History, Principle of CT Scan, Generations, Image quality, Radiation dose and Recent Advancement in CT scan

1. Rituraj Mishra.
B.Sc.MIT
BPKIHS,Dharan

2. CONTENTS
History of CT Scan
Basic principles of CT
Generation of CT
CT System Components
Image Reconstruction In CT
Helical and Multislice CT
Artefacts in CT
Image Quality in CT

3. HISTORY OF CT SCAN
 Sir Godfrey N. Hounsfield invented the CT scan in the
year 1972 and called computerized axial transverse
scanning.
 He presented a cross-sectional image of the head that
revealed the internal structures of the brain in a
manner previously only seen at surgery or autopsy and
for the first time pathologic processes such as blood
clots, tumors, and strokes could be easily seen
noninvasively.

4. COMMON NAMES
a. Computerized axial transverse scanning (Hounsfield,
1972)
b. Computerized axial tomography (CAT)
c. X-ray computed tomography (X-ray CT)
d. Computed/computerized tomography (CT)
Computed tomography (CT) is currently the
preferred name.

5. HISTORY LEADING TO CT
SCAN
 1917—Radon developed the basic mathematical equations.
 1940—Frank and Takahashi published the basic principles
of axial CT.
 1956—Cormack developed theory of image reconstruction.
 1972—Hounsfield developed the clinically useful CT scanner.
 1973—First clinical brain scanner in Mayo Clinic.
 Hounsfield and Cormack shared the 1979 Nobel
Prize for their development of CT.

6. INTRODUCTION
 Computed Tomography is a well accepted imaging
modality for evaluation of the entire body.
 Computed Tomography(CT) Scan Machines Uses X-
rays, a powerful form of Electromagnetic Radiation.
 The images are obtained directly in the axial plane of
varying tissue thickness with the help of a computer.
 Some pathology can be seen in saggital or coronal
plane by reconstruction of the images by computer.
 CT has undergone several evolutions and nowadays
multi- detectors CT scanners have been evolved which
have better application in clinical field.

7. COMPARISION OF CT WITH
CONVIENTIONAL RADIOGRAPHY
Conventional radiography suffers
from the collapsing of 3D structures
onto a 2D image.
CT gives accurate diagnostic
information about the
distribution of structures inside
the body.

8. A conventional X-ray image is
basically a shadow.
Shadows give you an
incomplete picture of an
object's shape.
This is the basic idea of computer aided tomography. In a
CT scan machine, the X-ray beam moves all around the
patient, scanning from hundreds of different angles
CONTD…

9. Comparison of CT with Conventional
Radiography
Radiographic procedure is qualitative and
not quantitative.

10. ADVANTAGE OF COMPUTED TOMOGRAPHY
OVER CONVIENTIONAL RADIOGRAPHY.
 To overcome superimposition of structures.
 To improve contrast of the image.
 To measure small differences in tissue
contrast.

11. TOMOGRAPHY
 Imaging of Layer/Slice.
Principle
Images of structures lying above and below the plane
are blurred out due to motion unsharpness while the
structures lying in plane of interest appear sharp in in
the image.

12. Tomography

13. PRINCIPLE OF COMPUTED
TOMOGRAPHY
 The internal structure of the object can
be reconstructed from multiple
projections of the object.
 Mathematically principle of CT was first developed in
1917 by Radon.
 Proved that image of unknown object could be
produced if one had several number of projections
throughout the object.

14. VARIOUS PARAMETERS OF CT
 SLICE
 MATRIX
 PIXEL
 VOXEL
 CT NUMBER
 WINDOWING
 WINDOW WIDTH
 WINDOW LEVEL
 PITCH

15. SLICE/CUT
 The cross section portion of body which is scanned for
production of CT image is called Slice.
 The slice has width and therefore volume.
 The width is determined by width of the x rays beam.

16. Cross Sectional Slices
Think like looking into the loaf of bread by cutting into the
thin slices and then viewing the slice individually.

17. MATRIX
 The CT image is represented as the Matrix of the
number.
 A two dimensional array of numbers arranged in rows
and columns is called Matrix.
 Each number represent
the value of the image at
that location.

