3
Definition
• Computed tomography(CT scan), also X-ray
computed tomography, computed axial
tomography (CAT scan) or computer assisted
tomography is a medical imaging procedure
that uses computer-processed X-rays to
produce tomographic images or 'slices' of
specific areas of the body. These cross-sectional
images are used for diagnostic and therapeutic
purposes in various medical disciplines.
4.
4
History
• In theearly 1900s, the Italian radiologist Alessandro
Vallebona proposed a method to represent a single
slice of the body on the radiographic film. This
method was known as tomography.
• The mathematical theory behind the tomographic
reconstruction dates back to 1917 by an Austrian
mathematician Johann Radon. He showed
mathematically that an image could be
reconstructed from an infinite set of its projections.
5.
5
At the annualcongress of British institute of
radiology in April’72 Sir Godfrey Newbold
Hounsfield from England announced the
invention of a revolutionary new imaging
technique which he called computerized axial
tomography.
Later in 1979 he shared the
Nobel Prize for Physiology or Medicine with
Allan McLeod Cormack for his part in
developing the diagnostic technique of X-ray
computed tomography (CT).
8
Technical development ofCT (yr/yr)
• 1917-Johann Radon demonstrates that the
image of a 3-dimensional object can be
reconstructed from an infinite number of 2-
dimensional projections of the object,
providing the mathematical basis for CT
image construction.
• 1969-
• 1971-
9.
9
• 1974- a.320x 320 image matrix
b. First body CT scan (of Hounsfield) in a
prototype of the EMI body scanner
• 1975- 2nd
generation CT scanner were marketed.
• 1977- a. 5 second scan time for an image
b. 3rd
generation CT was introduced.
• 1979- 512 x 512 image matrix. 200 scanners sold
in USA.
• 1981- 3 seconds scan time available.
• 1985- 1 second scan time/superfast ct developed.
• 1987- 1024 x 1024 image matrix
10.
10
• 1989- Firstspiral (helical) CT, manufactured by
Siemens, Siemens enters into the market
• 1998- 4-slice scanners
- 0.5 second scan time
• 1999- PET/CT developed, developed by Dr
David Townsend and Dr Ron Nutt
• 2002- 8- and 16-slice scanners introduced
• 2007- 128 slice ct scanner is marketed
- 72 million ct performed in developed
countries only.
11.
11
Basic principle ofCT
The basic principle behind CT is that the internal
structure of an object can be reconstructed from multiple
projections of the object.
The ray projection are formed by scanning a thin cross
section of the body with a narrow X-ray beam and
measuring the transmitted radiation with a sensitive
radiation detector. The detector does not form the image.
It merely adds up the energy of all the transmitted
photons. The numerical data from multiple ray sums are
then computer processed to reconstruct an image.
13
Comparison of CTwith conventional radiography
• Conventional radiography suffers
from the collapsing of 3D structure
into 2D images.
• CT gives accurate diagnostic
information about the distribution
of structure within body.
14.
14
X
Basic principle ofCT
-Reconstruction of 2 dimensional image-
Simple Backprojection
Projection Data
Blur
x
y
x
y
curvilinear integral of absorption coefficient regarding Y
object
X-ray
tube
X
-
r
a
y
d
e
t
e
c
t
o
r
a
r
r
a
y
Data Acquisition field Reconstruction field
X
X
Y
15.
15
CT Scanner
• X-Raymodality used to the
body in cross section
• Used to determine
– extent of trauma
– location and type of tumors
– status of blood vessels
– pre surgical planning
24
Digital Projection
• X-raytube and detector remain stationary
• Patient table moves continuously
– With X-rays “on”
• Produces an image covering a range of
anatomy
– Similar to a conventional X-ray image, e.g. flat
plate of the abdomen
• Image is used to determine scan location
and image reconstruction.
25.
25
Axial CT
• X-raytube and detector
rotate 360°
• Patient table is stationary
– With X-ray’s “on”
• Produces one cross-sectional
image
• Once this is complete patient
is moved to next position
– Process starts again at the
beginning
26.
26
Volume CT
• X-raytube and detector rotate 360°
• Patient table moves continuously
– With X-ray’s “on”
• Produces a helix of image information
– This is reconstructed into 30 to 1000 images
27.
27
Advantages of VolumeCT
• More coverage in a breath-hold
– Chest, Vascular studies, trauma
• Reduced misregistration of slices
– Improved MPR, 3D and MIP images
• Potentially less IV contrast required
• Gapless coverage
• Arbitrary slice positioning
29
Tomographic acquisition
• Singletransmission measurement through
patient made by a detector at a given
moment in time is called a ray.
