CT SCAN

1. Computed
Tomography

2. Introduction

3. Computed tomography scan combines a series
of x-ray images taken from different angles and
uses computer processing to create cross-
sectional images, or slices, of the bones, blood
vessels and soft tissues inside your body. CT
scan images provide more detailed
information than plain x-rays do.
Introduction

4. History

5. Allessandro Vallebona – Italian
radiologist, proposed a method
to represent a single slice of
the body on the radiographic
film. This method was known
as tomography (stratigraphy).
1930s

7. Allan Cormack, a Tufts
University Medical physicist,
developed the mathematics
used to reconstruct CT images.
1956

8. -Awarded the 1979 Nobel Prize for
Physiology or Medicine for his work in
developing the powerful
new diagnostic technique of computerized
axial tomography (CAT).
-Cormack was unusual in the field of Nobel
laureates because he never earned a
doctorate degree in medicine or any other
field of science.
After graduating from the University
of Cape Town in 1944 Cormack pursued
advanced studies there and at
the University of Cambridge.
-He was a lecturer at Cape Town from 1950
to 1956 and then, after a year’s research
fellowship at Harvard University, became
assistant professor of physics at Tufts
University.

9. Godfrey Hounsfield; a physicist
and engineer with EMI, the
British company most famous
for recording the Beatles, first
demonstrated the CT
technique.
1970

10. English electrical engineer who shared the
1979 Nobel Prize for Physiology or Medicine
with Allan Cormack for his part in developing
the diagnostic technique of computerized axial
tomography (CAT), or computerized tomography
(CT).
1979

11. Principle

12. A patient is scanned by an x-ray tube rotating
around the body part being examined.
Principle

13. A detector assembly measures the radiation
exiting the patient and feeds back the
information, referred to as primary data, to the
host computer.
Principle

14. After the computer has compiled and
calculated the data according to a preselected
algorithm, it assembles the data in a matrix to
form an axial image. Each image, or slice, is
displayed in a cross-sectional format.
Principle

15. Principles of
Operation

16. - Plane of the image is parallel to the long axis
of the body
- Produces sagittal and coronal images
Conventional/ Axial Tomography

18. - Plane of the image is perpendicular to the
long axis of the body
- produces a transverse image
Computed Tomography

19. - Refers to the movement of source-detector
assembly across the patient
- results in a projection
Translation/Sweep

20. - Represents the attenuation pattern of patient
profile
Projection

21. - computer-adapted mathematical calculation
applied to raw data during image
reconstruction
Algorithm

22. - Creation of an image from data
Reconstruction

23. - Time needed for the computer to present a
digital image after an exposure has been
computed
- time between the end of imaging and the
appearance of an image
Reconstruction time

24. Generations

25. Characteristic: translate/rotate
X-ray Beam Shape: pencil beam
Detector: single detector
Imaging time: 5 minutes
Solely used for head scans
First

27. Characteristic: translate/rotate
X-ray Beam Shape: narrow fan beam
Detector: multiple detector (5-30)
Imaging time: 30 seconds
Solely used for head scans
Second

28. Advantage: speed
- consist of multiple detector
Disadvantage: increased scatter radiation
- increased radiation intensity
toward the edges of the beam
Bow-tie filter is used to equalize the radiation
intensity that reaches the detector array
Second

30. Characteristic: rotate/rotate
X-ray Beam Shape: wide fan beam
Detector: curvilinear detector array (30)
Imaging time: < 1 second
Third

31. Advantages:
- better x-ray beam collimation
- decreased scatter radiation
- good image reconstruction
Disadvantage: ring artifact
software-corrected image reconstruction
algorithm removes ring artifact
Third

33. Third generation designs have the advantage
that thin tungsten septa can be placed
between detector in the array and focused on
the x-ray source to absorb scatter radiation.
Third generation system are calibrated only
once every few hours.
Third

