Presentation on Computed tomography in english

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.
2

History
• In the early 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.
3

At the annual congress 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).
4

Basic principle of CT
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.
7

Cross sectional slices
Things are like cutting a bread into slices and viewing all
slices individually.
8

Comparison of CT with 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.
9

CT Scanner
• X-Ray modality 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
10

Basic CT scanner
components
• Gantry
• X-Ray Tube
• Detector
• Control Console
12

Gantry
• CT X-ray tube
• High voltage
generator
• Detector array
• Data acquistion
system
• Slip ring
13

The CT X-ray Tube
• Anode heat capacity
– 3.5 MHU up to 28 MHU
• Determines maximum mAs
• Determines volume length
• Dictates generator size
14

Detector Elements
• Capture energy that has been
attenuated by the patient
15

Control console
• Set scan parameters
– kVp, mA, scan time,
reconstruction filter, etc.
• Set scan mode
– Surview, Axial or Spiral
• IRS (Image reconstruction
System)
• Review and archive images
• Post-processing
17

Scanning methods
• Surview
– AP,Lat
– Surview, Scanogram , Topogram….
• Conventional CT
– Axial
• Start/stop
• Volumetric CT
– Helical or spiral CT
• Continuous acquisition
18

Digital Projection
• X-ray tube 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.
19

Axial CT
• X-ray tube 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
20

Volume CT
• X-ray tube 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
21

Advantages of Volume CT
• 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
22

PHYSICS OF IMAGE
FORMATION
• Data accumulation
• Image reconstruction
23

Tomographic acquisition
• Single transmission 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 24

CT generations
CT scanner has 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
26

First generations
The first generation 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.
27

Second generations
X-ray beam is 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.
30

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
32

Third generations
X-ray tube is 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.
33

Fourth generations
 In fourth 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.
36

Fifth generations
 A novel 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.
38

Design: X-Ray tube rotates 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
43

 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
46

Three technological developments:
 Slip-ring gantry designs
 Very high power x-ray tubes
 Interpolation algorithms to handle projection data
Spiral/Helical CT
47

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
48

2. High Power X-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
50

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
51

New Technology, single row 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
53

• “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
54

 Cone Beam & multiple parallel rows of detectors.
 Widened (z-direction) x ray beam & detector array to
acquire multiple (4-64) slices simultaneously.
 Advantage: reducing scan time/ increases spatial
resolution.
 Disadvantage: less scatter rejection compared to
single slice, very expensive.
56

Generation Source Source
collimation
Detector
1st
G Single X-ray
Tube
Pencil Beam single
2nd
G Single X-ray
Tube
Fan Beam
(not enough
to cover FOV)
30
3rd
G Single X-ray
Tube
Fan Beam
(enough to
cover FOV)
300-750
57

4th
G Single X-ray
Tube
Fan Beam 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
58

• A basic data acquisition technique
scheme consist of---
a. X-Ray tube
b. Filter
c. Collimator
d. Detector
59

Detector
• The detector gather 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)
60

Scintillation crystal detector used in 1 ,2 generation
61

• Scintillation crystal detector used in 3rd & 4th generation
62

Reconstruction process
intensity
attenuated
:
)
(
intensity,
incident
:
)
(
[Np/m]
ted)
reconstruc
be
(to
t
coefficien
absorption
:
)
,
(
0 x
I
x
I
y
x
f





























0
0
)
,
(
0
)
,
sin
cos
(
)
,
(
:
projection
Back
)
,
(
of
FFT
1D
is
)
,
(
e
wher
)
,
(
2
1
)
,
(
:
data
Filtered
)
(
)
(
ln
)
,
(
)
,
(
:
data
projection
)
(
)
(
:
intensity
attenuated
d
y
x
p
y
x
f
X
p
U
P
dX
e
U
U
P
X
p
X
I
X
I
dY
y
x
f
X
p
e
X
I
X
I
f
jUX
f
dY
y
x
f
67

Reconstruction process
)
,
(
of
FFT
-
2D
is
)
,
(
where
)
0
,
(
)
,
(
:
Transform
Radon
y
x
f
v
u
F
U
F
U
P 

Data acquisition at angle : 0 – 180 degree
Obtain F(u,v) and then 2D IFFT -> reconstruction
Radon Transform is equivalent to Filtered backprojection !
68

