CT Overview

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  • CT Overview

    1. 1. CTCT • Seeram: Chapter 1 Computed Tomography - An Overview
    2. 2. George David Early HistoryEarly History • “tomostomos” ♦Greek word meaning sectionsection • Sectional imaging methods firstSectional imaging methods first developed in 1920’sdeveloped in 1920’s
    3. 3. Early History: Conventional Tomography Early History: Conventional Tomography • first used in 1935 • image produced on film • Image plane oriented parallel to film • Anatomy in plane of fulcrum stays in focus • anatomy outside of fulcrum plane mechanically blurred
    4. 4. Conventional vs Axial Tomography Conventional vs Axial Tomography Conventional Cut CT Axial Cut
    5. 5. Conventional Tomography BlurringConventional Tomography Blurring • Image produced on film • Objects above or below fulcrum plane change position on film & thus blur
    6. 6. CT ImageCT Image • Not produced on film • Mathematically reconstructed from many projection measurements of radiation intensity • Digital Image calculated Acme Mini- Compu- ter Digital Image
    7. 7. How Did We Go From…How Did We Go From…
    8. 8. The story concerns these men. What was their Link? The story concerns these men. What was their Link? Paul, Ringo, George, & John Geoff ???
    9. 9. It Was the Late 1960’sIt Was the Late 1960’s
    10. 10. A lot of the money was going hereA lot of the money was going here
    11. 11. Follow the MoneyFollow the Money
    12. 12. Measure Intensity of a Pencil BeamMeasure Intensity of a Pencil Beam X-Ray Source Radiation Detector
    13. 13. CT ImageCT Image • Measure a bunch of pencil beam intensities
    14. 14. CT ImageCT Image • Now make measurements from every angle
    15. 15. CT ImageCT Image • When you get done, multiple pencil beams have gone through every point in body
    16. 16. George David Digital ImageDigital Image • 2-dimensional array of individual image points calculated • each point called a pixelpixel ♦picture element • each pixel has a value ♦value represents x-ray transmission (attenuation)
    17. 17. Digital Image MatrixDigital Image Matrix 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 125 120 145 301 256 223 287 256 225 178 322 325 299 353 333 300
    18. 18. George David Numbers / Gray ShadesNumbers / Gray Shades • Each number of a digital image corresponds to a gray shade for one pixelpixel
    19. 19. Image ReconstructionImage Reconstruction Acme Mini- Computer Projection (raw) Data Pixel (calculated) Data X-Ray Source Radiation Detector
    20. 20. George David Image ReconstructionImage Reconstruction • CT math developed in 1910’s • Other Applications ♦astronomy (sun spot mapping) ♦electron microscope imaging ♦Nuclear medicine emission tomography ♦MRI
    21. 21. CT HistoryCT History • First test images in 1967 • First clinical images ~ 1971 • First commercial scanner 1972
    22. 22. CT HistoryCT History CT math developed in 1910’s • First commercial scanner 1972 • What took so long?
    23. 23. CT HistoryCT History • CT made possible by high speed minicomputer
    24. 24. CT ComputersCT Computers • Old mainframe computers too expensive & bulky to be dedicated to CT
    25. 25. George David The 1st Computer BugThe 1st Computer Bug
    26. 26. CT history - Obsolete Terminology CT history - Obsolete Terminology • CTAT ♦computerized transverse axial tomography • CAT ♦computerized axial tomography • CTTRT ♦computerized transaxial transmission reconstructive tomography • RT ♦reconstructive tomography
    27. 27. George David Data AcquisitionData Acquisition • cross sectional image reconstructed from many straight line transmission measurements made in different directions Tube Detector
    28. 28. George David Translate / RotateTranslate / Rotate
    29. 29. CT Early UnitsCT Early Units • 4 minute scans • 5 minute reconstruction • 80 X 80 matrix • head only ♦water bag fit tightly around head ♦Original computer software couldn’t deal with transition from skull to air
    30. 30. Beam TranslationBeam Translation • beam collimated to small round spot ♦collimated at tube and collimator X-ray Tube Detector
    31. 31. Beam TranslationBeam Translation • Tube/detector translates left to right • Entire assembly rotates 1o to right • Tube/detector translates right to left X-ray Tube Detector
    32. 32. Translate - RotateTranslate - Rotate • 180 translations in alternate directions • 1 degree rotational increments between translations
    33. 33. Projection MeasurementsProjection Measurements • Radiation detector generates a voltage proportional to radiation intensity
    34. 34. Image ReconstructionImage Reconstruction • Minicomputer does its thing Analog to Digital (A to D) conversion