35. Characteristic: rotate/stationary
X-ray Beam Shape: wide fan beam
Detector: fixed circular detector array (4000)
with detector ring
Imaging time: < 1 second
Fourth

36. Advantage: no ring artifacts
Disadvantage:
- increased patient dose
- high cost
Fourth

37. The detectors are no longer coupled to the x-
ray source and hence cannot make use of
focused septa to absorb scatter radiation.
However, detectors are calibrated twice during
each rotation of the x-ray source, proving a
self-calibrating system.
Fourth

39. - Electron beam CT
- Ultra fast CT scanner
- x-ray tube rotation is mechanical
- no moving parts
- developed specifically for use in cardiac
tomography imaging
- referred as cine-CT scanners
Fifth

40. Electron gun produces a focused electron
beam that generates a rotating x-ray fan beam
after being steered along tungsten target rings.
Fifth

42. - Helical/Spiral CT
- Volumetric scanners
- Introduced by Will Kalender and Kazuhiro Katada
- uses slip ring technology
slip ring: electromechanical device that conduct
electricity and electric signals through rings and
brushes across a rotating surface onto a fixed surface
Sixth

43. The main drawback of helical CT scanner lies in
the nature in which the data is collected. Since
the data is acquired in a helical formation, no
full slices of data are available because the
scanner is not producing planar sections
This problem can be compensated through the
use of reconstruction process
Sixth

44. - 64-Slice CT
- Multiple detector array
- Can acquire an outstanding amount of
information in a very short time span
- cone shaped beam
Seventh

45. ¤ MRI images exhibited better low-contrast
resolution than CT images
¤ CT shows bony structures better than MRI
¤ CT does not affect metal in a patient
CT vs MRI

46. ¤ CT is useful for scanning patients (who are
extremely claustrophobic, combative, or
uncooperative) quickly and easily because of
short gantry length, relatively large aperture,
and short scanning time
¤ CT often is a less costly examination than
MRI
CT vs MRI

47. Components

48. The most prominent part of the CT scanner. It
includes the x-ray tube, the detector array, the
high-voltage generator, the patient’s support
couch, and the mechanical support.
Gantry

49. - Special requirements:
- Power capacity: must be high
- >120 kVP
- 400 mA
- High speed rotors: for heat dissipation
- Anode heat capacity: 7 MHU (Spiral CT)
- Heat storage capacity: 8 MHU
- Anode cooling rates: 1 MHU/min
X-ray tube

50. - Focal spot size: small
- Take note: CT scanners designed for
high spatial resolution imaging not for
direct projection imaging
- Limiting characteristics:
- Focal spot design: must be robust or
strong
- Heat dissipation
- X-ray tube life: approx. 50,000 exposures
X-ray tube

51. - Focal spot Cooling Algorithms:
- Design to predict the focal spot
thermal state
- to adjust the mA setting accordingly
X-ray tube

52. - Group of detectors
- The image receptors in CT
- Absorbs radiation and converts it to electrical
signals
Detector Array

53. - Types
- Gas-filled detectors: previously used
- Scintillation and solid-state detectors:
recently used
Detector Array

54. - Gas detector:
- Basis: ionization of gas
- Three types:
- Ionization chamber
- Proportional counter
- Geiger-muller counter
- Characteristics:
- Excellent stability
- Large dynamic range
- Low quantum efficiency
Detector Array

55. - Scintillation detector:
- Characteristic: high x-ray detection efficiency
(90%)
- Reduces patient dose
- Allows faster imaging time
- Improves image quality
- Crystals used: Sodium Iodide
- Replaced by:
- Bismuth Germanate (BGO)
- Cesium Iodide (CsI)
Detector Array

56. - Current crystal of choice:
- Cadmium Tungstate (CdWO4)
- Special Ceramics
Detector Array

57. - Photodiode: converts light into electrical signal
- Characteristics:
- Small
- Cheap
- Does not require power supply
Detector Array

