Computed Tomography Basic principles Tamer M. Nassef

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

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 and it was publicly announced in 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 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

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 patient couch patient couch 10. ECG gating monitor

CT Gantry –Internal structure

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

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

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

Applications of X-ray attenuation & detection Conventional X-ray (Radiography) Conventional X-ray (Radiography) Conventional Tomography Conventional Tomography Computed Tomography Computed Tomography

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

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

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

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

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