3. History
Internal structure of an object can be reconstructed
from multiple projections to object.
“Tomos:Greek: to cut section”
& “Graphein:to write”
J.Radon (1917)
“Two dimension and three dimension object can be
reconstructed from the infinite set of projection data”.
1972 G.N.Housefeild(EMI) announced
‘COMPUTERIZED AXIAL TRANSVERSE
SCANNING’
4. CT MAIN SYSTEMS
IMAGING SYSTEM
COMPUTER SYSTEM
DISPLAY, RECORDING, STORAGE
SYSTEM
DATA ACQUISITION SYSTEM
30. VOXEL SIZE DEPENDS
-FOV
-MATRIX SIZE
-SLICE THICKNESS
Each VOXEL- a
number.
Extent of Xray
attenuation passing.
31. CT
ELIMINATES SUPERIMPOSITION
INCREASES CONTRAST OF LOW
SUBJECT CONTRAST TISSUES
HELPS IN DISTINGUISHING BETWEEN
HOMOGENOUS OBJECTS OF NON-
UNIFORM THICKNESS.
ABILITY TO ADJUST & MANIPULATE
IMAGE AFTER SCANNING
32. Computed tomography
X-rays irradiated on body, some rays are absorbed and some
pass through the body to produce an image.
In plain X-ray imaging, the film directly absorbs penetrated X-
rays.
In CAT scanning, an electronic device called a "detector array"
absorbs the penetrated X-rays, measures the X-ray amount, and
transmits the data to a computer system. A sophisticated
computer system, in turn, calculates and analyzes data from each
detector in each level, and finally reconstructs multiple, two-
dimensional, cross-sectional images.
33. Gantry tilt: depend on examination.
Lateral scout view.
Scout view is used for planning
34. Plain film imaging reduces the 3D patient
anatomy to a 2D projection image
Density at a given point on an image
represents the x-ray attenuation properties
within the patient along a line between the x-
ray focal spot and the point on the detector
corresponding to the point on the image
35. Tomographic image-picture of a slab of patient’s anatomy
2D CT image corresponds to 3D section of pt
CT slice thickness is very thin (1 to 10 mm)
and is approximately uniform
The 2D array of pixels in the CT image
corresponds to an equal number of 3D
voxels (volume elements) in the patient
Each pixel on the CT image displays the
average x-ray attenuation properties of the
tissue in the corrsponding voxel
37. Ray, Ray sum, View & Attenuation ProfileRay, Ray sum, View & Attenuation Profile
RayRay – Imaginary line– Imaginary line
between Tube &between Tube &
DetectorDetector
Ray SumRay Sum – Attenuation– Attenuation
along a Rayalong a Ray
ViewView – The set of ray– The set of ray
sums in one directionsums in one direction
The attenuation for eachThe attenuation for each
ray sum when plottedray sum when plotted
as function of itsas function of its
position is called anposition is called an
attenuation profileattenuation profile
View Attenuation
profile
Ray
Ray sums
39. AMPLIFIER
SIGNAL FROM DETECTORS GOES TO
AMPLIFIERS FOR SIGNAL
MAGNIFICATION AND THEN IS SENT TO
SAMPLE/HOLD UNIT
40. ADC
CONVERTS ANALOG SIGNAL OUTPUT
FROM THE SCANNING EQUIPMENT TO A
DIGITAL SIGNAL SO IT CAN BE
PROCESSED BY A COMPUTER.
41. SAMPLE/HOLD UNIT (S/H)
LOCATED BETWEEN AMPLIFIERAMPLIFIER AND ADCADC
PERFORMS SAMPLING AND ASSIGNS
SHADES OF GRAY TO THE PIXELS IN THE
DIGITAL MATRIX CORRESPONDING TO THE
STRUCTURES
44. DATA FLOW IN CT
REFERENCE DETECTOR
REFERENCE DETECTOR
ADC
PREPROCESSOR
COMPUTER
RAW DATA
CONVOLVED DATA
BACK
PROJECTORRECONSTRUCTED DATA
PROCESSORS
DISK TAPE
DAC CRT DISPLAY
45.
46. COLLIMATION IN CT
ADC
PRE-PATIENT COLLIMATION
(Tube/Source)
POST-PATIENT COLLIMATION
(Detector)
• Width of beam Leaving tube
• Shapes beam
• Defines Slice Thickness
• Width of ATTENUATED beam-
after passing pt- before reaching
detector.
• Fine tuning of beam profile
• Removes scatter Radiation.
52. SCINTILLATION CRYSTALS USED
WITH PM TUBES:
SODIUM IODIDE –AFTERGLOWAFTERGLOW
+Hygroscopic+ LOW DYNAMIC RANGE+Hygroscopic+ LOW DYNAMIC RANGE
( USED IN THE PAST)( USED IN THE PAST)
CALCIUM FLUORIDE
Cesium Iodide(CsI)
BISMUTH GERMANATE
Short decay time-Afterglow not a
problem.
53. S. CRYSTAL USED WITH
PHOTODIODE
CALCIUM TUNGSTATE
RARE EARTH OXIDES – CERAMIC
(requires less dose)-faster
55. COMPUTER SYSTEM
RECONSTRUCTION AND
POSTPROCESSING
CONTROL OF ALL SCANNER
COMPONENTS
CONTROL OF DATA ACQUSITION,
PROCESSING, DISPLAY.
