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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. IMAGE RECONSTRUCTION PERFORMED BY: • ARRAY PROCESSOR OR PROCESSORS

  6. RECONSTRUCTION WHEN ONE OR MORE TECHNICAL FACTORS ARE MODIFIED IS CALLED: • RETROSPECTIVE RECONSTRUCTION

  7. FACTORS THAT CAN BE CHANGED DURING RECONSTRUCTION • DFOV • MATRIX • SLICE THICKNESS • SLICE INCREMENTATION • ANGLE

  8. SFOV LARGE DFOV

  9. SFOV SMALL DFOV TARGETED (ZOOMED) RECON

  10. SFOV VERY SMALL DFOVTARGETED (ZOOMED) RECON

  11. DIFFERENT MATRICES 80 X 80 512 X 512 USUALLY MATRIX IS PERMANENTLY SET AT 512 X 512

  12. SLICE THICKNESS • RECONSTRUCTION IN DIFFERENT THICKNESS ONLY POSSIBLE IN THE MULTISLICE UNIT

  13. SLICE INCREMENTATION • RECONSTRUCTION IN DIFFERENT SLICE INCREMENTATION POSSIBLE IN SINGLE AND MULTISLICE UNITS

  14. 2MM 2MM 2MM SLICE SLICE SLICE 2MM 1MM 4MM INCREMENT INCREMENT INCREMENT INCREMENTATION CONTIGUOUS 50% OVERLAP 100% GAP

  15. SLICE THICKNESS RECON INCREMENT IMAGE QUALITY IMAGE QUALITY SLICE vs RECONSTRUCTIONINCREMENTATION

  16. TOMO ANGLE 360º TUBE

  17. TOMO ANGLE 180º TUBE

  18. TOMO ANGLE • ANGLE IMAGE QUALITY

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

  20. PREREQUISITE FOR DATA RECONSTRUCTION • DATA AVAILABLE RAW

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

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

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

  24. MATHEMATICAL FILTER CAN ALSO BE CALLED: • KERNEL • ALGORITHM • PASS FILTER

  25. TYPES OF FILTERS • SMOOTH • STANDARD • SHARP • EXTRA SHARP

  26. TYPES OF FILTERS (SIEMENS) • B20 (SMOOTH) • B40 (STANDARD) • A80 (SHARP) • A91 (VERY SHARP)

  27. SMOOTH SHARP

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

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

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

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

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

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

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

  35. ITERATIVE RECON DEFINITION • IT STARTS WITH ASSUMPTION AND COMPARES THIS ASSUMPTION WITH A MEASURED VALUE, MAKES CORRECTIONS TO BRING THE TWO VALUES IN AGREEMENT. THIS PROCESS REPEATS OVER AND OVER.

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

  37. ITERATIVE TECHNIQUES ARE NOT USED IN TODAY’S COMMERCIAL SCANNERS THEY ARE VERY SLOW BETTER THAN FILTERED BACK PROJECTION IN METAL ARTIFACT REDUCTION AND NOISE REDUCTION

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

  39. 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.

  40. 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

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

  42. ALGORITHM (FILTER) vs NOISE DETAIL HIGH PASS FILTER SHARP EDGE-ENHANCEMENT STANDARD LOW PASS FILTER SMOOTHING NOISE

  43. 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)

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