• A series of ray passing through the patient is
called projection.
• Two projection geometry is being used in CT
a. Parallel beam gepmetry
b. Fan beam geometry
31
• Purpose ofct scanner hardware is to acquire a large
number of transmission measurement at different
position.
• Single ct image may involve approximately 800 rays
taken at 100 different projection angle.
• 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)
• Image quality is related to the number of ray
projections used to reconstruct each ct scan image.
This statement ensures that each ray contains new
informations.
32.
32
• It isto be noted that number of ray projection
already increased from 28800(earlier scanner)
to more than one millions(newer scanners)
• Data gathering techniques have developed by
stages. These stages have been called
generations.
• These development not only reduces patients
exposure but also reduces exposure time and
thereby increased image quality.
33.
33
CT generations
CT scannerhas gone through a number of design changes
since the technology was first introduced in 1972.
a. First generation : ( Translate/Rotate, one detector, Pencill
beam.)
b. Second generation : ( Translate/Rotate, Multi detector,
narrow fan beam)
c. Third generation : ( Rotate/Rotate, wide fan beam)
d. Fourth generation : ( Rotate/Fixed)
e. Fifth generation : ( Stationary/Stationary)
f. Sixth generation : 6th
generation: helical
g. Seventh generation : 7th generation: multiple detector array
34.
34
First generations
The firstgeneration of CT scanner was rotate – translate,
pencil like X-ray beam system and a single detector, i.e
one detector per tomographic section. The x-ray tube
detector movement were both linear and rotatory.
Advantages : It accept only very small pencil ray of X-
rays.
Disadvantages :
• Time consuming (e.g A five view study of the head took
25-30 minutes.)
• Significant number of afterglow
• Could not accommodate the huge dynamic range in X-
ray intention.
35.
35
First generations
• EMIMark I scanner (1973)
• Earliest versions:4.5 minutes for a single scan and thus
were restricted to some regions (patient motion
controlled).
• Later versions: Procedures~ series of scans procedure
time reduced some what by using two detectors so that
two parallel sections were acquired in one scan
• Contrast resolution of internal structures was
unprecedented, images had poor spatial resolution.
38
Second generations
X-ray beamis of narrow fan angel(10°)
Scanning time– 18-90 sec/section
Advantages :
Shorten the scanning time than first generation.
15 times faster than first generation system.
Improved image quality.
First designs to permit scans of the trunk
Disadvantages :
• More scattered radiation than the pencil beam used in
first generation.
• Increase radiation dose to patient.
41
Number Of detectors: (300-750detectors, usually circular,
Shorter scanning time (2 sec)
Pure rotational scanning motion could be used , then it
would be possible to use higher-power, rotating anode x-
ray tubes and thus improve scan speeds in thicker body
parts.
“Slam-bang translational motion” was replaced with
smooth rotational motion :
-higher-output rotating anode x-ray tubes could be used
-greatly reducing scan times.
Third generations
42.
42
Third generations
X-ray tubeis collimated to a wide x-ray beam (fan-shaped )
Directed toward an arc-shaped row of detectors
Tube and detector array rotate around patient
Different projections are obtained during rotation by
pulsing x-ray source or by sampling the detectors at a very
high rate.
Type of detector : both xenon & scintillation crystal can be
used.
Improvement in detector and data acquisition technology
-detector array with enough, high spatial resolution
to allow measurement of a fan-beam projection of entire
patient cross-section.
43.
43
Third generations
Sampling considerationsrequired scanning an
additional arc of one fan angle beyond 180°, although
most scanners rotate 360°for each scan.
Current helical scanners are based on modifications
of rotate-rotate designs.
Scan times = few seconds or less, and recent
versions are capable of sub-second scan times.
Imaging process is significantly faster than 1st or 2nd
generation systems.
44.
44
Third generations
Number ofdetectors increased substantially (to more
than 800 detectors).
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 ½second.
Cons: very high performance detectors are needed to
avoid ring artefacts and the system is more sensitive to
aliasing than 1st or 2nd generation scanners.
47
Fourth generations
Infourth generation design, the detectors are removed
from the rotator gantry and are placed on a stationary
annulus around the patient. The detector does not
move. The X-ray tube rotate in a circle the detector ring
and the X-ray beam is collimated to form a fan beam.
Modern fourth generation CT system uses from 1200 to
4800 individual detector.
