Computed
Tomography
Basic principles
V.G.Wimalasena
Principal
School of Radiography
Introduction
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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 threedimensional images of the object from those slices.
History
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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
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gantry aperture (720mm diameter)
microphone
sagittal laser alignment light
patient guide lights
x-ray exposure indicator light
emergency stop buttons
gantry control panels
external laser alignment lights
patient couch
ECG gating monitor
CT Gantry –Internal structure
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x-ray tube
filters, collimator, and reference detector
internal projector
x-ray tube heat exchanger (oil cooler)
high voltage generator (0-75kV)
direct drive gantry motor
rotation control unit
data acquisition system (DAS)
detectors
slip rings
Understanding Basic factors
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Absorption :-stopping of
x-rays with transfer of
energy
Scatter:- deflection of xrays
Incident Intensity :- No.
of x-ray photons falling
on an object
Transmitted Intensity:Incident xNo. of photons passing ray beam
through
Scattered x-rays
Transmitted
X-ray beam
Attenuation
The reduction of the beam
intensity on passing
through the material due
to absorption plus scatter
The degree of attenuation
is obtained by measuring
and comparing the
incident and transmitted
intensities
More dense
material
Less
transmitted
x-rays
More
transmitted
x-rays
Less dense
material
Applications of X-ray attenuation &
detection
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Conventional X-ray (Radiography)
Conventional Tomography
Computed Tomography
Conventional X-Ray
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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
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The higher the
attenuation the lesser is
the blackness
The structure which
results more attenuation
or more transmission
predominates in the
image
Conventional Tomography
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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
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CT scan produces axial
sections/cuts /Slices
The CT image is
recorded through a
SCAN.
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Scan?
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A scan is made up of
multiple X-Ray
attenuation
measurements around an
objects periphery
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X-ray tube
Detector
Slice / Cut
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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