1. CT Scanning Dr. Craig Moore
Medical Physicist & Radiation Protection Adviser
Radiation Physics Service
CHH Oncology

2. Brief History of CT Scanning

3. First CT Scanner - 1972 Originally called CAT A = axial
80 x 80 resolution
4 min. per rotation
8 grey levels
overnight reconstruction

4. Here and Now 512 x 512 or 1024 x 1024 resolution Sub second rotation
4096 grey levels
100’s slices per rotation

5. Components of a CT Scanner

9. Principal components of CT scanner
X-ray tube, collimator, and detector array on a rotating gantry
Rotation axis is referred to as Z axis
Fan beam wide enough to cover patient cross-section
Narrower width in the z-axis
Behind the patient is a bank of detectors
Patient lies on a couch that is moved longitudinally through the gantry

12. X-ray Tube
Tube parallel to patient movement – minimise anode heel effect
X-rays are produced by firing electrons at a metal target – typically tungsten
Capable of producing long exposure times at high mA – get very hot (require heat capacities up to 4MJ and active cooling mechanisms)
Continuous scanning limited to around 90s
Focal spot size typically mm
High kVps (80 – 140) and beam heavily filtered (6-10 mm Al filters) to optimise spectrum
Stops attenuation coefficients varying with depth via beam hardening

13. Collimation and Filtration
Want a monoenergetic beam to avoid beam hardening artefacts
As beam passes through patient low energies are filtered
This results in the apparent reduction of attenuation and CT number of tissues
Computer reconstruction assumes monoenergetic beam
Not possible with X-ray tubes so they are heavily filtered
At least 6 mm aluminium or copper and typically high kVp
Some manufacturers use shaped filters such as bow-tie filters to even out the dose distribution (conform to the shape of an elliptical patient)
Pre-patient collimator is mounted on the X-ray tube
Beam is approx 50cm wide to cover cross section of patient
The size is variable in the z-axis
Multi-slice scanners between 0.5 and 40 mm thick beams
Post patient collimation is not used with multi-slice scanner

17. Bow Shaping Filters Body is approx circularly shaped
Shape of beam that reaches detector is therefore non-uniform which can lead to image artefacts
Shaping filters are applied to the beam prior to make the dose distribution more uniform

19. Detectors Requirements: Small enough to allow good spatial resolution
Up to 1000 detectors per scanner
Typically 1.5 mm width but can be a small as 0.5mm
High detection efficiency
Fast response
Wide dynamic range – massive variation in X-ray intensity
Stable and noise free
No afterglow
There needs to be separation between detectors to prevent light crossover
This reduces efficiency from 98% to 80%

20. Detectors Modern CT detectors are fabricated as detector modules
A side view shows individual detector elements coupled to photodiodes that are layered on electronics
Spaces in between filled with optical filler to reduce cross talk

21. Detectors

22. Generations of CT Scanner

26. CT Imaging
Conventional radiography suffers from the collapsing of 3D structures onto a 2D image
However, CT scanning has extremely good low contrast resolution, enabling the detection of very small changes in tissue type
Almost true depiction of subject contrast
CT gives accurate diagnostic information about the distribution of structures inside the body
Generation of images in transaxial section
Perpendicular to the axis of rotation of the X-ray tube about the body
Perpendicular to the craniocaudal axis of patient

27. Number of detectors and projections
Typically, for a 3rd generation scanner:
650 – 900 detectors
1000 to 2000 projections per rotation

28. Collapse of 3D Data into 2D Plane
Image contrast 2:1
Planar imaging
2D representation of 3D Distribution of Tissue
No depth information
Structures at different depths are superimposed
Loss of contrast
Subject Contrast 4:1
X rays

29. Typical CT Image

31. CT Images
Commonly calculated on 512x512 matrix, but 256x256 and 1024x1024 are also used
Each pixel is more accurately described as a voxel, because it has depth information
The value stored in each voxel is referred to as the CT number which is related to the attenuation of a particular tissue:
CTn = 1000 x (µt - µw)/ µw
Sometimes referred to as Hounsfield Units
Each CT number is assigned a certain shade of grey in the resulting image
CT number represents x-ray attenuation coefficient of the corresponding voxel within the patient

32. CT Numbers Tissue Range of CT Numbers Bone 500-3000 Muscle 40-60
Brain (grey matter)
35-45
Brain (white matter)
20-30
Fat
-60 to -150
Lung
-300 to -800

