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Computed Tomography

describes computed tomography and related algorithms.

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Computed Tomography

  1. 1. COMPUTED TOMOGRAPHY (CT) Represented by Kadriye Doğan
  2. 2. Introduction Computed tomography (CT) is an imaging method that uses special x-ray equipment to create detailed scans of areas inside the body. The term “tomography” comes from the Greek words tomos (a slice) and graphein (to write or record).
  3. 3. History  Invented by Godfrey Hounsfield in 1967,UK and announced in 1972.  Allan McLeod Cormack in US, independently invented similar process,and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.
  4. 4. Terms Attenuation is a reduction of the beam intensity on passing through the body section. Scan is made up of multiple x-ray attenuation measurements around a section. Slice is the cross sectional portion of the body. Voxel is a 3D version of the pixels. CT number is a number which represents the x- ray attenuation.
  5. 5. CT Scan When X-rays are irradiated on the human body, some of the rays are absorbed and some pass through the body to produce an image. X-ray detectors absorbs the penetrated X-rays, measures the X-ray amount, and transmits the data to a computer system. Computer system calculates and analyzes data from each detector in each level, and finally reconstructs multiple, two-dimensional, cross-sectional images.
  6. 6. Components of CT System  The scanning unit is consists of the gantry with tube and detectors.  The patient table  The computer system for image reconstruction  Monitor and record system
  7. 7. Image Reconstruction-1  X-rays density is measured before and after passing through the body section.  The difference between measurements are recorded as attenuation of section which across the detector.  The computer system converts digital data which comes from detectors to CT numbers of voxels for each section.
  8. 8. Image Reconstruction-2 The pixel value is colored white,black or gray according to the mean attenuation of the section that it corresponds to the Hounsfield scale.
  9. 9. Image Reconstruction Algorithms There are numerous reconstruction algorithms. Filtered Back Projection is most widely used in clinical CT scanners. 1)Back Projection 2)Iterative Reconsruction 3)Fouier Reconstruction 4)Filtered Back Projection
  10. 10. Back Projection
  11. 11. Back Projection
  12. 12. Iterative Reconstruction  Projection data are predicted based on an assumption about the initial attenuation coefficients of all voxels.  These predicted data are compared to measured projection data.  The voxel attenuation values are modified until an acceptable level of error between the predicted and measured data is achieved.
  13. 13. Iterative Reconstruction
  14. 14. Fourier Reconstruction  Calculate the 1D Fourier transform of all projections.[p(r) = P(k)]  Place P(k) on polar grid to get P(k,Θ)  Resample in Cartesian space to get F(kx,ky)  Calculate the 2D inverse Fourier transform of F(kx,ky) to get f(x,y) – image.
  15. 15. Fourier Reconstruction
  16. 16. Fourier Reconstruction
  17. 17. Filtered Back Projection  A problem that is immediately apparent is the blurring that occur in other parts of the reconstructed image.  The combination of back projection and ramp filtering is known as filtered back projection.
  18. 18. Filtered Back ProjectionFiltered Back Projection
  19. 19. Filtered Back Projection

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  • dr.youssif

    Feb. 4, 2018
  • ASHIQRAINA

    Mar. 10, 2020
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    May. 25, 2020
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    Jun. 1, 2020

describes computed tomography and related algorithms.

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