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 sections/cuts /Slices/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