Example of Simulation
Model Image Simple
Backprojection
Filtered
Backprojection
69

Method for image
reconstruction
There are three mathematical methods of image
reconstruction are used in CT.
 Back projection or summation method.
 Iterative or repetition method.
 Analytical method
71

Method for image
reconstruction
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.
72

Method for image
reconstruction
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.
73

Method for image
reconstruction
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.
74

Method for image
reconstruction
75

CT generations
CT scanner has 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)
77

First generations
The first generation 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.
78

First generations
• EMI Mark 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
79

Second generations
Type of X-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.
82

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
84

Third generations
X-ray tube is 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.
85

Third generations
Sampling considerations required 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.
86

Third generations
Number of detectors 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.
87

Fourth generations
 In fourth 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.
89

Fifth generations
 A novel 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.
91

Design: x-ray tube rotates 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.
Spiral/Helical CT
95

 Speed : patient movement continuous…………shorter
exam time ; entire abdomen or chest: 30 sec (1BH)
 Improved detections : differences in BH sin standard
CT, 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
97

Three technological developments:
 Slip-ring gantry designs
 Very high power x-ray tubes
 Interpolation algorithms to handle projection data
Spiral/Helical CT
98

1. Slip-Ring Technology
• Alternative to cabling system = slip-ring.
• 1989 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
99

2. High Power X-ray Tube
• Thermal load in CT
• 1stand 2nd, stationary tube(lowheat, slow scans)
• Oil cooling thermal systems around tube, fast scans
• scan time vs. Heat capacity increased x 5
• thermal demands on the x-ray tube
• Tubes with much higher thermal capacities were
required to withstand continuous operation over multiple
rotations
• New design: ceramic insulators ,oil cooling of bearing,
compact metal envelop E
• Expected life of tube 10,000-40,000 hrs vs. 1000 regular
one
Spiral/Helical CT
101

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
102

New Technology, single row had its limitation.
Design: 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.
104

• “turbo-charged” spiral
• Up to 8 rows of detectors
• 4 rows, large volume of patient scanned 1 BH
(thorax, abdomen, pelvis) at once
• Allows 1mm sections though chest in 20 sec
• Improvement in details
• Problem with PACS, stain on storage system
105

 Cone Beam & multiple parallel rows of detectors.
 Widened (z-direction) x ray beam & detector array to
acquire multiple (4-64) slices simultaneously.
 Advantage: reducing scan time/ increase z-resolution.
 Disadvantage: less scatter rejection compared to
single slice, very expensive.
108

Generation Source Source
collimation
Detector
1st
G Single X-ray
Tube
Pencil Beam single
2nd
G Single X-ray
Tube
Fan Beam
(not enough
to cover FOV)
30
3rd
G Single X-ray
Tube
Fan Beam
(enough to
cover FOV)
300-750
109

4th
G Single X-ray
Tube
Fan Beam
covers FOV
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
110

Tomography
In ordinary radiograph a three dimensional object - the
body is represented on two dimensional film. In
effect the object has been compressed virtually to
zero thickness in the direction of x-ray beam so that
the images of a number of structures are
superimposed on each other in the radiograph.
We have three method of separating such
superimposed image through modification of
ordinary radiograph –
1. Increased distortion
2. Decreased focus film distance
3. Motion of a part.
112

Tomography
Above method are failed completely when the desired
structure is buried within a denser one. Here we
make use of principle that departs from ordinary
radiography and is called TOMOGRAPHY.
Tomo-Combining from indicating section or layer
Graphin-To write
Simply,Body section radiography.
113

Tomography
Various types of Tomography
 CAT (Computerized axial tomography) – also known
as CT (Computed tomography).
 PET ( Positron emission Tomography) – Positron
emitting radionuclides are used here. By PET we can
measure cerebral blood flow, blood volume, oxygen
uptake, glucose transport & metabolism and locate
neurotransmitter receptor.
 SPECT (Single photon emission computed
Tomography) – used in diagnosing & treating stroke,
dementia, epilepsy etc.
114

MSCT
Multi slice CT is a specialized CT system equipped with
a multiple detector array that simultaneously obtains
tomographic data at different slice locations. MSCT
provides unparalleled capabilities for detailed
analysis of normal & abnormal anatomy and
pathology.
115

