PRINCIPLES OF CT. TOMOGRAPHY TOMOS ---SECTION. RADIOGRAPHY LIMITATIONS. SUPERIMPOSITION DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS. . SUPERIMPOSITION. RADIOGRAPHY LIMITATIONTISUE DIFFERENCE SENSITIVITY >10%. TOMOGRAPHY (CONVENTIONAL).

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Tomography tomos section l.jpg
TOMOGRAPHYTOMOS---SECTION


Radiography limitations l.jpg
RADIOGRAPHY LIMITATIONS

  • SUPERIMPOSITION

  • DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS.




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TOMOGRAPHY (CONVENTIONAL)

  • ELIMINATES TISSUE SUPERIMPOSITION

  • INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES






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LIMITATIONS OF CT

  • UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH SLIGHT CONTRAST DIFFERENCES < 1%.


Goals of ct l.jpg
GOALS OF CT

  • MINIMAL SUPERIMPOSITION

  • IMAGE CONTRAST IMPROVEMENT

  • SMALL TISSUE DIFFERENCE RECORDING



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TRANSMISSION

RELATIVE TRANSMISSION=Io/I



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Sir Godfrey Newbold Hounsfield CBE (28 August 1919 – 12 August 2004) was an English electrical engineer who shared the 1979 Nobel Prize for Physiology or Medicine with Allan McLeod Cormack for his part in developing the diagnostic technique of X-ray computed tomography (CT).



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CONSTRUCTION OF FIRST CT

  • RADIATION SOURCE – AMERICUM GAMMA SOURCE

  • SCAN—9 DAYS

  • COMPUTER PROCESSING—2.5 HOURS

  • PICTURE PRODUCTION 1 DAY



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1972

FIRST CLINICAL PROTOTYPE CT BRAIN SCANNER

FIRST SCANS—20 MIN.

LATER REDUCED TO 4.5 MIN.



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1974

DR. ROBERT LEDLEY DEVELOPED THE FIRST WHOLE BODY CT SCANNER .


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SCANNER GENERATIONS

  • I

  • II

  • III

  • IV







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CT MAIN SYSTEMS

  • IMAGING SYSTEM

  • COMPUTER SYSTEM

  • DISPLAY, RECORDING, STORAGE SYSTEM

  • DATA ACQUISITION SYSTEM


Imaging system l.jpg
IMAGING SYSTEM

  • PRODUCTION OF X-RAYS

  • SHAPING OF X-RAY BEAM ENERGY

  • FILTERING X-RAY BEAM


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SCANNER

GANTRY

TABLE/COUCH



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COMPUTER SYSTEM

  • RECONSTRUCTION AND POSTPROCESSING

  • CONTROL OF ALL SCANNER COMPONENTS

  • CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.

  • DATA FLOW DIRECTION


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COMPUTER SYSTEM IN CT

  • MINICOMPUTERS


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IMAGE DISPLAY, RECORDING, STORAGE

  • DISPLAYS IMAGE ( OUTPUT FROM COMPUTER)

  • PROVIDES HARD COPY OF THE IMAGE

  • FACILITATES THE STORAGE AND RETRIEVAL OF DIGITAL DATA

  • COMMUNICATES IMAGES IN THE NETWORK


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DATA ACQUISITION SYSTEM (DAS)

  • SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER.


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CT COMPONENTS

GANTRY

COMPUTER

TABLE/COUCH

CONSOLE


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ORIGINAL CLINICAL CT SCANS COMPOSED OF

80 X 80 MATRIX

PIXELS

6400


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EARLY DAYS vs TODAY

80 x 80

512 x 512






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SCAN FOV

SFOV

DETECTORS


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DFOV – DISPLAYED FIELD OF VIEW

  • SIZE DISPLAYED ON THE MONITOR


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PIXEL SIZE

PIXEL SIZE= DFOV (mm)/ MATRIX SIZE


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RECONSTRUCTION

RECONSTRUCTION


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PIXEL vs VOXEL

PIXEL

VOXEL


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PIXEL SIZE DEPENDS ON:

  • MATRIX SIZE

  • FOV


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VOXEL SIZE DEPENDS

  • FOV

  • MATRIX SIZE

  • SLICE THICKNESS



In ct digital reconstructed image is converted image is converted into a gray scale image l.jpg
IN CT DIGITAL RECONSTRUCTED IMAGE IS CONVERTED IMAGE IS CONVERTED INTO A GRAY SCALE IMAGE.



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CT #

1000


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CT #

- 500





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TECHNIQUE

  • kVp

  • mA

  • TIME

  • SLICE THICKNESS

  • SLICE INCREMENTATION


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PATIENT ORIENTATION

  • HEAD FIRST

  • FEET FIRST


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SCANNING

  • TOPOGRAM

  • REGULAR SCAN


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TOPOGRAM (SCOUT)

  • TUBE DOES NOT REVOLVE AROUND THE PATIENT


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AP SCOUT

  • TUBE SUSPENDED ABOVE PATIENT DURING SCOUT GENERATION

TUBE


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LAT SCOUT

  • TUBE AT THE 90º ANGLE TO PATIENT


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AXIAL SCAN

  • TABLE STOPS AT THE SCANNING POSITION AND THE TUBE ROTATES AROUND A PATIENT.


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SPIRAL

  • PATIENT CONTINUOUSLY MOVES IN THE Z-AXIS DIRECTION WHILE THE TUBE ROTATES AROUND.









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