33. Image Display
CT image represented by a range of CT numbers from to (ie 4000 levels of grey)
Human eye dose not have the capacity to distinguish so many grey levels
If 4000 shades of grey displayed altogether there would be very little difference between different tissues

34. Window Width and Level
The appearance of the image on the screen can be changed by altering the window width and level
Window width refers to the range of CT numbers selected for display
This range of CT numbers is centred at a particular level called the window level
e.g. if imaging bone window level should be ~1000
Can spread a small range of CT numbers over a large range of grayscale values

35. Only see CT numbers +/- 250 around -593 Window Level –12
Window Width 500
Good contrast in lungs
Only see CT numbers +/- 250 around -593
Window Level –12
Window Width 400
Good soft tissue contrast
Only see CT numbers +/- 200 around -12

36. How do we get the images?
Tube and detector rotate smoothly around the patient
X-rays are produced continuously and the detectors sample the X-ray beam approx 1000 times during one rotation
Typically 2 to 4 revolutions per second

39. In reality not always parallel to detectors
Each voxel is traversed by one or more x-ray beams for every measurement (1000 per rotation)
Number of measurements taken in single axial section depends on
number of detectors
Number of samples per rotation
Assume 800 detectors measured at 0.5° intervals per 360 ° rotation
This is 576,000 measurements
More than needed as we only need 260,000 measurements (512 x 512)

40. How do we get the picture?
Back Projection
Reverse the process of measurement of projection data to reconstruct image
Each projection if smeared back across the reconstructed image

41. Back Projection – the basics
Consider cylindrical uniform body with a hole down the centre
A beam passing through this body from one direction will have a transmitted profile in its central region
This single measurement cannot determine the position of the hole other than identifying that it is in the line of the pencil beam passing through the centre of the body
Pixel values along this line are decreased by the amount of attenuation measured
These values are projected back along the field of view
A second projection at 90° provides a second band of grey
This is then projected back across field of view
Progressive projections are shown in the final figure – a star like pattern
We now have an image that looks similar to what we are scanning

42. Back Projection
Back Project each planar image onto three dimensional image matrix 3 6 3 3 3 6 6 3 3 3 6 3

43. Back Projection
Back Project each planar image onto three dimensional image matrix 3 6 3 1 2 1 1 2 1 1 2 1

44. Back Projection
Back Project each planar image onto three dimensional image matrix 3 6 3 3 2 2 1 3 2 1 2 1 2 1 6 3 4 3 3 2 1 2 3 2 1

45. Back Projection
Back Project each planar image onto three dimensional image matrix 3 6 3 3 4 6 4 3 6 6 8 6 6 3 4 6 4 3 3 6 3

46. Back Projection
Back Project each planar image onto three dimensional image matrix 3 6 3 3 4 6 4 3 6 6 8 6 6 3 4 6 4 3 3 6 3

47. Back Projection More views – better reconstruction
1/r blurring, even with infinite number of views

48. Filtered Back Projection
Back projection produces blurred transaxial images
Projection data needs to be filtered before BP
Different filters can be applied for different diagnostic procedures
Smoother filters for viewing soft tissue
Sharp filters for high resolution imaging
After filtration, back projection same as before
Data from neighbouring beams are used
Some data is subtracted
Some data is added
Filters are convolved with the blurred image data in Fourier Space

49. Filtered Back Projection

54. Filtered Back Projection
Usually done in frequency (Fourier) space
Filter planar views prior to back projection
Correction of 1/r blurring requires ‘Ramp’ Filter
Gives increasing weight to higher spatial frequencies
Amplifies Noise
signal

55. Problems with Filtered Back Projection
Back projection is mathematically correct, but real life images require Filtered Back Projection
Back Projection can introduce noise and streaking artefacts
Not good with attenuation correction
Filtered Back Projection can reduce noise and artefacts, but may degrade resolution

56. Iterative Reconstruction
FBP was primary method for reconstructing CT images for many decades
Modern computing power now enables CT scanners to use iterative reconstruction
This process begins with an initial estimate of what the object may look like
This guess is used to compute forward projected data sets, which are compared with the measured projection data sets
The difference between the computed and measured projections creates an ‘error matrix’
The error matrix is used to correct each iteration until the CT image looks like the thing that was scanned

