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Seeram: Chapter 1 Computed Tomography - An Overview

CT. Seeram: Chapter 1 Computed Tomography - An Overview. Early History. “ tomos ” Greek word meaning section Sectional imaging methods first developed in 1920’s. Early History: Conventional Tomography. first used in 1935 image produced on film Image plane oriented parallel to film

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Seeram: Chapter 1 Computed Tomography - An Overview

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  1. CT • Seeram: Chapter 1 Computed Tomography - An Overview

  2. Early History • “tomos” • Greek word meaning section • Sectional imaging methods first developed in 1920’s

  3. 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. Conventional vs Axial Tomography Conventional Cut CT Axial Cut

  5. Conventional Tomography Blurring • Image produced on film • Objects above or below fulcrum plane change position on film & thus blur

  6. CT Image • Not produced on film • Mathematically reconstructed from many projection measurements of radiation intensity • Digital Image calculated Acme Mini- Compu- ter Digital Image

  7. How Did We Go From…

  8. The story concerns these men. What was their Link? ??? Geoff Paul, Ringo, George, & John

  9. It Was the Late 1960’s

  10. A lot of the money was going here

  11. Follow the Money

  12. Measure Intensity of a Pencil Beam X-Ray Source Radiation Detector

  13. CT Image • Measure a bunch of pencil beam intensities

  14. CT Image • Now make measurements from every angle

  15. CT Image • When you get done, multiple pencil beams have gone through every point in body

  16. Image Reconstruction X-Ray Source Acme Mini- Computer Pixel (calculated) Data Radiation Detector Projection (raw) Data

  17. Digital Image • 2-dimensional array of individual image points calculated • each point called a pixel • picture element • each pixel has a value • value represents x-ray transmission (attenuation)

  18. 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 Image Matrix

  19. Numbers / Gray Shades • Each number of a digital image corresponds to a gray shade for one pixel

  20. Image Reconstruction • CT math developed in 1910’s • Other Applications • astronomy (sun spot mapping) • electron microscope imaging • Nuclear medicine emission tomography • MRI

  21. CT History • First test images in 1967 • First clinical images ~ 1971 • First commercial scanner 1972

  22. CT History • CT math developed in 1910’s • First commercial scanner 1972 • What took so long?

  23. CT History • CT made possible by high speed minicomputer

  24. CT Computers • Old mainframe computers too expensive & bulky to be dedicated to CT

  25. The 1st Computer Bug

  26. CT history - Obsolete Terminology • CTAT • computerized transverse axial tomography • CAT • computerized axial tomography • CTTRT • computerized transaxial transmission reconstructive tomography • RT • reconstructive tomography

  27. Data Acquisition • cross sectional image reconstructed from many straight line transmission measurements made in different directions Tube Detector

  28. Translate / Rotate

  29. CT Early Units • 4 minute scans • 5 minute reconstruction • 80 X 80 matrix • head only • water bag fit tightly around head

  30. X-ray Tube Detector Beam Translation • beam collimated to small round spot • collimated at tube and collimator

  31. X-ray Tube Detector Beam Translation • Tube/detector translates left to right • Entire assembly rotates 1o to right • Tube/detector translates right to left

  32. Translate - Rotate • 180 translations in alternate directions • 1 degree rotational increments between translations

  33. Projection Measurements • Radiation detector generates a voltage proportional to radiation intensity

  34. Image Reconstruction Analog to Digital (A to D) conversion • Minicomputer does its thing

  35. 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 Image Matrix • Digital Matrix contains many numbers which may be • Displayed on CRT • Manipulated • Stored

  36. Digital Image Manipulation • Window • Level • Smoothing • Edge enhancement • Slice reformatting • 3D • derived from multiple axial slices

  37. Digital 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. CT - 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. 2nd Generation CT Tube • 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 Detectors

  40. 2nd Generation CT 10o • arc beam allowed 10 degree rotational increments • scan times reduced • 20 sec - 2 min • 2 slices obtained simultaneously • double row of detectors

  41. 3rd 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. 3rd Generation CT Z-axis orientation perpendicular to page Patient

  43. 4th Generation CT • Fixed annulus of detectors • tube rotates (no translation) inside stationary detector ring • only a fraction of detectors active at once

  44. 3rd & 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. 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. Spiral 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. Spiral CT Z-axis orientation perpendicular to page Patient

  48. Multi-slice CT • Multiple rows of fan beam detectors • Wider fan beam in axial direction • Table moves much faster • Substantially greater throughput

  49. Computer Improvements • Reconstruction time • Auto-printing protocols • Image manipulation • Backup time • Slice reformatting • 3D reconstruction And the ability to do it all simultaneously

  50. 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

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