18. PIXEL
 Each square in a matrix is called a pixel.
 Also known as picture element.

19. VOXEL
 Each individual element or number
in the image matrix represents a three
dimensional volume element in object
called VOXEL.
• Since a CT section has a finite
thickness, each pixel actually
represents a small volume element, or
voxel. The size of this voxel depends
on the matrix size, the selected field
of view (FOV), and the section
thickness

20. CT NUMBER
 The numbers in the image matrix is called CT
NUMBER.
 Each pixel has a number which represents the x-ray
attenuation in the corresponding voxel of the object.

22. HOUNSFIELD UNITS(HU)
 Related to different composition and nature of Tissue.
 X-ray attenuation depends on both the density and
atomic number(z) of materials and the energy of the
x-ray photons. For CT imagining a high KV(like 120-
140) and heavy beam filtration is used. This minimizes
the photoelectric interactions that are influenced by z
of the material. Therefore, CT number are determined
by the density of the tissues or materials.

23. Air - 1000
Fat -100
Pure water 0
CSF 15
White matter 45
Gray matter 40
Blood 20
Bone/calcification +1000
TISSUE AND CT NUMBER APPROXIMATE

24. WINDOWING is a system where the CT no. range of
interest is spread cover the full grey scale available on
the display system
WINDOW WIDTH –Means total range of CT no.
values selected for gray scale interpretation.
WINDOW LEVEL– represents the CT no. selected for
the centre of the range of the no. displayed on the
image.

25. Pitch
 The relationship between patient and tube motion is
called Pitch.
 It is defined as table movement during each
revolution of x-ray tube divided by collimation
width.
 For example: For a 5mm section, if patient moves
10mm during the time it takes for the x-ray tube to
rotate through 360˚, the pitch is 2.
 Increasing pitch reduces the scan time and patient
dose.

26. STEPS OF CT IMAGE FORMATION

27. Phase of CT scanning
1.Scanning the patient or data Acquisition
a)X-ray Generator
b)X-ray Tube
c)X-ray Filtration System
d)Detector System
2.Reconstruction
a)Simple back projection
b)Iterative method
c)Analytical method
3.Display

28. DATA ACQUISTION
The scanning process begins with data
acquisition
Data Acquisition refers to a method by
which the patient is systematically
scanned by the X ray tube and detectors
to collect enough information for image
reconstruction

29. Major components of Data Acquisition
System(DAS)
X-ray Generators
Generators are located on rotating scan frames within the
CT gantry to accommodate slip Ring.
Power: 50 to 80kw
Frequency: 5 to 50kHz
KVp: 120-140
mA:80-500

30. Gantry Assembly
 The first major component of a CT system is referred
to as the scan or imaging system. The imaging system
primarily includes the gantry and patient table or
couch. The gantry is a moveable frame that contains
the X-ray tube including collimators and filters,
detectors, data acquisition system (DAS), rotational
components including slip ring systems and all
associated electronics such as gantry angulation
motors and positioning laser lights.

31. X-ray Tube
 The three main parts of any X-ray tube are the anode, cathode and the
filament. When the filament is heated, electrons are ejected from its
surface. A large voltage between the cathode and the anode force
electrons to accelerate towards the anode. The electrons hitting the
anode (tungsten) produce Bremstrahlung radiation at an efficiency of
only 1 percent. The other 99 percent of the electrons energy is converted
into heat.
Rotating anode x-ray tube with unique cooling.
Small focal spot size (0.6mm) to improve spatial
resolution.
Anode heating capacity:1MHU to 8MHU
Cooling rate:1MHU per minute.
 Most modern system use tubes with two focal spots
small spot is used for high resolution examination. And
large spot is used for larger anatomic coverage.