58. Three Important Factors
Contributing to Detector
Efficiency

59. The area of the detectors sensitive to radiation
as a fraction of the total exposed area
1. Geometric Efficiency

60. The fraction of incident x-rays on the detector
that area absorbed and contribute to the
measured signal
2. Quantum Efficiency

61. The ability to accurately convert absorbed x-ray
signal to electrical signal
3. Conversion Efficiency

62. - The product of geometric, quantum and
conversion efficiency
Overall/Dose Efficiency

63. - Computer-controlled electronic amplifier and
switching device
- Where signal from each radiation detector is
connected
Data Acquisition System

64. - High frequency power
- High voltage step-up transformer
- Power: 50 kW
- Accommodates higher x-ray tube rotor speeds
- Accommodates instantaneous power surges
characteristic of pulsed system
High-Voltage Generator

65. - Supports the patient comfortably
- Construction: low-Z (atomic number) material (
Carbon Fiber)
- Rationale: it does not interfere with x-ray
beam transmission and patient imaging
- Features:
- Smoothly and accurately motor driven
- Rationale: precise positioning
possible
- Capable of automatic indexing
- Rationale: operator does not have to
enter the room between each scan.
Patient Couch

66. - Restricts the volume of tissue irradiated
- reduces patient dose
- improves image contrast
- Types: Post patient collimator & Pre patient
Collimator
Collimation

67. - Prepatient Collimator
- Limits the area of the patient that
intercepts the useful beam
- Mounted on the x-ray tube housing or
adjacent to it
- Purpose: to decrease patient dose
- Determines:
- Dose profile
- Patient dose
Collimation

68. - Predetector/Post patient Collimator
- Restricts the x-ray beam viewed by the
detector array
- Purpose:
- to decrease scatter radiation
- to improve contrast
- Determines:
- slice thickness
- sensitivity profile
Collimation

70. - Contains meters and controls
- For selection of proper imaging technique
factors
- For proper mechanical movement of the
gantry and patient couch
- For the use of computer commands
- Allow image reconstruction and transfer
Operating Console

71. - 2-3 Operating consoles
- 2 for CT radiologic technologists
- 1st: to operate imaging system
- 2nd: to post-process images for
filming and filing
Operating Console

72. - 1 for Physician
- to view the images
- to manipulate contrast, size and
general visual appearance
- accepts the reconstructed image
from operator’s console
- displays reconstructed image for
viewing and diagnosis
Operating Console

73. - Two monitors:
- 1st: provided for operator
- to annotate patient data on the
image (e.g. hospital identification,
name, patient number, age, gender)
- to provide identification for each
image (e.g. number, technique, couch
position)
Operating Console

74. - 2nd: allows the operator to view the
resulting image before transferring it to
hard copy or physician’s viewing console
Operating Console

75. - Technique factors:
- kVp: <120
- mA: 400 (maximum)
- varied according to patient thickness
to reduce patient dose
- slice thickness: 0.5-5 mm
Operating Console

76. - Physician’s work station: allows the physician
- to call up any previous image
- to manipulate image to optimize
diagnostic information
- Scan time: length of time required per scan
Operating Console

78. - Unique subsystem of the CT imaging system
- Microprocessor and primary memory: heart of the
computer
- determines reconstruction time
- Array processors:
- mostly used in CT instead of
microprocessors
- Rationale:
- does many calculations
- faster than microprocessors (<1 sec
reconstruction time)
Computer

79. - Computer memories: ROM and RAM
- Read only Memory: for storage data only and cannot
be overwritten
- Random Access Memory: temporary memory that
stores information while software is used
- Central Processing Unit (CPU): performs calculations
and logical operations under control of software
instruction
- heart of the computer
Computer

80. - Special requirements:
- controlled environment
- relative humidity: <30%
- temperature: <20°C
- High humidity and temperature:
- contribute to computer failure
Computer

82. - Electromechanical device that conducts
electricity and electrical signals through rings
and brushes
- allows the gantry to rotate continuously
without interruption
- made Multi Slice CT possible
Slip Ring Technology

83. - Brushes: transmit power to the gantry
components
- composition: silver graphite alloy
- used as sliding contact
- replacement of brushes
- every year
- during preventive maintenance
Slip Ring Technology

84. When the examination begins, the x-ray tube
rotates continuously. While the x-ray tube is
rotating, the couch moves the patient through
the plane of the rotating x-ray beam.

