COMPUTED TOMOGRAPHY

1. COMPUTED TOMOGRAPHY
CT SCAN
BY SAJID EJAZ

2. INTRODUCTION
• Computer tomography (CT), originally
known as computed axial computed axial
tomography (CAT or CT scan) and body
section rentenography.
• It is a medical imaging method employing
tomography where digital geometry
processing is used to generate a three-
dimensional image of the internals of an object
from a large series of two-dimensional X-ray
images taken around a single axis of rotation.

3.  The word "tomography" is derived from the
Greek tomos (slice) and graphein (to write).
CT produces a volume of data which can be
manipulated, through a process known as
windowing, in order to demonstrate various
structures based on their ability to block the
Xray beam.

4. HISTORY
 The first commercially viable CT scanner
was invented by Godfrey Newbold
Hounsfield in Hayes, England at Thorn EMI
Central Research Laboratories using X-rays.
 Hounsfield conceived his idea in 1967, and
it was publicly announced in 1972.
 It is claimed that the CT scanner was "the
greatest legacy" of the Beatles; the massive
profits from their record sales enabled EMI
to fund scientific research.

5. ADVANTAGE OF COMPUTED
TOMOGRAPHY OVER
CONVENTIONAL RADIOGRAPHY
 To overcome superimposition of structures.
 To improve contrast of the image.
 To measure small differences in tissue
contrast.

6. PRINCIPLE OF 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.

7. VARIOUS PARAMETER OF CT
 SLICE
 MATRIX
 PIXEL
 VOXEL
 CT NUMBER
 WINDOWING
 WINDOW WIDTH
 WINDOW LEVEL
 PITCH

8. 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.

9. 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.

10. PIXEL
 Each square in a matrix is called a pixel.
 Also known as picture element.
VOXEL
 Each individual element or number in
the image matrix represents a three
dimensional volume element in object
called VOXEL.

11. 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.

12. HOUNSFIELD UNIT (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.

13. WINDOWING
 It 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 .

14. 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.

15. GENERATION OF CT
 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.

16. FIRST GENERATION CT SCAN
 Head kept enclosed in
a water bath
 Paired detectors
 A reference detector

17. SECOND GENERATION OF CT
SCAN
 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)

18. THIRD GENERATION OF CT
SCAN
 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

19. THIRD GENERATION OF CT
SCAN

20. FOURTH GENERATION OF CT
SCAN
 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.

21. FOURTH GENERATION OF CT
SCAN

22. FIFTH GENERATION OF CT
SCAN
 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.

23. FIFTH GENERATION OF CT
SCAN

24. CT BASICHARDWARE
A-X-ray tube
B-Collimators
C-Detectors – Scintillation crystals
-Xenon gas ionisation chambers
-Ceramic scintillators

25. X-RAY TUBE
• Focal spot:0.6mm
• Annode heat capacity

26. TWO COLLIMATORS IN CT
PREPATIENT COLIIMATORS
• Determine slice
thickness
• Reduce patient dose
PREDECTOR COLLIMATORS
• Reduce scattered
radiation to improve
contrast
• Helps determine slice
thickness

27. Thank you…