MSCT
Fundamentally, MSCT scanner is equipped with a
multiple detector array that concurrently collect data
at different slice locations, a defining features that
brings in numerous advantageous spin off like rapid
scanning, large patient coverage volume, high z-axis
resolution, generation of true isotropic datasets
which in combination facilitates 3D imaging,
perfusion imaging, CT Fluoroscopiy and so on.
MSCT provides a huge gain in performance that can
be reduce scan times, reduce scan collimation or to
increase scan length substantially.
116

General principles of MSCT
The basic principles of MSCT are relatively simple.
The X-ray point source and the detector array are placed on
opposite sides of the patient on a ring like structures
called the gantry. The gantry rotate around the patient,
who is located on the table at its center. The table moves
at constant speed along the axis of the gantry.
117

General principles of MSCT
X-rays are emitted towards the patient, penetrate the patient,
and are captured by more than one detectors. This
process generates a series of helical projections of
patients attenuations properties. Image representing X-
ray attenuation at each point in the volume traversed by
the photons are than mathematically reconstructed from
the helical projection data.
118

The physics of detection design
The fundamental difference between MSCT scanner and its
predecessors i.e. single slice spiral or helical scanners,
lies in the detectors array design. The beneficial strategy
in redesigning Ct scanner detectors of replacing a single
detector row by four or more rows allows a dramatic
increase in data acquisition capability besides numerous
other spin-offs Traditionally, a single slice, spiral CT
scanner has a single tube source that irradiates one row
of detectors measuring about 20 mm in length ( along z or
long axis).
119

The physics of detection design
Regardless of the X-ray beam collimation, there is only one
row of detectors. In MSCT this is replaced by a multiple
row of detectors called a detectors array that enables
simultaneous acquisition of 6 or 64 or 128 or 256 or 512
slices during one gantry rotation.
120

1. Faster acquisition and scanning time
2. Simultaneously data acquisition by using multiple
detectors
3. Sub-second rotation ( 0.5-0.8 sec) of gantry
4. Faster table translation speed
5. Larger anatomic coverage
6. Greater tube loading capacity
7. More data generation and processing.
Technical advances in MSCT
123

Broadly MSCT technology has improved existing
clinical applications while bringing in many novel
clinical applications.
These are—
A. Existing applications improved by MSCT
B. New applications pioneered by MSCT
Clinical applications of MSCT
124

Existing applications improved by MSCT
—
 CT angiography
 3D imaging
 Virtual endoscopy
 Virtual labyrinthoscopy
 3D dental CT etc
125

New applications pioneered by MSCT—
 Perfusion CT imaging of brain
 Cardiac imaging
 CT fluoroscopy
 Fusion medicine
 Screening CT
126

Perfusion CT imaging of brain :
It is a quick and convenient method of assessing
perfusion disturbance in acute stroke patient. Three
colour image maps with quantitative results related to
patient regional cerebral blood volume (rCBV),mean
transit time (MRTT) and regional cerebral blood flow
(rCBF)are generated that displays stroke much earlier
than the conventional CT images.
New applications pioneered by
MSCT
127

Perfusion CT imaging of brain :
It is useful for –
 quick & reliable identification of stroke signature
 Improved selection of patient for thrombolytic
therapy
 Identifying the vascular origin of ischemic insult
 Mapping the sequelae of stroke like final infarct size
& hemorrhagic risk.
 Besides of brain- body perfusion is increasingly
applied for lesions in kidney, liver, lungs.
MSCT
128

Cardiac imaging:
Fundamentally, MSCT has been utilized in three novel
areas of cardiac imaging :
1. Cardiac calcification scoring
2. Coronary angiography
3. Assessment of cardiac function
MSCT
129

Cardiac imaging:
Cardiac calcification scoring is a promising technique
utilizing the numerical quantifications of calcium in
coronary artery as an indicator of coronary heart
disease. CT coronary angiography is essentially a non-
invasive imaging technique of coronary arteries by
retrospectively ECG gated cardiac techniques that
enable of detection of stenosis and visualization and
differentiation of soft plaque. It is useful in following up
of post CABG, particularly in case with stent
placement.
MSCT
130

