Seeram: Chapter 1 Computed Tomography - An Overview

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

5. Conventional vs Axial Tomography Conventional Cut CT Axial Cut

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? ??? Godfrey Paul, Ringo, George, & John

9. It Was the Late 1960’s

10. A lot of the money was going here

11. Electronic and Musical Industries LTD 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 image points • 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 • Math developed in 1910’s • Other Applications • Astronomy (sun spot mapping) • Electron microscope imaging • Nuclear medicine emission tomography • MRI Acme Mini- Compu- ter Digital Image

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

27. Translate / Rotate

28. CT Early Units • 4 minute scans • 1 slice • 5 minute reconstruction • 80 X 80 matrix • head only • water bag fit tightly around head

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

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

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

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

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

34. 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 monitor • Manipulated • Stored

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

36. 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

37. 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

38. 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

39. 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

40. 3rd Generation CT Z-axis orientation perpendicular to page Patient

41. 3rdGeneration (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

42. 3rdGeneration Image Quality Improvements • Faster scan times • reduces motion artifacts • Improved spatial resolution • Improved contrast resolution • Increased tube heat capacity • less delay between scans / patients • Increased throughput

43. Spiral CT (late 1980’s) • Continuous rotation of gantry • Slip ring technology • Patient moves slowly but continuously through gantry • No dead time as gantryreverses • Much faster

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

45. Multi-slice CT (2000’s) • Multiple rows of fan beam detectors • Wider fan beam in axial direction • Table moves much faster • Substantially greater throughput

46. Computer Improvements • Virtually instantaneous reconstruction time • Auto • Window protocols • Transmission to PACS • Backup • Image manipulation • Slice reformatting • 3D reconstruction And the ability to do it all simultaneously

47. Noise Dose Resolution Fundamental CT Tradeoff To improve one requires compromise on another • Typically phantom dose: 1-2 rad (10-20 mSv)

48. CT Usage • 16% of imaging procedures • 23% of total per capita exposure • 49% of medical exposure

49. CT Usage • Annual growth • U.S. Population: <1% • CT Procedures: >10% • ~ 67,000,000 procedures in 2006 • about 10% pediatric CT Computed Tomography — An Increasing Source of Radiation Exposure David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc. New England Journal of Medicine, 2007

50. 6/19/2001 “Each year, about 1.6 million children in the USA get CT scans to the head and abdomen — and about 1,500 of those will die later in life of radiation-induced cancer … How many children’s lives are saved by CT?