85. Reconstruction of an image at any z-axis
position is possible because of a mathematical
process called interpolation.
Interpolation Algorithm

86. If one wishes to estimate a value between
known values, that is interpolation; if one
wishes to estimate a value beyond the range of
known values, that is extrapolation.

87. Data interpolation is performed by a special
computer program called an interpolation
algorithm.

88. When these images are formatted into sagittal
and coronal views, prominent blurring can occur
compared with conventional CT reformatted
views.

89. The solution to the blurring problem is
interpolation of values separated by 180
degrees – half a revolution of the x-ray tube.

90. This results in improved z-axis resolution and
greatly improved reformatted sagittal and
coronal views.

91. Relationship between patient couch movement
and x-ray beam width.
Pitch

93. Image Characteristics

94. The image obtained in CT is different from that
obtained in conventional radiography. It is
created from data received and is not a
projected image. In radiography, x-rays form an
image directly on the image receptor. With CT
imaging systems, the x-rays form a stored
electronic image that is displayed as a matrix of
intensities.

95. The CT image format consists of many cells,
each assigned a number and displayed as an
optical density or brightness level on the
monitor.

96. - Original EMI: 80x80 matrix (6400 individual
cells of information)
- Current imaging system: 512x512 matrix
(262,144 cells of information)

97. Each cell of information is a pixel and the
numerical information contained in each pixel is
a CT number, or Hounsfield unit (HU).

98. A two-dimensional representation of a
corresponding tissue volume.
Pixel

101. The diameter of image reconstruction.
FOV

102. When the FOV is increased for a fixed matrix
size, for example, from 12 cm to 20 cm, the size
of each pixel is increased proportionately.

104. When the matrix size is increased for a fixed
FOV, for example, 512x512 to 1024x1024, the
pixel size is smaller.

106. Compute the pixel size for the following
characteristics of CT images:
a. FOV 20 cm, 128x128 matrix
b. FOV 20 cm, 512x512 matrix
c. FOV 36 cm, 512x512 matrix
(before solving convert cm to mm)
Pixel size = FOV/Matrix size

108. The tissue volume is known as voxel (volume
element), and it is determined by multiplying
the pixel size by the thickness of the CT image
slice
Voxel

110. If each of the three scans in the preceding
question was conducted at a 5-mm slice
thickness, what would be the respective voxel
slices?
Voxel size (mm3)=pixel size (mm2) x
slice thickness (mm)

111. Computed Tomography
Numbers

112. Each pixel is displayed on the monitor as a level
of brightness. These levels correspond to a
range of CT numbers from -1000 to +3000 for
each pixel.

113. Tissue CT Number
Dense bone 3000
Bone 1000
Liver 40-60
Muscle 50
White matter 45
Gray matter 40
Blood 20
Cerebrospinal fluid 15
Water 0
Fat -100
Lungs -200
Air -1000

114. CT Number = k(µ1 - µw / µw )
µ1 = attenuation coefficient of the tissue in the
voxel under analysis
µw = attenuation coefficient of water
k= constant that determines the scale factor for
the range of CT numbers
The value of a CT number if given by
the following:

115. The projections acquired by each detector
during CT are stored in computer memory. The
image is reconstructed from these projections
by a process called filtered back projection.
Image Reconstruction

116. Here, the term filter refers to a mathematical
function rather than to a metal filter for the x-
ray beam. This process is much too complicated
to be discussed here, but a simple example
helps to explain how it works.
Image Reconstruction