CT Fluoroscopy:
It is particularly useful for interventional techniques
such as biopsy and drainage of thoracic, abdominal,
pelvic and retroperitoneal lesions, drainage and
aspiration of intracranial hematomas, rod and seed
placement for bracytherapy, motion analysis and
bolus tracking CTA. A real time information display
remove the impact of patient breathing and motion on
image quality and permits accurate depth and
direction demarcation of the needle during
procedures. It also offers increased accuracy of fine
needle control therapy reducing complications while
reducing the procedure time.
MSCT
131

Fusion medicine:
Fusion imaging essentially combines anatomical and
physiological mapping of lesions, making it a
powerful tool for clinicians and radiologist in the
better understanding of disease. MSCT is a core
modality in fusion imaging since it provide accurate
spatial and density information. It is combined with
nuclear scans, PET or MRI scans and operates at
either system levels or data acquisition or processing
level.
MSCT
132

Screening test :
This is an emerging concept targeting early detection of
disease entities like lung cancer, colon cancer and
coronary artery disease. Low dose CT lung cancer
screening features computer-aided detection of
nodules, which automatically matches and compares
areas of suspicion while detecting new small lesion.
MSCT
133

Two types of detector are used in CT –
a. Scintillation crystal – used in fourth Gen CT i.e
rotate fixed.
b. Xenon gas ionization chamber– used in 3rd
Gen CT
rotate-rotate.
Detector used in CT
134

Scintillation crystal are any of an extremely large
number of materials that produce light as a result of
some external influence. More specifically ,these
materials are those that will produce light
(scintillate)when ionizing radiation react with them.
This is just what an X-ray intensifying screen does.
On a single interaction of a photon with crystal the
energy of X-ray photon will be converted to light
photons.
Scintillation crystal
135

Some of these light photon will be emitted promptly
and produce desired signal. Some light photon will
be delayed and produce afterglow. All crystal must
be matched to a light detector to convert the light
output to an electrical signal. The combination of a
scintillation crystal and the light detector is called
scintillation detector. All rotate-fixed and some
rotate-rotate units use scintillation crystal detector.
136

Types of crystal—
A. Thalium activated sodium iodide--- not use in recent
scanner due to
 It is hygroscopic
 Require an air tight container
 Very long afterglow
B . Cesium iodide (CsI)
C. Bismuth germinate (BGO)
D. Cadmium tungstate (CdWO4)
137

X-ray photon
crystal
Light photon
Desired signal
Afterglow
Photomultiplier tube/Diode
Analysis by computer
Interact
convert
Emitted promptly
Delayed emission
Convert light signal-an electron flow
Image on
Monitor 138

CT number
It is defined as a relative comparison of the X-ray
attenuation of each voxel of tissue with an equal
volume of water.
This number is compared to the attenuation value of
water and displayed on a scale of arbitrary units
named Hounsfield units (HU) after sir Godfrey
Hounsfield.
This scale assigns water as an attenuation value (HU)
of zero.
The range of CT numbers is 2000 HU wide although
some modern scanners have a greater range of HU
upto 4000.
139

CT number
Each number represent a shade of gray with +1000(white)
and -1000 (black) at either end of the spectrum.
140

CT number
Name of tissue/Organ CT number
Bone +400 +1000
Liver +50 +70
Soft tissue +40 +80
Blood +50 +60
Pancreas +30 +50
Kidney +20 +40
141

CT number
Name of tissue/Organ CT number
Water 0
Fat - 60 - 100
Lung - 400 - 600
Air - 1000
142

Equipment &
methodology
The major areas and equipment for generating a CT
image include –
1. Patients area
2. Operator’s console
3. Computer room and
4. Diagnostic viewing console.
143

Equipment &
methodology
Patient area :
It is a separate room containing patient table and gantry.
Patient table is automated in vertical & horizontal
direction so that the patient can be properly positioned
with in the gantry. Patient is usually placed in supine
position and is immobilized. Positioning is assisted by
positioning light. Gantry house X-ray tube and detector
and it can be angulated 20° to 30° cranially or caudally.
144

Equipment &
methodology
Operator’s console :
It is placed by the side of the patient area separated by
lead glass so that the technologist can see the patient
area from his room. After positioning of the patient the
technologist control the CT system from operator’s
console. Patient date ( name, age, sex ) & some
technical parameters (pixel size, slice thickness etc )
selected by the technologist must be enter into the
computer before the scanning is started.
145

