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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


TOMOGRAPHYTOMOS---SECTION


RADIOGRAPHY LIMITATIONS

  • SUPERIMPOSITION

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


SUPERIMPOSITION


RADIOGRAPHY LIMITATIONTISUE DIFFERENCE SENSITIVITY>10%


TOMOGRAPHY (CONVENTIONAL)

  • ELIMINATES TISSUE SUPERIMPOSITION

  • INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES


TOMOGRAPHY


TOMOGRAPHY


TOMOGRAPHY LIMITATIONSMOTION BLURR


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


TRANSMISSION

RELATIVE TRANSMISSION=Io/I


HISTORY OF CT


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).


HOUNSFIELD’S SKETCH


CONSTRUCTION OF FIRST CT

  • RADIATION SOURCE – AMERICUM GAMMA SOURCE

  • SCAN—9 DAYS

  • COMPUTER PROCESSING—2.5 HOURS

  • PICTURE PRODUCTION 1 DAY


HOUNSFIELD’S LATHE BED SCANNER


1972

FIRST CLINICAL PROTOTYPE CT BRAIN SCANNER

FIRST SCANS—20 MIN.

LATER REDUCED TO 4.5 MIN.


CLINICALLY USEFUL CT SCANNER


1974

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


SCANNER GENERATIONS

  • I

  • II

  • III

  • IV


180 DEG ROTATION


180 DEG ROTATION


360 DEG ROTATION


360 DEG ROTATION


MODERN SCANNER


CT MAIN SYSTEMS

  • IMAGING SYSTEM

  • COMPUTER SYSTEM

  • DISPLAY, RECORDING, STORAGE SYSTEM

  • DATA ACQUISITION SYSTEM


IMAGING SYSTEM

  • PRODUCTION OF X-RAYS

  • SHAPING OF X-RAY BEAM ENERGY

  • FILTERING X-RAY BEAM


SCANNER

GANTRY

TABLE/COUCH


GANTRY INSIDE


COMPUTER SYSTEM

  • RECONSTRUCTION AND POSTPROCESSING

  • CONTROL OF ALL SCANNER COMPONENTS

  • CONTROL OF DATA ACQUSITION, PROCESSING, DISPLAY.

  • DATA FLOW DIRECTION


COMPUTER SYSTEM IN CT

  • MINICOMPUTERS


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


DATA ACQUISITION SYSTEM (DAS)

  • SET OF ELECTRONICS BETWEEN DETECTORS AND HOST COMPUTER.


CT COMPONENTS

GANTRY

COMPUTER

TABLE/COUCH

CONSOLE


ORIGINAL CLINICAL CT SCANS COMPOSED OF

80 X 80 MATRIX

PIXELS

6400


EARLY DAYS vs TODAY

80 x 80

512 x 512


COORDINATE SYSTEM IN CT

X


COORDINATE SYSTEM IN CT

Y


COORDINATE SYSTEM IN CT

Z


COORDINATE SYSTEM IN CT

ISO-CENTER


SCAN FOV

SFOV

DETECTORS


DFOV – DISPLAYED FIELD OF VIEW

  • SIZE DISPLAYED ON THE MONITOR


PIXEL SIZE

PIXEL SIZE= DFOV (mm)/ MATRIX SIZE


RECONSTRUCTION

RECONSTRUCTION


PIXEL vs VOXEL

PIXEL

VOXEL


PIXEL SIZE DEPENDS ON:

  • MATRIX SIZE

  • FOV


VOXEL SIZE DEPENDS

  • FOV

  • MATRIX SIZE

  • SLICE THICKNESS


IMAGE DISPLAY


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


CT # vs BRIGHTNESS LEVEL

+ 1000

-1000


CT #

1000


CT #

- 500


CT # OF CYST

5


CT # OF LIPOMA ( FATTY TUMOR)

-100


SCANNING


TECHNIQUE

  • kVp

  • mA

  • TIME

  • SLICE THICKNESS

  • SLICE INCREMENTATION


PATIENT ORIENTATION

  • HEAD FIRST

  • FEET FIRST


SCANNING

  • TOPOGRAM

  • REGULAR SCAN


TOPOGRAM (SCOUT)

  • TUBE DOES NOT REVOLVE AROUND THE PATIENT


AP SCOUT

  • TUBE SUSPENDED ABOVE PATIENT DURING SCOUT GENERATION

TUBE


LAT SCOUT

  • TUBE AT THE 90º ANGLE TO PATIENT


AXIAL SCAN

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


SPIRAL

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


CONVENTIONAL AND SPIRAL/HELICAL CT


ADVANTAGE OF SPIRAL IMAGING OVER CONVENTIONAL

  • SPEED


CT SPECIAL APPLICATIONS


CARDIAC ANGIOGRAPHY


VIRTUAL ENDOSCOPY


RADIATION TREATMENT


3D IMAGING


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