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Computed Computed TomographyTomography
Basic principlesBasic principles
V.G.Wimalasena
Principal
School of Radiography
IntroductionIntroduction Computed tomography (CT)Computed tomography (CT) is a is a medical
imagingimaging method employing method employing tomographytomography. . The word "tomography" is derived from the The word "tomography" is derived from the
GreekGreek tomostomos (slice) and (slice) and grapheingraphein (to write). (to write). A large series of two-dimensional A large series of two-dimensional X-rayX-ray
images (slices) of the inside of an object are images (slices) of the inside of an object are taken around a single taken around a single axis of rotationaxis of rotation. .
Digital geometry processingDigital geometry processing is used to is used to generate generate three-dimensional imagethree-dimensional imagess of the of the object from those slices.object from those slices.
HistoryHistory The first commercially viable CT The first commercially viable CT
scanner was invented by Sir scanner was invented by Sir Godfrey HounsfieldGodfrey Hounsfield in in HayesHayes, , United KingdomUnited Kingdom at at EMIEMI Central Central Research Laboratories using X-rays. Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. Hounsfield conceived his idea in 1967. and it was publicly announced in 1972. and it was publicly announced in 1972.
Allan McLeod CormackAllan McLeod Cormack of of Tufts UniversityTufts University in in MassachusettsMassachusetts independently invented a similar independently invented a similar process, and both Hounsfield and process, and both Hounsfield and Cormack shared the 1979 Cormack shared the 1979 Nobel Prize in MedicineNobel Prize in Medicine..
Prototype CT scannerPrototype CT scanner
Historic EMI ScannerHistoric EMI Scanner
Modern CT scannerModern CT scanner
LabelLabel1.1. gantry aperture (720mm diameter)gantry aperture (720mm diameter)2.2. microphone microphone 3.3. sagittal laser alignment light sagittal laser alignment light 4.4. patient guide lights patient guide lights 5.5. x-ray exposure indicator light x-ray exposure indicator light 6.6. emergency stop buttons emergency stop buttons 7.7. gantry control panels gantry control panels 8.8. external laser alignment lights external laser alignment lights 9.9. patient couchpatient couch 10.10. ECG gating monitor ECG gating monitor
CT Gantry –Internal CT Gantry –Internal structurestructure
LabelLabel1.1. x-ray tubex-ray tube2.2. filters, collimator, and reference filters, collimator, and reference
detector detector 3.3. internal projectorinternal projector4.4. x-ray tube heat exchanger (oil x-ray tube heat exchanger (oil
cooler)cooler)5.5. high voltage generator (0-75kV)high voltage generator (0-75kV)6.6. direct drive gantry motordirect drive gantry motor7.7. rotation control unitrotation control unit8.8. data acquisition system (DAS)data acquisition system (DAS)9.9. detectorsdetectors10.10. slip ringsslip rings
Understanding Basic Understanding Basic factorsfactors
AbsorptionAbsorption :- :-stopping of x-rays stopping of x-rays with transfer of with transfer of energyenergy
ScatterScatter:- deflection :- deflection of x-raysof x-rays
Incident IntensityIncident Intensity :- :- No. of x-ray photons No. of x-ray photons falling on an objectfalling on an object
Transmitted Transmitted IntensityIntensity:- No. of :- No. of photons passing photons passing throughthrough
Incident x-ray beam
Transmitted X-ray beam
Scattered x-rays
AttenuationAttenuationThe reduction of the The reduction of the
beam intensity on beam intensity on passing through the passing through the material due to material due to absorption plus absorption plus scatterscatter
The degree of The degree of attenuation is attenuation is obtained by obtained by measuring and measuring and comparing the comparing the incident and incident and transmitted transmitted intensitiesintensities
More dense material
Less dense material
Less transmitted x-rays
More transmitted x-rays
Applications of X-ray Applications of X-ray attenuation & detectionattenuation & detection
Conventional X-ray Conventional X-ray (Radiography)(Radiography)
Conventional TomographyConventional Tomography Computed TomographyComputed Tomography
Conventional X-RayConventional X-Ray Conventional x-ray Conventional x-ray
produces a produces a compression of a compression of a volume to a planevolume to a plane
The detector is the The detector is the Silver halide crystal Silver halide crystal on a x-ray filmon a x-ray film
The degree of The degree of blackening blackening represents the total represents the total attenuation through attenuation through the path of x-ray the path of x-ray photonsphotons
The The higherhigher the the attenuationattenuation the the lesserlesser is the is the blacknessblackness
The The structurestructure whichwhich results results more more attenuationattenuation or or more transmission more transmission predominatespredominates in in the imagethe image
Conventional Conventional TomographyTomography
The The sourcesource and and detector movesdetector moves
Produces Images Produces Images of coronal or of coronal or sagittal sections sagittal sections (cuts) of areas of (cuts) of areas of interestinterest
Eliminates the Eliminates the superimposition of superimposition of structures above structures above and belowand below
CT ScanCT Scan CT scanCT scan produces produces
axial axial sections/cuts /Slicessections/cuts /Slices
The CT image is The CT image is recorded through a recorded through a SCAN. SCAN.
Scan?Scan? A A scanscan is made up is made up
of of multiple X-Ray multiple X-Ray attenuation attenuation measurementsmeasurements around an objects around an objects peripheryperiphery
X-ray tube
Detector
Slice / CutSlice / Cut The The cross sectionalcross sectional
portion portion of the body of the body which is scanned for which is scanned for the production of CT the production of CT image is called a image is called a slice.slice.
The slice has The slice has width width andand therefore therefore volumevolume..
The width is The width is determined by the determined by the width width of the of the x-ray x-ray beambeam
To be continued To be continued ….CTComplementary2….CTComplementary2