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
Electronic and Musical Industries LTD

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 Pixel (calculated) Data Projection (raw) Data
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. Digital Image Matrix
125 25
311
111
182
222
176
199
192 85 69
133
149
112 77
103
118
139
154
120
145
301
256
223
287
225
178
322
325
299
353
333
300

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 5 minute reconstruction 80 X 80 matrix
1 slice
5 minute reconstruction
80 X 80 matrix
head only
water bag fit tightly around head

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

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

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 Minicomputer does its thing
Analog to Digital (A to D) conversion

34. Digital Image Matrix Digital Matrix contains many numbers which may be
Displayed on monitor
Manipulated
Stored
125 25
311
111
182
222
176
199
192 85 69
133
149
112 77
103
118
139
154
120
145
301
256
223
287
225
178
322
325
299
353
333
300

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

41. 3rd 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

42. 3rd Generation 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 gantry reverses
Much faster

44. Z-axis orientation perpendicular to page
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. Fundamental CT Tradeoff
Typically phantom dose: 1-2 rad (10-20 mSv)
To improve one requires compromise on another
Resolution
Noise
Dose

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

49. CT Usage Annual growth ~ 67,000,000 procedures in 2006
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. How many children’s lives are saved by CT?
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 …

51. Medical imaging procedures should be appropriate & conducted at the lowest radiation dose consistent with acquisition of desired information
Discussion of dose risks should be accompanied by acknowledgement of procedure benefits

52. Risks of medical imaging at effective doses < 50 mSv (5 rad) for single procedures or 100 mSv(10 rad) for multiple procedures over short time periods are too low to be detectable & may be nonexistent.
Predictions of hypothetical cancer incidence and deaths in patients exposed to such low doses are
highly speculative
should be discouraged

53. These predictions are harmful because they lead to sensationalistic articles … that cause some patients & parents to refuse imaging procedures, placing them at substantial risk by not receiving the clinical benefits of the prescribed procedures