57. Iterative Reconstruction

59. Helical and Multi-Slice Scanning

60. Helical Scanning Have discussed simplest form of CT scanning
Can produce transaxial slices with the patient being moved along the z-axis between each rotations
‘Step and shoot’
This is now very rare
Helical scanning is now the standard
Slip rings
Continuous table feed through gantry

61. Helical Scanning Slip ring technology
X-ray tube has to be supplied with constant power
Detectors have to pass signals to computer
Not possible if gantry was wired – cables would become entangled and overstretched
Slip ring is a metal ring mounted on the gantry
Good connection while the gantry is free to rotate

64. First we need the ‘scout view’
Scout views are needed prior to the scan
Performed to allow the planning of the CT sequence
Scout views are produced lines by line at a fixed projection angle
Typically AP

65. Helical Scanning
Patient moves continuously through the gantry as the X-ray tube and detectors rotate
Continuous acquisition of data in a single exposure
Can be visualised as a ribbon wrapped around the body
This technology minimises slice misregistration

66. Contiguous scan

69. Helical Scanning
The position at which sections can be reconstructed can be anywhere within scanned volume other than at the ends
For example:
300 mm long volume scanned
10 mm slice width
Pitch = 2
Only 15 rotations required
From the measured data, 30 contiguous slices, each 10mm thick can be reconstructed (for a single slice scanner)
For thinner slices we need multi-slice scanners

70. Advantages of Helical Scanning
Speed
No need to pause between scans for table movement
Pitches greater than 1 allowed (reduction in dose)
Longer scan lengths within breath hold
Reduced patient movement artefacts
Increased throughput
Reduced use of contrast medium

71. Disadvantages of Helical Scanning
Broadening of Slice profile
Effective slice thickness increases – poorer z axis resolution
Higher noise
Helical artefacts not seen in axial scanning
Possibility of very high dose if pitch < 1
Lot of tube heating and loading

72. Multi Slice CT

73. Multiple detectors in single row
Remember!!
Multiple detectors in single row

74. Multislice CT
In its original form, CT scanning was two-dimensional – 2D slices through the body
True 3D imaging requires isotropy
Voxel size must be equal in all directions
Under these circumstances, data generated in a 3D matrix can be reconstructed in any plane

75. Multislice CT
Voxel size in axial plane is dependent on matrix size and field of view
Typically 1mm
Single slice helical scanners have the capability of collimating the beam width (in the patient direction) to 1mm, but this is restricted by scan time
Not possible in practice so we need to have multi-slice technology

77. Multislice CT Key to 3D scanning is the multislice scanner
These scanners use solid state detectors with multiple rows of detectors
Typical configuration for an 8 slice scanner
12 curved detector rows
Each row has approx 800 detectors
Each row has minimum possible gap between them
Central rows have approx half the length of 2 outer rows
Length of central rows 0.5 – 1mm
Rows can be used separately or in combination

78. Multislice CT Four possible combinations here are possible:
(a) 8 x 1mm slices
(b) 8 x 2mm slices
(c) 4 x 4mm slices
(d) 2 x 8mm slices
Eight slice scanner here is capable of producing scans at four slice widths, 1,2,4, or 8mm
In each case, slice width is determined by the detector size and by collimation
Scanners up to 256 slices now available

79. Beam width is varied

80. Computer reconstruction
Physical acquisition
Computer reconstruction

82. Single slice
Multi slice
Single slice
Multi slice
3 rotations
One rotation

84. Dose and Multi-Slice Scanners
Considerations similar to those of single slice scanners
Dose utilisation on z axis usually poorer than with single slice scanners
X ray beam width is generally broader than the total imaged width
Geometric efficiency down to 50% for very small slice thicknesses (sub mm)

85. Geometric Efficiency

86. Dose and Helical CT
All helical scanning requires extra irradiation at the end of each run to obtain sufficient interpolation data to reconstruct the required volume
On multi-slice scanners this extra length can be quite long

87. Helical & Multi-Slice in the UK
Helical & Multi-Slice scanning represent significant steps forward in CT
Better scanning of previous scans
Expansion of workload
Nearly all scanners sold in UK are multi-slice
Technology is still advancing
32/40/64/256 slice scanners now available
More slices in the future?

88. Next Lecture: Dosimetry
Operator controls and affects on dose and image quality
artefacts