    35. 35. Digital Image MatrixDigital Image Matrix 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 125 120 145 301 256 223 287 256 225 178 322 325 299 353 333 300 • Digital Matrix contains many numbers which may be ♦Displayed on CRT ♦Manipulated ♦Stored
    36. 36. Digital Image Manipulation Digital Image Manipulation • Window • Level • Smoothing • Edge enhancement • Slice reformatting • 3D ♦derived from multiple axial slices
    37. 37. Digital Image StorageDigital Image Storage • Magnetic Disk • CD • Tape • Optical Disk • PACS archive ♦picture archival and communications system » not part of CT » contains images from many modalities » allows viewing on connected computers
    38. 38. CT - ImprovementsCT - Improvements • all CT generations measure same multi-line transmission intensities in many directions • Improvements ♦Protocol for obtaining many line transmissions ♦# of line transmissions obtained simultaneously ♦detector location ♦Overall acquisition speed
    39. 39. 2nd Generation CT2nd Generation CT • arc beam used instead of pencil beam • several detectors instead of just one ♦detectors intercepted arc ♦radiation absorbent septa between detectors » reduced scatter » acted like grid Tube Detectors
    40. 40. 2nd Generation CT2nd Generation CT • arc beam allowed 10 degree rotational increments • scan times reduced ♦20 sec - 2 min • 2 slices obtained simultaneously ♦double row of detectors 10o
    41. 41. 3rd Generation CT3rd Generation CT • Wide angle fan beam • rotational motion only / no translation ♦detectors rotate with tube • 30o beam • Many more detectors • scan times < 10 seconds
    42. 42. 3rd Generation CT3rd Generation CT Patient Z-axis orientation perpendicular to page
    43. 43. 4th Generation CT4th Generation CT • Fixed annulus of detectors ♦tube rotates (no translation) inside stationary detector ring • only a fraction of detectors active at once
    44. 44. 3rd & 4th Generation (Non-spiral) CT3rd & 4th Generation (Non-spiral) CT • Tube rotates once around patient ♦Table stationary ♦data for one slice collected • Table increments one slice thickness • Repeat ♦Tube rotates opposite direction
    45. 45. 3rd / 4th Generation Image Quality Improvements 3rd / 4th Generation Image Quality Improvements • Faster scan times ♦reduces motion artifacts • Improved spatial resolution • Improved contrast resolution • Increased tube heat capacity ♦less wait between scans / patients ♦better throughput
    46. 46. Spiral CTSpiral CT • Continuous rotation of gantry • Patient moves slowly through gantry • cables of old scanners allowed only 360o rotation (or just a little more) ♦tube had to stop and reverse direction ♦no imaging done during this time • no delay between slices ♦dynamic studies now limited only by tube heating considerations
    47. 47. Spiral CTSpiral CT Patient Z-axis orientation perpendicular to page
    48. 48. Multi-slice CTMulti-slice CT • Multiple rows of fan beam detectors • Wider fan beam in axial direction • Table moves much faster • Substantially greater throughput
    49. 49. George David Computer ImprovementsComputer Improvements • Reconstruction time • Auto-printing protocols • Image manipulation • Backup time • Slice reformatting • 3D reconstruction And the ability to do it all simultaneously
    50. 50. Cine CT (Imatron)Cine CT (Imatron) • four tungsten target rings surround patient ♦replaces conventional x-ray tube ♦no moving parts ♦like 4 moving focal spots • electron beam sweeps over each annular target ring ♦can be done at electronic speeds • 2 detector rings ♦2 slices detected • maximum scan rate ♦24 frames per second
    51. 51. Imatron Cine CT (scanned from Medical Imaging Physics, Hendee)
    52. 52. George David CT Patient DoseCT Patient Dose • In theory only image plane exposed • In reality adjacent slices get some exposure because ♦x-ray beam diverges ♦interslice scatter
    53. 53. George David Dose ProtocolsDose Protocols • Plain X-ray ♦entrance skin exposure • Mammography ♦mean glandular dose • CT ♦Computer tomography dose index (CTDI) ♦Multiple-scan average dose (MSAD)
    54. 54. George David CT Dose depends onCT Dose depends on • kVp • mA • time • slice thickness • filtration • Noise • detector efficiency • collimation • matrix resolution • reconstruction algorithm
    55. 55. CT Patient DoseCT Patient Dose • Typically 2 - 4 rad ♦AAPM has a single slice protocol for measuring head & body doses • More dose required to improve spatial resolution for same noise level ♦smaller pixels ♦need to increase does to get same dose / pixel • More dose required to improve noise for same spatial resolution Resolution Noise Dose
    56. 56. George David New StuffNew Stuff • CT Angiography • CT fluoroscopy • CT virtual endoscopy / colonoscopy / ??scopy

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