32. X-ray Beam Filtration System
CT employs monochromatic beam but radiation from CT
X-ray tube is polychromatic. so, X-ray beam is shaped by
compensation filter.
a)Pre patient Collimators: Reduces the patient dose.
 Depends on the focal spot size
 Mounted on the tube housing
 Creates more parallel beam
b)Post patient Collimators: Reduces the scattered radiation
detectors.
 Restricts the field of view of detectors
 Reduces the scatter radiation on the detector
 Aperture width helps determine the slice thickness

33. Overall Functions of
Collimators.
 To decrease scatter
radiation
 To reduce patient
dose
 To improve image
quality
 Collimator width
determines the slice
thickness

34. Detectors
The detectors gather information by measuring the x-ray transmission
through the patient.
 Two types:
Scintillation crystal detector
 Use solid materials in which the energy of X-rays is converted to
light photons. Then, the emitted light is converted into an electrical
current by using a photomultiplier tube or a silicon photodiode. The
material which produces light when the X-ray energy is absorbed is
named scintillator and the combination of a scintillator and the
device converting light into a current, is named scintillation
detector.
(Cadmium tungstate+ Si Photodiode)
Xenon gas ionisation chamber
 These are based on the ionization of a gas inside a closed chamber
when the X-ray energy is absorbed into a gas. The main
disadvantage is the low efficiency of gas detectors.

35. 2)Reconstruction
 Reproduction of an image from raw data is called
Reconstruction.
A)Simple back projection
The image is created by reflecting the attenuation
profiles back in same direction they were obtained.

36. B)Iterative method
 It start with assumption that all point in matrix have same
value and it was compared with measured value and make
correction until Values come with in acceptable range.
Today commonly used .
 It contain three correction factor
1. SIMULTANEOUS RECONSTRUCTION
2. RAY BY RAY CORRECTION
3. POINT BY POINT CORRECTION

37. C)Analytical Method
Two popular method used in that method are:-
1. 2-D FOURIER ANALYSIS
2.FILTERED BACK PROJECTION

38. 2-D FOURIER ANALYSIS
In it any function of time or space can be represented by the
sum of various frequencies and amplitude of sine and
cosine waves.
For example the actual projected image of original object is
more rounded than those shown which would be slowly
simplify and corrected by Fourier transformation.

39. 2-D FOURIER ANALYSIS

40. FILTERED BACK PROJECTION
Same as back projection except that the image is filtered,
or Modified to exactly counterbalance the effect of
sudden density Changes , which cause blurring(star
like pattern) in simple back projection.

42. 3)Display
 The reconstructed image is displayed on the monitor.
It is a digital image.
 It consists of 2D representation of 3D object in the
form of pixels.
 CT pixel size is determined by dividing the FOV by
matrix Size which is generally 512*512.
 PIXEL SIZE= FOV (mm)/ MATRIX SIZE

43. Generations of CT Scan
First Generation
 Narrow pencil beam
 Single or, Two detectors.
 Detector used is made up of NaI.
 Translate –Rotate movements of
Tube- detector combination
 Scan time-5mins.
 Designed only for evaluation
of brain.

44. CONTD…
•Head kept enclosed in a water
bath
•Paired detectors
•A reference detector

45. Second Generation CT Scanner

46. Second Generation
 Narrow fan beam
 Linear detector array(5 to30 detectors) in detector
assembly.
 Translate-Rotate movements of Tube-Detector
combination
 Fewer linear movements are needed as there are more
detectors to gather the data.
 Between linear movements, the gantry rotated 30o
 Scan time~30secs(advantage over first generation)
 Bow tie filter was used which compensate the increased
radiation intensity that occurs towards the edge of the
beam due to body shape.(Narrow fan Beam.)

47. Third Generation
•Rotate(tube)Rotate(detectors)
Motion.
•Pulsed wide fan beam.
•Arc of detectors(600-900)
•Detectors are perfectly aligned
with the X-Ray tube
•Both Xenon and scintillation
crystal detectors can be used
•Scan time< 5secs(i.e.20secs or
longer.)
•Disadvantage: Ring Artifacts
due to electronic drift between
many detectors.

