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 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 individual image points calculated
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 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. Beam Translation beam collimated to small round spot
collimated at tube and collimator
X-ray Tube
Detector

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

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

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

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 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. 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

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. Z-axis orientation perpendicular to page
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. Z-axis orientation perpendicular to page
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

52. (scanned from Medical Imaging Physics, Hendee)
Imatron Cine CT
(scanned from Medical Imaging Physics, Hendee)

53. CT Patient Dose In theory only image plane exposed
In reality adjacent slices get some exposure because
x-ray beam diverges
interslice scatter

54. Dose Protocols Plain X-ray Mammography CT entrance skin exposure
mean glandular dose CT
Computer tomography dose index (CTDI)
Multiple-scan average dose (MSAD)

55. CT Dose depends on Noise detector efficiency collimation
kVp mA
time
slice thickness
filtration
Noise
detector efficiency
collimation
matrix resolution
reconstruction algorithm

56. CT Patient Dose Typically 2 - 4 rad
AAPM has single slice protocol for measuring head & body doses
More dose required at higher resolution for same noise level
More dose required to improve noise at same spatial resolution
Resolution
Noise
Dose

57. Fundamental CT Tradeoff
To improve one requires compromise on another
Resolution
Noise
Dose

58. New Stuff CT Angiography CT fluoroscopy
CT virtual endoscopy / colonoscopy / ??scopy