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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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PRINCIPLES OF CT

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Principles of ct l.jpg

PRINCIPLES OF CT


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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SUPERIMPOSITION


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RADIOGRAPHY LIMITATIONTISUE DIFFERENCE SENSITIVITY>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 LIMITATIONSMOTION BLURR


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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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TRANSMISSION

RELATIVE TRANSMISSION=Io/I


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HISTORY OF CT


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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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HOUNSFIELD’S SKETCH


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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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HOUNSFIELD’S LATHE BED SCANNER


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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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CLINICALLY USEFUL CT SCANNER


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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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180 DEG ROTATION


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180 DEG ROTATION


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360 DEG ROTATION


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360 DEG ROTATION


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MODERN SCANNER


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CT MAIN SYSTEMS

  • IMAGING SYSTEM

  • COMPUTER SYSTEM

  • DISPLAY, RECORDING, STORAGE SYSTEM

  • DATA ACQUISITION SYSTEM


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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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GANTRY INSIDE


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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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COORDINATE SYSTEM IN CT

X


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COORDINATE SYSTEM IN CT

Y


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COORDINATE SYSTEM IN CT

Z


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COORDINATE SYSTEM IN CT

ISO-CENTER


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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


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IMAGE DISPLAY


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IN CT DIGITAL RECONSTRUCTED IMAGE IS CONVERTED IMAGE IS CONVERTED INTO A GRAY SCALE IMAGE.


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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 CYST

5


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CT # OF LIPOMA ( FATTY TUMOR)

-100


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SCANNING


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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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CONVENTIONAL AND SPIRAL/HELICAL CT


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ADVANTAGE OF SPIRAL IMAGING OVER CONVENTIONAL

  • SPEED


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CT SPECIAL APPLICATIONS


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CARDIAC ANGIOGRAPHY


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VIRTUAL ENDOSCOPY


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RADIATION TREATMENT


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3D IMAGING


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