Scanning time : 1-10 sec.
Type of detector : scintillation crystal.
Type of X-ray beam: wide fan beam.
The main advantage is the speed and disadvantage is
an increased amount of scatter radiation.
49
Fifth generations
Anovel CT scanner was developed for cardiac
tomographic imaging. This ‘cine –CT scanner does not
use a conventional X-ray tube but rather a large ring of
tungsten that circle the patient, which lies directly
opposed to the detector ring.
The X-ray is produced from the focal track as a high-
energy electron beam strikes the tungsten. There are no
moving parts to this scanner gantry. The electron beam
is produced in a cone like structure( A vacuum
enclosure) behind the gantry and is electronically
steered around the patient. So that it strikes the annular
ring of tungsten.
Scan time : 50 mili sec.
Number of slice : 17 CT slice/sec.
54
Design: X-Ray tuberotates as patient is moved
smoothly into x-ray scan field.
Simultaneous source rotation, table translation
and data acquisition.
Produces one continuous volume set of data for
entire region.
Data for multiple slices from patient acquired at
1sec/slice.
6th
generation/Helical CT
57
Speed :Patient movement continuous…………shorter
exam time ; entire abdomen or chest: 30 sec
Improved detections : Even small lesions fall out of
plane for each continuous slice
Improved contrast : Image a region in a short period,
contrast can be timed
Improved reconstruction & manipulation : Volume of
data collected, transverse data can be reconstructed in
any plane-strip away skin, muscles, etc….
Advantages of Spiral/Helical CT
58.
58
Three technological developments:
Slip-ring gantry designs
Very high power x-ray tubes
Interpolation algorithms to handle projection data
Spiral/Helical CT
59.
59
1. Slip-Ring Technology
•Alternative to cabling system = slip-ring.
• 1989 by Kalender.
• Electromechanical devices: circular electrical conductive
rings and brushes.
• Transmit electrical energy across a moving interface.
• All power and control signals from the stationary parts of
the scanner system are communicated to the rotating
frame through slip ring.
• Allow scan frame to rotate continuously with no need to
stop between rotations to rewind system cables
Spiral/Helical CT
61
2. High PowerX-ray Tube
• Thermal load in CT is more
• Oil cooling thermal systems present around
tube, fast scans.
• Scan time vs. Heat capacity increased x 5 times
• Thermal capacity is more thus tubes with much
higher thermal capacities able to withstand
continuous operation over multiple rotations.
•Expected life of tube 10,000-40,000 hrs vs. 1000
regular one
Spiral/Helical CT
62.
62
3. Interpolation Algorithms
•Kalender developed interpolation methods to generate
projections in a single plane.
• Overlapping sections generated by math, not beam,
improve z-axis with no increase in dose.
• Improved image quality.
Spiral/Helical CT
64
New Technology, singlerow had its limitation.
Designed with multiple detector array.
The collimator spacing is wider and more of the x-rays
that are produced by the tube are used in producing image
data :
--Opening up the collimator in a single array scanner
increases slice thickness, reducing spatial resolution in
the slice thickness dimension.
--With multiple detector array scanners, slice thickness is
determined by detector size, not by the collimator.
Seventh generations CT
65.
65
• “Turbo-charged” spiral
•Up to 8 rows of detectors
• Large volume of patient scanned (thorax,
abdomen, pelvis) at once
• Allows 1mm sections though whole chest in
20 sec
• Improvement in details can be gained
Seventh generations CT
69
4th
G Single X-ray
Tube
FanBeam 1200-4800
5th
G Many
tungsten
anodes in
single
large tube
Fan Beam Stationary
Ring of
Detectors
6th
G 3G/4G 3G/4G 3G/4G
7th
G Single X-ray
Tube
Cone beam Multiple
array of
detectors
70.
70
• A basicdata acquisition technique scheme
consist of---
a. X-Ray tube
b. Filter
c. Collimator
d. Detector
71.
71
Detector
• The detectorgather information by measuring
the X-Ray transmission through patient.
• Two types—
a. Scintillation crystal detector
( Cadmium tungstate + Si photodiode)
Used in 3rd
& 4th
generation scanners
b. Xenon gas ionization chamber
Used in earlier CT scanner(1st
& 2nd
generation)
82
Method for imagereconstruction
There are three mathematical methods of image reconstruction
are used in CT.
Back projection or summation method.
Iterative or repetition method.
Analytical method
83.
