Computed tomography (CT) is a medical imaging method employing tomography.

1. Computed Tomography
Presented by :
120130103071-Nandasana Jay
120130103084-Prajapati Jay
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2. Introduction
 Computed tomography (CT) is a medical
imaging method employing tomography.
 The word "tomography" is derived from the
Greek tomos (slice) and graphein (to write).
 A large series of two-dimensional X-ray images
(slices) of the inside of an object are taken
around a single axis of rotation.
 Digital geometry processing is used to
generate three-dimensional images of the
object from those slices.
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3. History
 The first commercially viable CT scanner was
invented by Sir Godfrey Hounsfield in Hayes,
United Kingdom at EMI Central Research
Laboratories using X-rays. Hounsfield
conceived his idea in 1967. and it was publicly
announced in 1972.
 Allan McLeod Cormack of Tufts University in
U.S. independently invented a similar process,
and both Hounsfield and Cormack shared the
1979 Nobel Prize in Medicine.
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4. CT Scan
 CT scan produces axial
sections/cuts /Slices
 The CT image is
recorded through a
SCAN.
 Scan?
A scan is made up of
multiple X-Ray
attenuation
measurements around an
objects periphery
X-ray tube
Detector
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5. Understanding Basic factors
 Absorption :-stopping
of x-rays with transfer
of energy
 Scatter:- deflection of x-
rays
 Incident Intensity :- No.
of x-ray photons falling
on an object
 Transmitted Intensity:-
No. of photons passing
through
Incident x-
ray beam
Transmitted
X-ray beam
Scattered x-rays
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6. Attenuation
 The reduction of the
beam intensity on
passing through the
material due to
absorption plus scatter
 The degree of
attenuation is obtained
by measuring and
comparing the incident
and transmitted
intensities
More dense
material
Less dense
material
Less
transmitted
x-rays
More
transmitted
x-rays
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7. Slice / Cut
 The cross sectional
portion of the body
which is scanned for the
production of CT image
is called a slice.
 The slice has width and
therefore volume.
 The width is
determined by the
width of the x-ray beam
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8. Matrix
 The image is
represented as a
MATRIX of numbers.
 Matrix :- A two
dimensional array of
numbers arranged in
rows and columns.
 Each number
represents the value of
the image at that
location
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9. VOXEL
 Each individual element
or number in the image
matrix represents a
three dimensional
volume element in the
object, called a VOXEL
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10. PIXEL
 The VOXEL is
represented in the
image as a two-
dimensional element
called PIXEL - (picture
element)
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11. CT numbers
 The numbers in the
image matrix are called
CT numbers.
 Each pixel has a number
which represents the x-
ray attenuation in the
corresponding voxel of
the object
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12. Visual image & Gray Scale
 To obtain a visual
image, the CT numbers
are assigned different
shades of gray on a gray
scale.
 Each shade of gray
represents the x-ray
attenuation within the
corresponding voxel
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13. 13
Tissue Types And Appearance
*Cerebrospinal fluid (CSF)

14. CT Image
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15. Phases of CT imaging
1. Scanning the patient
2. Data Acquisition
I. Tube or tube and detector move
II. Multiple attenuation measurements are taken
around the object
3. Image reconstruction
4. Image Display
5. Image archival (recording)
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16. DATA ACQUISITION
 Basic components
 X-ray tube
 Collimators
 Detector/s
 Collimated x-ray beam
traverses the object and
enters the detector.
X-ray
tube
Collimators
Detector
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17. Practical method of Generation of
Attenuation profiles
 Attenuation profiles are a recording of X-ray attenuation
verses position in the object.
 Attenuation is related to the ratio between incident
intensity (I0) and transmitted intensity (It)
I0
It
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18.  X-ray intensity is measured by a DETECTOR which
converts x-ray photons to electrical current
 The reference (Input) detector measures the incident
intensity (I0)
 The output detector measures the transmitted
intensity (It )
I0= incident intensity = tube output
It
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19.  I0 should remain constant
 It will change depending on the attenuation of the
object scanned (e.g. centered, round object of
uniform density)
Io=1000
It=1000
It
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20. Calibration profile
 If the object scanned is a centered, round water bath
each attenuation profile is called a CALIBRATION PROFILE.
and the set of profiles are called the Calibration File or
Cal file
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21. DATA Calibration
 Subtraction of
calibration file
from the
attenuation
profiles of the
object called data
calibration
 (consider an
object containing
two areas of
different densities
as shown)
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22. Scan Data File
 The difference profile are stored as numerical values as a
function of position in the profile.
 This data is used in the image reconstruction process
 The set of difference profiles for a complete scan is called
SCAN DATA FILE or raw data
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23. Back projection of scan data
1. Back projection of
the corrected
attenuation profiles
is accomplished by
feeding the
numerical values of
each point along the
profile into a matrix.
(Since the scanning
motion is circular,
the matrix is usually
round)
--
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24. Formation of Star artifact and streaks
 Consider a
scan of a
single high
density object
suspended in
air
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25.  The attenuation profile for this object has a single
impulse signal
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26.  Back projections are crated for each profile
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27.  Addition of the attenuation profiles create an image
with star and streak artifacts
To be continued – CT Complementary 3 27

28. Generations of CT Scan
1st generation: rotate/translate, pencil beam
2nd generation: rotate/translate, narrow fan beam
3rd generation: rotate/rotate, wide fan beam
4th generation: rotate/stationary
5th generation: stationary/stationary
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29. 1st generation: rotate/translate,
pencil beam
• Only 2 x-ray detectors used (two different slices)
• Parallel ray geometry
• About 4.5 minutes/scan with 1.5 minutes to
reconstruct slice
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30. 2nd generation: rotate/translate,
narrow fan beam
• Incorporated linear array of
30 detectors
• More data acquired to
improve image quality
• Shortest scan time was 18
seconds/slice
• Narrow fan beam allows
more scattered radiation to
be detected
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31. 3rd generation: rotate/rotate, wide fan
beam
• Number of detectors
increased substantially (to
more than 800 detectors)
• Angle of fan beam
increased to cover entire
patient.
• Mechanically joined x-ray
tube and detector array
rotate together
• Newer systems have scan
times of ½ second
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32. 4th generation: rotate/stationary
• Designed to overcome the problem of ring
artifacts.
• Stationary ring of about 4,800 detectors
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33. 5th 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.
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35. Latest update:-
6th generation: helical
7th generation: multiple detector array
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36. Modern CT scanner
1. gantry aperture (720mm
diameter)
2. microphone
3. sagittal laser alignment light
4. patient guide lights
5. x-ray exposure indicator light
6. emergency stop buttons
7. gantry control panels
8. external laser alignment lights
9. Patient couch
10. ECG gating monitor
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37. Advantages
•Quick and painless
•Can help diagnose and guide
treatment for a wider range of
conditions than plain X-rays
•Can detect or exclude the presence
of more serious problems
•Can be used to check if a previously
treated disease has recurred
Disadvantages
•Small increased risk of cancer in
future from exposure to ionising
radiation (X-rays). Risk is greater for
children
•Uses higher doses of radiation, so the
risks (while still small) are in general
greater than other imaging types
•Injection of a contrast medium (dye)
can cause kidney problems or result in
allergic or injection-site reactions in
some people
•Some procedures require anaesthesia
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38. Questions?
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39. Thank You……………
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