121. In CT, we would have not four cells (pixels) but
rather more than 250,000. Consequently, CT
image reconstruction requires the solution of
more than 250,000 equations simultaneously.
Image Reconstruction

122. Recently, a more robust reconstruction
algorithm, iterative reconstruction, has been
introduced.
Image Reconstruction

123. Iterative reconstruction requires more
computer capacity but can result in improved
contrast resolution at lower patient radiation
dose.
Image Reconstruction

124. Image Quality

125. - Ability to image small objects that have high
subject contrast
- expressed in line pairs/millimeter (lp/mm)
Spatial Resolution

126. 1. Pixel size
2. Slice thickness
3. Voxel size
4. Design of prepatient and predetector
collimator
5. Detector size
Factors Affecting/Influencing SR

127. Factors Effect in Spatial Resolution
Thick Slice Thickness Poor SR
Large Pixel Size Poor SR
Large Voxel Size Poor SR
Large Detector Size Poor SR
Prepatient and Predetector Collimator influences SR
by affecting the contrast of the system

128. - One bar and its interspace of equal width
Line Pair

129. - Used to describe CT SR
- Low SF= represents large objects
- High SF= represents small objects
Spatial Frequency

130. - Mathematical expression of the ability of the
CT scanner to reproduce a high-contrast edge
with accuracy
Edge Response Function (ERF)

131. - Mathematical expression for measuring
resolution
- The ratio of the image to the object as a
function of spatial frequency
- used to describe CT SR
Modulation Transfer Function
(MTF)

132. - MTF = 1; faithfully represents the object
- MTF = 0; image is blank and contain no
information
- MTF = intermediate values; intermediate levels
of fidelity
Modulation Transfer Function
(MTF)

133. 1. Collimation
2. Detector size and concentration
3. Mechanical/ Electrical gantry control
4. Reconstruction algorithm
Characteristics of CT Imaging
System Contributing to Image
Degredation

134. - Measured by determining the OD along the
axis of the image
Image Fidelity

135. - Spatial frequency at MTF equal to 0.1
Limiting Resolution

136. 1. Artifacts generation
2. Contrast resolution
3. Spatial resolution
Important Measures of Imaging
System Performance

137. - Ability to distinguish one soft tissue from
another without regard for size or shape
- Contrast resolution is superior in CT principally
because of x-ray beam collimation
Contrast Resolution

138. - Ability to image low-contrast objects is limited
by:
- size and uniformity of the object
- noise of the system
Contrast Resolution

139. - Determined by the mass density of the body
part
- Characterized by x-ray linear attenuation
coefficient
X-ray Absorption in Tissue

140. - A function of x-ray energy and atomic number
of the tissue
X-ray Linear Attenuation
Coefficient

141. - The percentage standard deviation of a large
number of pixels obtained from a water bath
image
- the variation in CT number above or below the
average values
- appears as graininess
Noise

143. - The resolution of low-contrast objects is
limited by the noise of the CT imaging system
- Should be evaluated daily through imaging of a
20-cm diameter water bath.
Noise

145. Pixel Value Noise
Equal 0
Large variation High
Small variation Low

146. 1. kVp and filtration
2. Pixel size
3. Slice thickness
4. Detector efficiency
5. Patient dose -> primary control of noise
Factors Affecting Noise

147. - Describes the amount to which the CT number
of a material is exactly proportional to the
density of the material (in HU)
- A check calibration that can be made daily
uses the five-pin performance test object
Linearity

149. - The consistency of the CT numbers of an
image of a homogenous material across the
scan field
Uniformity

150. - Constancy of pixel values in all region of the
reconstructed image
- can be tested easily with an internal software
package that allows the plotting of CT numbers
along any axis of the image as a histogram or as
a line graph
- Acceptable value: +/- 2 mean value (SD)
Spatial Uniformity