83
Method for imagereconstruction
Iterative or repetition method
An iterative reconstruction starts with an assumption
and compares this assumption with measured
values, makes correction to bring the two into
agreement, and than repeats the process over and
over until the assumed and measured values are the
same or within acceptable limits.
84.
84
Method for imagereconstruction
Iterative or repetition method
There are three variation of iterative reconstruction.
a. Simultaneous reconstruction – all projections for
the entire matrix are calculated at the beginning of
the iteration, and all correction are made
simultaneously for iteration.
b. Ray by ray correction – one ray sum is calculated
and corrected, and this correction are incorporated
into future ray sums, with the process being
repeated for every ray in each iteration.
85.
85
Method for imagereconstruction
Iterative or repetition method
c. Point by point correction – the calculation and correction are
made for all rays passing through one point, and these
correction are used in ensuing calculations, again with
process being repeated for every point.
88
CT generations
CT scannerhas gone through a number of design
changes since the technology was first introduced in
1972.
a. First generation : ( Translate/Rotate, one detector,
Pencill beam.)
b. Second generation : ( Translate/Rotate, Multi detector,
narrow fan beam)
c. Third generation : ( Rotate/Rotate, wide fan beam)
d. Fourth generation : ( Rotate/Fixed)
e. Fifth generation : ( Stationary/Stationary)
89.
89
First generations
The firstgeneration of CT scanner was rotate – translate,
pencil like X-ray beam system and a single detector, i.e
one detector per tomographic section. The x-ray tube
detector movement were both linear and rotatory.
Advantages : It accept only very small pencil ray of X-
rays.
Disadvantages :
• Time consuming (e.g A five view study of the head took
25-30 minutes.)
• Significant number of afterglow
• Could not accommodate the huge dynamic range in X-
ray intention.
90.
90
First generations
• EMIMark I scanner (1973)
• Earliest versions:4.5 minutes for a single scan and thus
were restricted to some regions (patient motion
controlled).
• Later versions: procedures = series of scans procedure
time reduced some what by using two detectors so that
two parallel sections were acquired in one scan
• Contrast resolution of internal structures was
unprecedented, images had poor spatial.
• Resolution very poor
93
Second generations
Type ofX-ray beam– narrow fan angel of 10°
Scanning time– 18-90 sec/section
Advantages :
Shorten the scanning time than first generation.
15 times faster than first generation system.
Improved image quality.
First designs to permit scans of the trunk
Disadvantages :
• More scattered radiation than the pencil beam used in
first generation.
• Increase radiation dose to patient.
95
Number Of detectors: (300-750detectors, usually circular,
Shorter scanning time (2 sec)
Pure rotational scanning motion could be used , then it
would be possible to use higher-power,rotating anode x-
ray tubes and thus improve scan speeds in thicker body
parts.
“Slam-bang translational motion” was replaced with
smooth rotational motion :
-higher-output rotating anode x-ray tubes could be used
-greatly reducing scan times.
Third generations
96.
96
Third generations
X-ray tubeis collimated to a wide x-ray beam (fan-shaped )
Directed toward an arc-shaped row of detectors
Tube and detector array rotate around patient
Different projections are obtained during rotation by
pulsing x-ray source or by sampling the detectors at a very
high rate.
Type of detector : both xenon & scintillation crystal can be
used.
Improvement in detector and data acquisition technology
-detector array with enough, high spatial resolution
cells to allow measurement of a fan-beam projection of
entire patient cross-section.
97.
97
Third generations
Sampling considerationsrequired scanning an
additional arc of one fan angle beyond 180°, although
most scanners rotate 360°for each scan.
Current helical scanners are based on modifications
of rotate-rotate designs.
Scan times = few seconds or less, and recent
versions are capable of sub-second scan times.
Imaging process is significantly faster than 1st or 2nd
generation systems.
98.
98
Third generations
Number ofdetectors increased substantially (to more
than 800 detectors).
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 ½second.
Cons: very high performance detectors are needed to
avoid ring artefacts and the system is more sensitive to
aliasing than 1st or 2nd generation scanners.
100
Fourth generations
Infourth generation design, the detectors are removed
from the rotator gantry and are placed on a stationary
annulus around the patient. The detector does not
move. The X-ray tube rotate in a circle the detector ring
and the X-ray beam is collimated to form a fan beam.
Modern fourth generation CT system uses from 1200 to
4800 individual detector.
Scanning time : 1-10 sec.
Type of detector : scintillation crystal.
Type of X-ray beam: wide fan beam.
The main advantage is the speed and disadvantage is
an increased amount of scatter radiation.
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