Computed Tomography

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Computed Tomography Powered By Docstoc
					 Computed
Tomography
  Basic principles

            V.G.Wimalasena
            Principal
            School of Radiography
                     Introduction
   Computed tomography (CT) is a medical imaging
    method employing tomography.
   The word "tomography" is derived from the Greek
    tomos (slice) and graphein (to write).
   A large series of two-dimensional X-ray images (slices)
    of the inside of an object are taken around a single axis
    of rotation.
   Digital geometry processing is used to generate three-
    dimensional images of the object from those slices.
                     History
   The first commercially viable CT scanner was
    invented by Sir Godfrey Hounsfield in Hayes,
    United Kingdom at EMI Central Research
    Laboratories using X-rays. Hounsfield conceived
    his idea in 1967. and it was publicly announced
    in 1972.
   Allan McLeod Cormack of Tufts University in
    Massachusetts independently invented a similar
    process, and both Hounsfield and Cormack
    shared the 1979 Nobel Prize in Medicine.
Prototype CT scanner
Historic EMI Scanner
Modern CT scanner
                      Label
1.    gantry aperture (720mm diameter)
2.    microphone
3.    sagittal laser alignment light
4.    patient guide lights
5.    x-ray exposure indicator light
6.    emergency stop buttons
7.    gantry control panels
8.    external laser alignment lights
9.    patient couch
10.   ECG gating monitor
CT Gantry –Internal structure
                    Label
1.    x-ray tube
2.    filters, collimator, and reference detector
3.    internal projector
4.    x-ray tube heat exchanger (oil cooler)
5.    high voltage generator (0-75kV)
6.    direct drive gantry motor
7.    rotation control unit
8.    data acquisition system (DAS)
9.    detectors
10.   slip rings
        Understanding Basic factors
   Absorption :-stopping of
    x-rays with transfer of
    energy                                   Scattered x-rays
   Scatter:- deflection of x-
    rays
   Incident Intensity :- No.
    of x-ray photons falling
    on an object                                   Transmitted
   Transmitted Intensity:-                        X-ray beam
                               Incident x-
    No. of photons passing ray beam
    through
                     Attenuation
The reduction of the beam      More dense
  intensity on passing         material      Less
  through the material due                   transmitted
  to absorption plus scatter                 x-rays
The degree of attenuation
  is obtained by measuring                  More
  and comparing the                         transmitted
  incident and transmitted                  x-rays
  intensities

                               Less dense
                               material
Applications of X-ray attenuation &
             detection

      Conventional X-ray (Radiography)
      Conventional Tomography
      Computed Tomography
               Conventional X-Ray
   Conventional x-ray
    produces a compression
    of a volume to a plane
   The detector is the Silver
    halide crystal on a x-ray
    film
   The degree of
    blackening represents the
    total attenuation through
    the path of x-ray
    photons
   The higher the
    attenuation the lesser is
    the blackness
   The structure which
    results more attenuation
    or more transmission
    predominates in the
    image
       Conventional Tomography
   The source and detector
    moves
   Produces Images of
    coronal or sagittal
    sections (cuts) of areas
    of interest
   Eliminates the
    superimposition of
    structures above and
    below
                      CT Scan
   CT scan produces axial
    sections/cuts /Slices         X-ray tube
   The CT image is
    recorded through a
    SCAN.
   Scan?
   A scan is made up of
    multiple X-Ray
    attenuation
    measurements around an
    objects periphery
                                Detector
                       Slice / Cut
   The cross sectional
    portion of the body
    which is scanned for the
    production of CT image
    is called a slice.
   The slice has width and
    therefore volume.
   The width is determined
    by the width of the x-ray
    beam

To be continued
  ….CTComplementary2

				
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