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CT Physics

CT Physics. V.G.Wimalasena Principal School of radiography. Slice / Cut. 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.

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CT Physics

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  1. CT Physics V.G.Wimalasena Principal School of radiography

  2. Slice / Cut • The cross sectionalportion 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

  3. Matrix • The image is represented as a MATRIX of numbers. • Matrix :- A two dimensional array ofnumbers arranged in rows and columns. • Each number represents the value of the image at that location

  4. VOXEL • Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL

  5. PIXEL • The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element)

  6. CT numbers • The numbers in the image matrix are called CT numbers. • Each pixel has a number which represents the x-rayattenuation in the corresponding voxel of the object

  7. Visual image & Gray Scale • To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale. • Each shade of grayrepresents the x-ray attenuation within the corresponding voxel

  8. CT Image

  9. Phases of CT imaging • Scanning the patient • Data Acquisition • Tube or tube and detector move • Multiple attenuation measurements are taken around the object • Image reconstruction • Image Display • Image archival (recording)

  10. Scanning the patient - Evolution of CT scanners 1st Generation 2nd Generation 3rd Generation 4th Generation

  11. DATA ACQUISITION • Basic components • X-ray tube • Collimators • Detector/s • Collimated x-ray beam traverses the object and enters the detector. • The attenuation is determined from the difference betweenincident intensity and transmitted intensity X-ray tube Collimators Detector

  12. Translate rotate system • In the basic CT system the x-ray tube and detector are translated linearly so that the beam scans the object (eg. A disc of uniform density)

  13. Ray, Ray sum, View & Attenuation Profile Ray – Imaginary line between Tube & Detector Ray Sum – Attenuation along a Ray View – The set of ray sums in one direction The attenuation for each ray sum when plotted as function of its position is called anattenuation profile Ray sums Ray View Attenuation profile

  14. Attenuation profile of different structures • Attenuation of objects with different densities will change the attenuation profile Object with low attenuation Object with high attenuation

  15. Attenuation profile at different positions • In a translate –rotate CT, after a view is recorded, the tube and detector rotate a small angle and the entire process is repeated until many views have been recorded for the same slice

  16. Views & attenuation profiles for a slice 1 2 4 3

  17. Image reconstruction • The image is created by reflecting the attenuation profiles back in the same direction they were obtained • This process is called BACK PROJECTION BACK PROJECTION

  18. Drawbacks in Back projection (Artifacts - Star shape & streaks) • The resultant image closely resembles the original object • But it shows star shaped patterns around objects and streaks • These are called ‘Star’ and ‘streak’ artifacts

  19. Formation of Star artifact and streaks • Consider a scan of a single high density object suspended in air

  20. The attenuation profile for this object has a single impulse signal

  21. The back projections take the form of a stripe through the center of the object

  22. Back projections are crated for each profile

  23. Addition of the attenuation profiles create an image with star and streak artifacts Final back projection To be continued – CT Complementary 3

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