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PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY. RADIOGRAPHY LIMITATIONS. SUPERIMPOSITION DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS. . SUPERIMPOSITION. TISUE DIFFERENCE SENSITIVITY 5%-10%. TOMOGRAPHY ( CONVENTIONAL).

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PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

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  1. PHYSICAL PRINCIPLES OF COMPUTED TOMOGRAPHY

  2. RADIOGRAPHY LIMITATIONS • SUPERIMPOSITION • DIFFICULTY IN DISTINGUISHING BETWEEN HOMOGENOUS OBJECTS OF NON-UNIFORM THICKNESS.

  3. SUPERIMPOSITION

  4. TISUE DIFFERENCE SENSITIVITY 5%-10%

  5. TOMOGRAPHY ( CONVENTIONAL) • ELIMINATES TISSUE SUPERIMPOSITION • INCREASES CONTRAST OF LOW SUBJECT CONTRAST TISSUES

  6. TOMOGRAPHY

  7. TOMOGRAPHY

  8. TOMOGRAPHY

  9. CT ADVANTAGES

  10. LIMITATIONS OF CT • UNABLE TO DIFFERENTIATE BETWEEN TISSUES WITH SLIGHT CONTRAST DIFFERENCES < 1%.

  11. GOALS OF CT • MINIMAL SUPERIMPOSITION • IMAGE CONTRAST IMPROVEMENT • SMALL TISSUE DIFFERENCE RECORDING

  12. CT DATA AQUISITION

  13. SLIP RINGS

  14. SEQUENTIAL-SLICE BY SLICE SCANNING

  15. SCANNING

  16. TRANSMISSION RELATIVE TRANSMISSION=Io/I

  17. Total # of trans. measurements=# of views X # of rays in each view

  18. ATTENUATION

  19. DATA AQUSITION GEOMETRIES • CONTINUOUS • STATIONARY

  20. CONTINUOUS

  21. STATIONARY

  22. CT 120-140 KVP • REDUCED DEPENDENCY ON ATTENUATION COEFFICIENT • REDUCED CONTRAST • INCREASED PHOTON FLUX

  23. ORIGINAL CLINICAL CT SCANS COMPOSED OF 80 X 80 MATRIX PIXELS 6400

  24. X

  25. Y

  26. Z

  27. ISOCENTER

  28. SCAN FOV SFOV DETECTORS

  29. SCAN FOV-SMALL SFOV DETECTORS

  30. SFOV - HEAD

  31. TOO SMALL OF SFOV – OUT OF FIELD ARTIFACT

  32. SCAN FOV-RESOLUTION SFOV

  33. RECONSTRUCTION RECONSTRUCTION

  34. SCAN FOV-RESOLUTION SFOV

  35. DISPLAY FOV vs SCANNING FOV • DFOV CAN BE EQUAL OR LESS OF SFOV • SFOV – AREA OF MEASUREMENT DURING SCAN • DFOV - DISPLAYED IMAGE

  36. PIXEL SIZE PIXEL SIZE= FOV (mm)/ MATRIX SIZE

  37. MOST SCANNERS PIXEL SIZE 1 TO 10mm

  38. EXAMPLE: • FOV= 40 CM= 40 X 10 MM=400 mm • MATRIX= 512 X 512 = 5122 400/512 = 0.78 mm 0.8 mm

  39. EACH PIXEL IN CT HAS RANGE OF GRAY SHADES • 2 8 = 256 SHADES • 2 12 = 4096 SHADES = -100 TO 3095 SHADES OF GRAY

  40. PIXEL vs VOXEL PIXEL VOXEL

  41. PIXEL SIZE DEPENDS ON: • MATRIX SIZE • FOV

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