DATA FLOW DIRECTION
56. Tomographic reconstruction
t)I/Iln(
II
t0
t
0t
µ
µ
=
= −
e
• Each ray acquired in CT is a transmission measurement through the
patient along a line
• The unattenuated intensity of the x-ray beam is also measured during
the scan by a reference detector
• Before the acquisition of the next slice, the table that the patient lies on
is moved slightly in the cranial-caudal direction (the “z-axis” of the
scanner)
•
57. Reconstruction
There are numerous reconstruction algorithms
Filtered backprojection reconstruction is most
widely used in clinical CT scanners
Builds up the CT image by essentially reversing the
acquistion steps
The µ value for each ray is smeared along this same
path in the image of the patient
As data from a large number of rays are
backprojected onto the image matrix, areas of high
attenutation tend to reinforce one another, as do
areas of low attenuation, building up the image
60. Image reconstructionImage reconstruction
The image is createdThe image is created
by reflecting theby reflecting the
attenuation profilesattenuation profiles
back in the sameback in the same
direction they weredirection they were
obtainedobtained
This process is calledThis process is called
BACK PROJECTIONBACK PROJECTION
BACK PROJECTION
62. Window width: is the range of CT
numbers that are used to map
signals into shades of gray
Window level: determines the
midpoint of the range of gray
levels to be displayed
Darker or Lighter
63. 1st
generation: rotate/translate,
pencil beam
Only 2 x-ray detectors used (two different
slices)
Parallel ray geometry
Translated linearly to acquire 160 rays
across a 24 cm FOV
Rotated slightly between translations to
acquire 180 projections at 1-degree intervals
About 4.5 minutes/scan with 1.5 minutes to
reconstruct slice
64. • LONG HIGH VOLTAGE CABELS WERE USED-
• 1 Clockwise rotation-STOP-Anticlockwise- Unwind High
Tension cabels
• Tube-detector array moves continousely around pt-covers
Volume
65. Slip Ring: : connects generator
with tube
Circular Electrical Conductive Ring:Overcame Voltage Cable Problem
66. 1st
generation (cont.)
Large change in signal due to increased x-
ray flux outside of head
– Solved by pressing patient’s head into a flexible
membrane surrounded by a water bath
NaI detector signal decayed slowly, affecting
measurements made temporally too close
together
Pencil beam geometry allowed very efficient
scatter reduction, best of all scanner
generations
67. 2nd
generation: rotate/translate,
narrow fan beam
Incorporated linear array of 30 detectors
Rotated in an arc upto 300
More data acquired to improve image quality
Shortest scan time was 18 seconds/slice
Practicle to scan large distances
Narrow fan beam allows more scattered
radiation to be detected
69. 3rd
generation: rotate/rotate, wide
fan beam
Detector & X-ray source rotate in unison
Number of detectors increased substantially
(more than 800 detectors-incr 30-40degree)
Angle of fan beam increased to cover entire
patient
– Eliminated need for translational motion
Mechanically joined x-ray tube and detector
array rotate together
Newer systems have scan times of ½ second
70. 4th
generation: rotate/stationary
Designed to overcome the problem of ring
artifacts
Stationary ring of about 4,800 detectors
around pt.
Wide FAN BEAM Geometry
X-ray tube moves in circular motion within
detector array.
71.
72. 3rd
vs. 4th
generation
3rd
generation fan beam geometry has the x-
ray tube as the apex of the fan; 4th
generation
has the individual detector as the apex
t)I/Iln(:gen4
t)I/Iln(:gen3
t0
th
t201
rd
µ
µ
=
=
gg
gg
73.
74. 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; can produce
fast-frame-rate CT movies of the beating
heart
75. Electron beam CT scanners:uses e-gun,linear
accelerators
Electron beam is focused & deflected along Tungsten Target in
FAN BEAM GEOMETORY
76. • Electron beam targeted on TUNGSTEN Target
ring
• Opposite to stationary Semi-circular detector
array.
77.
78. 6th
generation: helical
Helical CT scanners acquire data while the
table is moving
By avoiding the time required to translate the
patient table, the total scan time required to
image the patient can be much shorter
Allows the use of less contrast agent
In some instances the entire scan be done
within a single breath-hold of the patient
84. Ring artifacts
The rotate/rotate geometry of 3rd
generation
scanners leads to a situation in which each
detector is responsible for the data
corresponding to a ring in the image
Drift in the signal levels of the detectors over
time affects the µt values that are
backprojected to produce the CT image,
causing ring artifacts
88. Drawbacks in Back projectionDrawbacks in Back projection
(Artifacts - Star shape & streaks)(Artifacts - Star shape & streaks)
The resultant imageThe resultant image
closely resembles theclosely resembles the
original objectoriginal object
But it shows starBut it shows star
shaped patternsshaped patterns
around objects andaround objects and
streaksstreaks
These are calledThese are called
‘Star’ and ‘streak’‘Star’ and ‘streak’
artifactsartifacts
89. Formation of Star artifact andFormation of Star artifact and
streaksstreaks Consider aConsider a
scan of ascan of a
single highsingle high
densitydensity
objectobject
suspended insuspended in
airair
90. The back projections take the form of a stripeThe back projections take the form of a stripe
through the center of the objectthrough the center of the object
91. Back projections are crated for each profileBack projections are crated for each profile
92. Addition of the attenuation profiles create anAddition of the attenuation profiles create an
image with star and streak artifactsimage with star and streak artifacts
Final back projection