Equipment &
methodology
Computer room:
Computer is installed in separate room because air
conditioning and power requirements for it are
critical and must be optimized for reliable service.
Time require to reconstruct the image depends on a
number of factor such as the design of the
equipment’s and the software (computer programs
that control CT system).
146

Equipment &
methodology
Diagnostic viewing console
In most CT system it is combined with operators
console. Data of each scan is stored in the
computer. After completion of scanning technologist
review the scan on CRT (cathode ray tube) and
adjust the contrast by changing window width and
window level. The scan is than photographed on a
radiographic film by a multi format camera.
147

Artifacts in CT
Artifacts are display of incorrect anatomy resulting
from errors in imaging, and are manifested as
objects that are not real, not present, with incorrect
size or shape, of have incorrect location and or
relative brightness.
148

Artifacts in CT
The common artifacts are-
1. Motion artifacts
2. Streak artifacts
3. Beam hardening artifacts
4. Ring artifacts
5. Partial volume or volume averaging artifacts
6. Geometrical artifacts.
149

Artifacts in CT
1. Motion artifacts :
During scanning if the patient moves it will produce
vertical or diagonal bands of artifacts. Motion
artifacts occurs because the back projection
algorithm assumes stationary object geometry. If
motion occurs halfway through the CT scan
acquisition, the first half of the projection data will
back project the anatomy in a different orientation.
The image will appeared blurred in the area where
the motion occurred and give rise to image
ghosting ; i.e. the appearance of two superimposed
image.
150

Figure 5. Motion causes blurring
and double images (left), as well as
long range streaks (right).
152

Artifacts in CT
2 .Streak artifacts:
Each detector at every position will observe
transmitted radiation. If a high density material
severely reduces the transmission, some detector
may record no transmission. This violates basic
assumption. As a result, streaks will appear in the
image.
153

Artifacts in CT
3. Beam hardening artifacts:
Although we speak of average energy of a CT X-ray
beam about 70 KeV. As the heterogeneous X-ray
beam passes through the patient, the low energy
photons are rapidly absorbed. This means the X-ray
beam exiting the patient contains a lower
percentage of energy photons than the beam had
when in enters the patient. This effect is called
beam hardening.
155

Artifacts in CT
4. Ring artifacts :
These are the result of miscalibration of one detector
or detector failure. If one detector is miscalibrated,
it will record incorrect data in every projection. This
misinformation is reconstructed as a ring in the
image.
157

Artifacts in CT
5. Partial volume or volume averaging
artifacts:
CT can not reveal detaqil within a voxel. It measures the
average CT number of the contents of each voxel. A
high-contrast object occupying only part of a voxel
will raise the CT number for the corresponding pixel
and so appear larger than it is.
159

Partial volume/ volume average
artifacts 160

Artifacts in CT
6. Geometrical artifacts:
Because of the diverging beam, CT slices are narrower
at the center than at the edge. In consequence there
may be an overlap at the adges or an unscanned
region at the center.
161

Limitations of CT
 Time : 15-30 minutes are require to complete most
examination. So only 15 to 20 examinations could be
performed during an 8 hours office time.
 Real time imaging is not possible in CT.
 CT can not resolve pathological lesion less than 1
mm.
 Radiation dose is higher.
 High cost.
162

Advantages of CT
There are many advantages of CT over conventional
radiography such as—
 Here different body structures are not superimposed.
 Image quality and resolution is very high.
 Size, location and extent of pathological lesion can
be determined with extreme accuracy.
 Density of differences as low as 0.5 %can be
resolved and distinguished.
 The image can be stored in the computer, displayed
on monitor, printed on film, manipulated and altered
in many ways.
163

Conventional X-ray CT
Transmitted ray passes
through the patient and is
detected or imaged by an
X-ray film.
Presents a cross sectional
image, which is detected
by gas-filled or crystal
detector and image
reconstruction occurs in a
digital computer
Low density structures are
obscured by super-
imposed high density
structures.
Almost all structures of all
densities are visualized.
164

Size of the lesions and its
location can not be
assessed even the help of
contrast media & multiple
exposure.
Displays entire cross
section of the slice, so the
size & location of any
pathology can be detected
with extreme accuracy.
Patients dose rate is
relatively lower than CT
Patient’s dose rate is high.
After giving exposure image
can not be manipulated in
the film.
After giving exposure image
can be manipulated by
changing window levels and
than printed on the film.
165

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