48. Fourth Generation

49. Fourth Generation
 Complete circular array of about 1200 to 4800
stationary detectors
 Single x-ray tube rotates with in the circular array of
detectors
 Wide fan beam to cover the entire patient
 Scan time of newer scanners is about ½sec(sub
second.)
 Designed to address ring artifacts by keeping detector
assembly stationary.
 Free of Ring Artifact.
 Disadvantage: High cost.

50. Fifth Generation
 stationary/stationary
 Developed specifically for cardiac tomographic imaging
 No conventional x-ray tube; large arc of tungsten encircles
patient and lies directly opposite to the detector ring
 Electron beam steered around the patient to strike the
annular tungsten target
 Capable of 50-msec scan times; can produce fast-frame-
rate CT movies of the beating heart

51.  Electron gun
 Large Arcs of tungsten
targets
 Detector ring
 17 slices per second

53. SPIRAL/HELICAL CT
 Spiral/Helical scanning uses third generation or fourth
generation slip ring design.
 Spiral computed tomography (or helical computed
tomography) is a computed tomography(CT) technology in
which the source and detector travel along a helical path
relative to the object. Typical implementations involve
moving the patient couch through the bore of the scanner
whilst the gantry rotates. Spiral CT can achieve improved
image resolution for a given radiation dose, compared to
individual slice acquisition. Most modern hospitals
currently use spiral CT scanners.

54. SLIP RING TECHNOLOGY
 In conventional CT scanning there was a paused
between each gantry rotation. But in Helical CT, Slip
Ring technology is used which allows continuous
rotation of gantry without interruption.
 Slip Rings are electrical conducting brushes and
component of gantry transferring the data or,
electrical energy to and from the stationary part of
gantry to rotating part of gantry for continuous
rotation of gantry.

55. Contd….
 There are usually three slip rings made up of
conduction materials(i.e. Sliver and Graphite.)
 First Slip Ring provides high voltage power to X-ray
tube.
 Second provide low voltage to control system on
rotating gantry and
 Third Slip Ring transfers digital data from rotating
detectors arrays.

57. MULTISLICE/MULTIDETECTOR CT
 Multidetector computed tomography(MDCT) is a form of
computed tomography (CT) technology for diagnostic
imaging.
 In MDCT, a two-dimensional array of detector elements
replaces the linear array of detector elements used in
typical conventional and helical CT scanners. The two-
dimensional detector array permits CT scanners to acquire
multiple slices or sections simultaneously and greatly
increase the speed of CT image acquisition.
 Image reconstruction in MDCT is more complicated than
that in single section CT.

58. DUAL SOURCE CT
Dual Source CT (DSCT) is equipped
with two X-ray tubes.
Two corresponding detectors are
oriented in the gantry with an angular
offset of 90 degrees.
ADVANTAGES
1) High temporal resolution(in
response to time domain) for
cardiac imaging without β
blockers which means heart rate
is independent upon temporal
resolution.
2) Less radiation dose even for
obese patient.
3) Faster acquisition time with
shortest breathe hold.

59. DUAL ENERGY CT
Standard computed tomography (CT)
scanners use normal X-rays to make
cross-sectional ‘slice-like’ pictures or
images of the body.
A dual energy CT scanner is fairly new
technology that uses both the normal
X-ray and also a second less powerful X-
ray to make the images.
Two pictures are taken of the same
slice at different energies.(i.e. 80/140KV
,100/140KV Or,70/150KV)
This gives dual energy CT additional
advantages over standard CT for a wide
range of tests and procedures.
Most commonly used for CT
Angiography.

60. PORTABLE CT SCAN

61. PORTABLE CT SCAN
 As the world’s first portable, full-body, 32-slice CT (computed
tomography) scanner, BodyTom is a multi-departmental
imaging solution capable of transforming any room in the
hospital into an advanced imaging suite. The system boasts an
impressive 85cm gantry and 60cm field of view, the largest field
of view available in a portable CT scanner.
 The battery-powered BodyTom with an innovative internal drive
system can easily be transported from room to room and is
compatible with PACS, planning systems, surgical and robotic
navigation systems.
 Uniquely designed to accommodate patients of all sizes,
BodyTom provides point-of-care CT imaging wherever high-
quality CT images are needed, including the operating room,
intensive care unit, radiation oncology suites, and the emergency
department.

