1. tomos = slice, graphein = to write
Computed Tomography
CT, CAT
tomos = slice, graphein = to write
Magdalena Bazalova

2. 1. What is a CT scanner?
an X-ray device capable of cross-sectional imaging
creates images of slices through the patient

3. What is a CT scanner?
doughnut shaped gantry with moving patient table

4. Why CT?
conventional radiography suffers from collapsing of 3D structures onto a 2D image
although the resolution of CT is lower, it has extremely good low contrast resolution enabling the detection of very small changes in tissue type
CT gives accurate diagnostic information about the distribution of structures inside the body

5. CT scanning applications
very wide ranging – good for imaging bone and soft tissue
diagnostic imaging
radiotherapy planning
3D applications

6. CT imaging applications

7. CT imaging applications

8. CT imaging applications

9. Why CT for radiotherapy?
Radiation therapy planning is done on the basis of patient CT images and is therefore patient specific
the target and organs at risk are delineated in CT images (possibly with help of other imaging modalities – PET)
dose calculation algorithms use CT images for determination of dose delivered to the patient during treatment

10. Why CT for radiotherapy?
Tissue inhomogeneities can be taken into account in most treatment planning systems
Dose to soft tissue is different than dose to cortical bone - mass density variations between tissue types are the most important factor
Therefore, mass densities of tissues have to be known for an accurate dose calculation
CT images do not represent mass densities of patient body directly but they can be converted into mass densities using a calibration curve

11. 2. CT scanner components
X-ray tube
X-ray beam
detector ring

12. X-ray journey

13. X-ray tube

14. Beam shaping filter

15. Detectors

16. Detector arrangement

17. Philips CT simulator

18. Questions on CT apparatus
How do we call the device that produces X-ray beam?
(X-ray tube )
What have the X-rays pass through on their way to the detector ring?
(beryllium window, Al filters, bow-tie filter, patient, anti-scatter grid)

19. 3. CT image definition and formation

20. What are we measuring in CT?
the linear attenuation coefficient, µ, between the X-ray tube and
the detector
the linear attenuation
coefficient is a measure
of how rapidly are X-ray
attenuated

21. 2D-projection data set - sinogram
projections
I, I0 - intensities -d
x-ray source -d d p d
Projection angle

22. Reconstruction algorithms
Computer based
simple back-projection
filtered back-projection
iterative techniques

23. Simple back-projection
reverse the process of measurement of projection data to reconstruct an image
each projection is uniformly distributed across the reconstructed image

24. Simple back-projection
1/r blurring

25. Filtered back-projection
simple back-projection produces blurred images
projection data need to be filtered before reconstruction
different filters can be applied for different diagnostic purposes
smoother filters for viewing soft tissue
sharp filters for high resolution images
back-projection is the same as before

26. Filtered back-projection

27. Image reconstruction Simple back-projection Filtered back-projection
FBP

28. Patient image reconstruction

29. Patient filtered back-projection

30. CT number scale
HU represents the linear attenuation of a material.

31. CT number window

32. CT number window

33. CT for radiotherapy – calibration, HU to mass density conversion
HU do not represent mass density, needed for dose calculation, directly. To obtain mass densities of each voxel:
A set of tissue equivalent
materials with known
mass densities is scanned
and a calibration curve
is created

34. Calibration curve for treatment planning

35. Questions on reconstruction
How do we call picture and volume elements?
(pixels and voxels)
What do CT images represent?
(linear attenuation coefficients of voxels)
How do we call raw detector data?
(a sinogram)
Name two reconstruction techniques?
(simple and filtered back-projection)

36. 4. CT technology

37. Third generation CT scanners

38. Fourth generation CT scanners

39. Fifth generation CT scanners

40. Helical CT scanning

41. Advantages of helical mode

42. Questions on CT technology
How many CT generations exist?
5 (maybe more)
Which one is the third one?
rotate/rotate
What are the advantages of helical scanning?
arbitrary image position, faster scanning

43. 5. CT image quality

44. Image noise

45. Image noise

46. Image contrast

47. Image contrast

48. Factors affecting image noise

49. Reconstruction filters

50. Factors affecting detector signal

51. Radiation dose

52. Questions on image quality
Name three factors that influence image quality.
kVp, mA, time, filteration of the beam, slice thickness, reconstruction filter, pitch
Name three parameters that describe image quality
spatial resolution, contrast, noise
What is noise?
variation in HU in a uniform image
What is contrast
ability to resolve details without blurring

53. 6. Artefacts in CT
If not recognized, CT artifacts can cause misdiagnosis
and incorrect outcomes of radiotherapy treatment planning.

54. Definition of CT artefacts

55. Types of CT artefacts

56. Origin of artefacts

57. Beam hardening: cupping

58. Beam hardening: correction

59. Beam hardening: correction

60. Reduction of streaks and bands

61. Partial volume artefacts

62. Partial volume artefacts

63. Minimizing partial volume artefacts

64. Photon starvation

65. Avoidance using mA modulation

66. Metal artefacts

67. Metal artefact reduction

68. Patient motion artefacts
Voluntary and involuntary motion cause artefacts in the reconstructed image

69. Minimizing motion artefacts

70. Motion artefact correction

71. Detector sensitivity: ring artefacts

72. Ring artefacts

73. CT artefacts: summary

74. Questions on CT artifacts
Remedy for photon starvation?
mA modulation, adaptive filtering
What can cause metal artifacts in patients?
dental implants, surgical clips, electrodes, prostheses
In which CT generation can occur ring artifacts?
in the 3rd where detector ring rotates with X-ray tube