Principle and generation of Computed Tomography

1. Presented by :Rituraj Mishra.
B.Sc.MIT, 2013
Moderator: Mr. Ranjit Kumar Jha

2. 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.

3. 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.

4. COMPARISION OF CT WITH CONVIENTIONAL RADIOGRAPHY.
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

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

6. 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.

7. 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.

8. Tomography

9. 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.

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

11. 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.

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

13. 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.

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

15. VOXEL
 Each individual element or number in the image
matrix represents a three dimensional volume element
in object called VOXEL.

16. 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.

17. HOUNSFIELD UNITS(HU)
 Related to different composition and nature of Tissue.
 The CT NUMBER is also known as Hounsfield
units(HU).
 Represent the density of tissue.
 Different Tissue have different CT number Range in
HU.

18. 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

19. 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.
It corresponds to contrast of the image.
WINDOW LEVEL– represents the CT no. selected for
the centre of the range of the no. displayed on the
image. It corresponds to brightness of image .

20. 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.

21. STEPS OF CT IMAGE FORMATION

22. 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

23. 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

24. Major components of Data Acquisition
System(DAS)
a)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: 80-120
mA:80-500

25. b) X-ray Tube
 Rotating anode x-ray tube with unique cooling.
 Small focal spot size (0.6mm) to improve spatial resolution.
 Anode heating capacity:1MHU to 7MHU
 Cooling rate:1MHU per minute.
c)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.
b)Post patient Collimators: Reduces the scattered radiation detectors.

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

27. d)Detectors
The detectors gather information by measuring the x-ray
transmission through the patient.
 Two types:
Scintillation crystal detector
(Cadmium tungstate+ Si Photodiode)
Can be used in third and fourth generation scanners
Xenon gas ionisation chamber
Can be used in third generation scanners only

28. 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.

29. 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.
It contain three correction factor
1. SIMULTANEOUS RECONSTRUCTION
2. RAY BY RAY CORRECTION
3. POINT BY POINT CORRECTION

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

31. 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.

32. 2-D FOURIER ANALYSIS

33. 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.

35. 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

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

37. First generation CT Scanner
•Head kept enclosed in a water
bath
•Paired detectors
•A reference detector

38. Second Generation CT Scanner

39. Second Generation
 Narrow fan beam
 Linear detector array(5 to30)
 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)

40. 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
•Disadvantage: Ring Artifacts
due to electronic drift between
many detectors.

41. Fourth Generation

42. 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 ½ s or, <2s.
 Designed to address ring artifacts by keeping
detector assembly stationary.
 Disadvantage: High cost.

43. 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

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

45. CT SCAN IN OUR RADIOLOGY
DEPARTMENT.(16 SLICES)

46. Patient Positioning and Data
Acquisition

47. TECHNICAL SPECIFICATION
(Hitachi ECLOS 16)
 CT scanner mode: Multislice
 Slices per rotation: 16
 Other rotation speed options: 1.0, 1.5, 2.0, 3.0 seconds
 Minimum rotation speed: 0.8 seconds
 Gantry diameter: 70 cm
 Maximum beam width (cm): 2 cm
 Table weight limit: 495 lbs
 Table movement range vertically/longitudinally: 42 to 100 cm
vertical, 186 cm horizontal
 X-ray generator kV range: 100, 120, 130 kVp
 Maximum scan range: 175 cm
 X-ray tube heat capacity: 5.0 MHU
 Power requirements: 3-phase, 200 Amp max.

49. 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.

50. 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.

51. 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.

53. 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.

54. 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.

55. 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.

56. PORTABLE CT SCAN

57. 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.

58. 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.

60. 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