Computed Tomography Basic principles V.G.Wimalasena Principal
School of Radiography Slide 2 Introduction Computed tomography (CT)
is a imaging method employing tomography. Computed tomography (CT)
is a medical imaging method employing
tomography.imagingtomographymedical imagingtomography The word
"tomography" is derived from the Greek tomos (slice) and graphein
(to write). The word "tomography" is derived from the Greek tomos
(slice) and graphein (to write).Greek A large series of
two-dimensional X-ray images (slices) of the inside of an object
are taken around a single axis of rotation. A large series of
two-dimensional X-ray images (slices) of the inside of an object
are taken around a single axis of rotation.X-rayaxis of
rotationX-rayaxis of rotation Digital geometry processing is used
to generate three- dimensional images of the object from those
slices. Digital geometry processing is used to generate three-
dimensional images of the object from those slices. Digital
geometry processingthree- dimensional image Digital geometry
processingthree- dimensional image Slide 3 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. 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.Godfrey HounsfieldHayes United KingdomEMIGodfrey
HounsfieldHayes United KingdomEMI 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. 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.
Allan McLeod CormackTufts University MassachusettsNobel Prize in
Medicine Allan McLeod CormackTufts University MassachusettsNobel
Prize in Medicine Slide 4 Prototype CT scanner Slide 5 Historic EMI
Scanner Slide 6 Modern CT scanner Slide 7 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 patient couch patient couch 10. ECG gating
monitor Slide 8 CT Gantry Internal structure Slide 9 Label 1. x-ray
tube x-ray tube x-ray tube 2. filters, collimator, and reference
detector filterscollimator filterscollimator 3. internal projector
4. x-ray tube heat exchanger (oil cooler) 5. high voltage generator
(0-75kV) high voltage generator high voltage generator 6. direct
drive gantry motor 7. rotation control unit 8. data acquisition
system (DAS) data acquisition system data acquisition system 9.
detectors detectors 10. slip rings Slide 10 Understanding Basic
factors Absorption :-stopping of x-rays with transfer of energy
Absorption :-stopping of x-rays with transfer of energy Scatter:-
deflection of x- rays Scatter:- deflection of x- rays Incident
Intensity :- No. of x-ray photons falling on an object Incident
Intensity :- No. of x-ray photons falling on an object Transmitted
Intensity:- No. of photons passing through Transmitted Intensity:-
No. of photons passing through Incident x- ray beam Transmitted
X-ray beam Scattered x-rays Slide 11 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 dense material Less transmitted x-rays
More transmitted x-rays Slide 12 Applications of X-ray attenuation
& detection Conventional X-ray (Radiography) Conventional X-ray
(Radiography) Conventional Tomography Conventional Tomography
Computed Tomography Computed Tomography Slide 13 Conventional X-Ray
Conventional x-ray produces a compression of a volume to a plane
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
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 degree of blackening represents the total
attenuation through the path of x-ray photons Slide 14 The higher
the attenuation the lesser is the blackness The higher the
attenuation the lesser is the blackness The structure which results
more attenuation or more transmission predominates in the image The
structure which results more attenuation or more transmission
predominates in the image Slide 15 Conventional Tomography The
source and detector moves The source and detector moves Produces
Images of coronal or sagittal sections (cuts) of areas of interest
Produces Images of coronal or sagittal sections (cuts) of areas of
interest Eliminates the superimposition of structures above and
below Eliminates the superimposition of structures above and below
Slide 16 CT Scan CT scan produces axial sections/cuts /Slices CT
scan produces axial sections/cuts /Slices The CT image is recorded
through a SCAN. The CT image is recorded through a SCAN. Scan?
Scan? A scan is made up of multiple X-Ray attenuation measurements
around an objects periphery A scan is made up of multiple X-Ray
attenuation measurements around an objects periphery X-ray tube
Detector Slide 17 Slice / Cut The cross sectional portion of the
body which is scanned for the production of CT image is called a
slice. 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 slice has width and therefore volume. The
width is determined by the width of the x-ray beam The width is
determined by the width of the x-ray beam To be continued
.CTComplementary2
