1 / 21

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.

thi
Download Presentation

PAM2003 Lecture 5: Computed Tomography II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related