62. RADIATION DOSE IN CT
 Volume Computed Tomography Dose Index (CTDIvol) is a
standardized parameter to measure Scanner Radiation Output
 CTDIvol provides information about the amount of radiation
used to perform the study.
 CTDIvol is NOT patient dose
 CTDIvol is reported in units of mGy for either a 16-cm
(for head exams) or 32-cm (for body exams) diameter.
 AAPM (American Association of Physicts in Medicine)
introduces a parameter known as the Size Specific Dose
Estimate (SSDE) to allow estimation of patient dose based on
CTDIvol and patient size.
 For the same CTDIvol, a smaller patient will tend to have a
higher patient dose than a larger patient.

63. EFFECTIVE DOSE (mSv)
CT
 HEAD------------ 2
 CHEST ---------- 8
 ABDOMEN --10
 PELVIS -------10
GENERAL RADIOGRAPHY
 SKULL -------- 0.07
 CHEST PA ---- 0.02
 ABDOMEN --- 1
 PELVIS -------- 0.7

64. IMAGE QUALITY
 Spatial resolution.
 Contrast resolution.
 Noise.
 Image artifacts.

65. Spatial Resolution
 Ability of an image system to distinctly depict two
objects as they become smaller and closer together.
 Directly related to mAs =quantity of photons making
the image
 Other factors that influence spatial resolution include
pixel size, which is influenced by the chosen, scanned
field of view and matrix size, width of the detector,
spacing between detectors, number of projections or
views obtained and focal spot size.

66. Contrast Resolution
 It is the ability of a CT scanner to differentiate small
attenuation differences on the CT image.
 A soft tissue, standard or smooth algorithm is used
during the reconstruction process to enhance soft
tissue and contrast resolution.

67. Noise
 Noise is the portion of a signal that contains no
information.
 Noise is characterized by a grainy appearance of the
image.
 Local variations in contrast due to a background texture
called noise that does not represent the attenuation in
patient.
 Noise is considered to be the number one limiting
factor of CT image quality.
 The major types of noise include quantum noise and
quantum noise is a result of too few photons reaching a
detector after being attenuated by the body.

68. Image Artifacts
 An artifact is any distortion or error in the image that
is unrelated to the subject being studied.
Types of Artifacts
 Motion Artifact
 Metal Artifact
 Beam Hardening Artifact
 Partial Volume Artifact
 Ring Artifact

69. Motion Artifacts
 Motion can be voluntary or involuntary. No matter
which kind of motion we are dealing with, the most
efficient way to reduce motion artifact is to reduce our
scanning time. Methods to reduce patient motion
artifacts include patient immobilization, ECG gated CT,
and some correction algorithms.

70. Motion Artifacts

71. Metal Artifacts
 Metallic materials such as prosthetic devices, dental
fillings, surgical clips, and electrodes produce streak
artifacts on the image. Several methods have been
provided to remove the artifacts coming from metal.

72. Metal Artifacts

73. Beam Hardening Artifacts
 Beam hardening is a phenomenon results from the
increase of mean energy of the X-ray beam when it
passes through object. Therefore, the CT numbers of
certain structures change and induce some artifacts.
This kind of artifact can be reduced or eliminated
with a filter that ensures the uniformity of the beam at
the detectors.

74. Beam Hardening Artifacts

75. Partial Volume Artifacts
 Partial volume artifacts arise when a voxel contains
many types of tissue. It will produce a CT number as
an average of all types of tissue. This is the source of
partial volume effect and will appear as bands and
streaks. Using thinner slice and some computer
algorithms can reduce partial volume artifacts.

76. Partial Volume Artifacts

77. Ring artifacts
 Ring artifacts appear on a CT image as a ring or a number
of rings superimposed on the structures being scanned.
The artifact is commonly associated with third generation
or rotate-rotate CT systems. This artifact occurs due to one
or several misaligned or miscalibrated detectors in the
detector array of the rotate-rotate CT system.

78. Ring artifacts