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PAM2003 Lecture 5: Computed Tomography II

PAM2003 Lecture 5: Computed Tomography II. Image Processing. In this lecture. Briefly re-cap last week’s CT lecture Image Processing Back projection CT or Houndfields numbers Multiplanar Reformatting (MPR) Volume Rendering Partial Volume Effect Resolution Compromise. Last Time.

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PAM2003 Lecture 5: Computed Tomography II

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  1. PAM2003Lecture 5: Computed Tomography II Image Processing

  2. In this lecture • Briefly re-cap last week’s CT lecture • Image Processing • Back projection • CT or Houndfields numbers • Multiplanar Reformatting (MPR) • Volume Rendering • Partial Volume Effect • Resolution Compromise

  3. Last Time • Short comings of conventional radiography • What is CT • Advantage of CT • Applications of CT • Different Generation Scanners • Spiral CT • Multislice spiral CT

  4. Re-cap • Cross-sectional ‘slices’ • Eliminate superposition

  5. Sheet X-ray beam patient Array of detectors Re-cap: Principles • Use a series of 2D views through an object to calculate its contents • Slice defined by ‘sheet’ of x-rays, produced by a fan beam (typically 1 cm thick) • Thin slice also serves to reduce scatter Rotate tube & detectors through 360º Computer reconstruction of 2D slices

  6. Re-cap: Measurement • Total attenuation between tube & detector • Sum of attenuation coefficients in all voxels beam has travelled through • A measure of how rapidly x-ray are absorbed along line within material • Goal: To calculate attenuation within each individual voxel attenuation

  7. volume element (VOXEL) picture element (PIXEL) slice thickness Re-cap: CT Image • Slice subdivided into matrix of tissue voxels • Voxels correspond to locations in computer memory or pixels in image • Brightness of each pixel governed by x-ray attenuation in corresponding voxel

  8. What We Measure • Detectors measure X-ray intensity after attenuation through patient • Attenuation equal to sum of attenuation in each pixels beam has travelled through • Computer accounts for ‘fan’ shaped beam Total X-ray intensity transmitted through column

  9. Back Projection Simplistic case of sphere in centre of object Simple Analogy • Many projection angles • Detector records total attenuation • Columns filled with total • Overlying projections build up image • More projections increase image quality 8 projections 4 projections 2 projections 64 projections 32 projections 16 projections

  10. Back Projection Simple Arithmetic Example 15 • Simplistic case • 3 X 3 array of pixels 5 5 30 10 10 15 15 5 10 30 30 15 30 10 5 - Sum of all 4 projections ÷

  11. Back Projection Simple Arithmetic Example 15 • Simplistic case • 3 X 3 array of pixels 5 5 30 10 10 15 15 5 10 30 30 15 30 10 5 - 10 Sum of all 4 projections ÷11

  12. Filtered Back Projection • Back projection causes blurring • Compensated by computer using process called ‘filtering’ • Effectively modifies brightness near edge of each back-projected beam Back projected image Filtered back projected image

  13. CT Numbers • Attenuation coefficient of each pixel is compared to that of water, μW • Multiplier (1000) used to obtain whole numbers • Defined as: -1000 for air 0 for water • Varies with kV

  14. +100 Bone +1000 liver window level (WL) muscle blood window width (WW) 0 CT number lung fat Air -1000 -100 Windowing • 2000 CT numbers • Human eye can only perceive ~64 levels • Soft tissue (ex. Fat & lungs) only covers ~80 CT numbers • WL & WW set independently to differentiate different tissues • Pixels with CT numbers outside window are undifferentiated, displayed as black or white

  15. Windowing • Allows differentiation of different tissues WW: 530 WL:-590 WW: 400 WL: -12 • Same image, Different windowing

  16. Multiplanar Reconstruction (MPR) • Data acquired as a series of Axial slices Axial slices

  17. Multiplanar Reconstruction (MPR) • MPR reformats data into Coronal & Sagittal slices Sagittal slices Coronal slices

  18. 3D Image Reconstruction

  19. Partial volume effect • CT cannot reveal detail within voxel • Small high contrast objects raise CT of entire pixel • Tiny calcifications • Traces of contrast media • Reduce effect by using thinner slice

  20. Resolution Compromise • Max spatial and contrast resolution cannot be achieved without unacceptably high dose • E.g. Doubling contrast requires without increasing pixel size requires 4 times increase in dose

  21. Summary • Image Processing • Back projection • CT or Houndfields numbers • Multiplanar Reformatting (MPR) • Volume Rendering • Partial Volume Effect • Resolution Compromise

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