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IMAGE RECONSTRUCTION

IMAGE RECONSTRUCTION. ALGORITHM -A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM SPECIFIC INPUT. ALGORITHM MUST TERMINATE AFTER FINATE NUMBER OF STEPS . DETECTORS. PRE-PROCESING. REFORMATTED RAW DATA. CONVOLUTION WITH FILTER. IMAGE RECONSTRUCTION ALGORITHM. IMAGE: STORAGE

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IMAGE RECONSTRUCTION

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  1. IMAGE RECONSTRUCTION

  2. ALGORITHM-A SET OF RULES OR DIRECTIONS FOR GETTING SPECIFIC OUTPUT FROM SPECIFIC INPUT

  3. ALGORITHM MUST TERMINATE AFTER FINATE NUMBER OF STEPS

  4. DETECTORS PRE-PROCESING REFORMATTED RAW DATA CONVOLUTION WITH FILTER IMAGE RECONSTRUCTION ALGORITHM IMAGE: STORAGE DISPLAY RECORDING ARCHIVING RECONSTRUCTED IMAGE

  5. HISTORY OF IMAGE RECONSTRUCTION • 1917 RADON DEVELOPED MATHEMATICALSOLUTIONS TO THE PROBLEM OF IMAGE RECONSTRUCTION FROM A SET OF PROJECTIONS. UTILIZED IN SOLVING PROBLEMS IN ASTRONOMY AND OPTICS. • 1961 FINALLY THESE TECHNIQUES WERE USED IN MEDICINE.

  6. HOUNSFIELD’S IMAGES—NOISY BECAUSE OF HIS RECONSTRUCTION METHOD. CONVOLUTION BACK PROJECTION USED TO IMPROVE IMAGE QUALITY AND SHORTEN PROCESSING TIME.( USED BY Shepp & Logan)

  7. ORIGINAL DATA FILTERED DATA

  8. CONVOLUTION BACK PROJECTION DEVELOPED BY: • RAMCHANDRAN • LAKSHMINARAYANAN

  9. BEAM GEOMETRIES AND DATA ACQUSITION • PARALLEL –SLOW • FAN -FASTER

  10. Algorithms applicable to CT Back projection Iterative methods Analytic methods

  11. BACK PROJECTION • ALSO CALLED SUMMATION METHOD OR LINEAR SUPERPOSITION METHOD

  12. Example image of how a cube generates different projections depending on the angle of projection

  13. INCOMPLETE BACK-PROJECTION

  14. ITERATIVE ALGORITHMS • SIMULTANEOUS ITERATIVE RECONSTRUCTION TECHNIQUE • ITERATIVE LEAST SQUARES TECHNIQUE • ALGEBRAIC RECONSTRUCTION TECHNIQUE

  15. ART USED BY HOUNSFIELD IN HIS FIRST EMI BRAIN SCANNER

  16. ITERATIVE TECHNIQUES ARE NOT USED IN TODAY’S COMMERCIAL SCANNERS

  17. ANALYTIC RECONSTRUCTION ALGORITHMS • FOURIER RECONSTRUCTION • FILTERED BACK PROJECTION USED IN MODERN CT SCANNERS

  18. FILTERED BACK-PROJECTIONCONVOLUTION METHOD • PROJECTION PROFILE IS FILTERED OR CONVOLVED TO REMOVE THE TYPICAL STAR LIKE BLURRING THAT IS CHARACTERISTIC OF THE SIMPLE BACK PROJECTION.

  19. STEPS IN FILTERED BACK PROJECTION • ALL PROJECTION PROFILES ARE OBTAINED • THE LOGARITHM OF DATA IS OBTAINED • LOGARITHMIC VALUES ARE MULTIPLIED BY DIGITAL FILTER • FILTERED PROFILES ARE BACKPROJECTED • THE FILTERED PROJECTIONS ARE SUMMED AND THE NEGATIVE AND POSITIVE COMPONENTS ARE CANCELLED

  20. FOURIER RECONSTRUCTION • USED IN MRI • NOT USED IN CT BECAUSE OF COMPLICATED MATHEMATICS

  21. FOURIER TRANSFORM • RADIOGRAPH –AN IMAGE IN SPATIAL DOMAIN • IN FOURIER TRANSFORM IMAGE IN FREQUENCY DOMAIN • CAN BE RETRANSFORMED BACK TO SPATIAL DOMAIN BY WITH THE INVERSE FOURIER

  22. TYPES OF DATA • MEASUREMENT DATA • RAW DATA • CONVOLVED DATA • IMAGE DATA

  23. MEASUREMENT DATA(SCAN DATA) • DATA THAT ARISES FROM DETECTORS. IT NEEDS TO BE PREPROCESSED TO ELIMINATE ARTIFACTS.

  24. RAW DATA • IT’S THE RESULT OF SCAN DATA BEING PRE-PROCESSED

  25. CONVOLVED DATA • FILTERED BACKPROJECTION IS THE ALGORITHM USED BY MODERN CT. • IT REQUIRES FILTERING AND THEN BACKPROJECTION. RAW DATA IS FILTERED USING MATHEMATICAL FILTER OR KERNEL (CONVOLUTION) • IT REMOVES BLURR. CONVOLUTION CAN ONLY BE APPLIED TO RAW DATA.

  26. IMAGE DATA(RECONSTRUCTED DATA) • CONVOLVED DATA THAT HAVE BEEN BACKPROJECTED INTO THE IMAGE MATRIX TO CREATE CT IMAGES DISPLAYED ON THE MONITOR.

  27. DETECTORS PREPROCESSING SCAN DATA RAW DATA CONVOLUTION CONVOLVED DATA DATA BACK PROJECTION

  28. ALGORITHM vs NOISE DETAIL EDGE-ENHANCEMENT STANDARD SMOOTHING NOISE

  29. RECONSTRUCTION IN SPIRAL CT • FILTERED BACK PROJECTION USED + INTERPOLATION BACAUSE OF THE CONTINUOUS MOVEMENT OF THE PATIENT IN THE Z-DIRECTION. ( TO ELIMINATE MOTION BLURR)

  30. RECONSTRUCTION IN MULTISLICE SPIRAL CT • INTERLACED SAMPLING • LONGITUDINAL INTERPOLATION • FAN BEAM RECONSTRUCTION

  31. RECONSTRUCTION ALGORITHM COMPARISON • ANALYTIC METHODS ARE FASTER THAN ITERATIVE ALGORITHMS. • FILTERED BACK PROJECTION IS USED IN MODERN CT SCANNERS

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