ct physics final jhlgkuyfkytfukyguktfkytfkytfiyt

•Introduction-what is CT
•Terminology-ray,projection,fanbeam ,parallel beam
•Scanning principle-linear attenuation coefficient,voxel,solving
equation
•CT number
•Image display-windowing
•Equipment for CT---console,computer gantry (xray
tube,collimation,filtration,detectors ,generator)
•Generations of CT-1 -7

•image recontruction-iterative,back projection,filtered back projection
•Multiplanar reconstruction
•CT flurososcopy
•Helical and multislice tomography—xary tube and generator,interpolation,multidetector
array,pitch
•Dual source CT
•3D reconstruction
•Advantage and disadvantage
•Artifacts
•Radiation dose

INTRODUCTION
Designed by Godfrey N. Hounsfield
to overcome the visual representation
challenges in radiography and
conventional tomography by
collimating the X-ray beam and
transmitting it only through small
cross-sections of the body

G.N.HOUNSFIELD ALLAN M. CORMACK
In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology &
Medicine with Allan MacLeod Cormack, Physics Professor who
developed solutions to mathematical problems involved in CT.

C.T. scan
•Computed tomography (CT) scan
machines uses X-rays, a powerful form
of electromagnetic energy.
•CT combines X radiation and radiation
detectors coupled with a computer to
create cross sectional image of any part
of the body.

Cross-sectional slices
Think like looking into a loaf of bread by cutting it into
thin slices and then viewing the slices individually.

BASIC PRINCIPLE
•The internal structure of an
object can be reconstructed from
multiple projections of the object.
•CT scanning is a systematic
collection and representation of
projection data.

Comparison of CT with Conventional Radiography
•Conventional radiography
suffers from the collapsing of
3D structures onto a 2D
image
•CT gives accurate diagnostic
information about the
distribution of structures
inside the body

TOMOGRAPHY
It is a generic term formed from the Greek words tomos
meaning “slice” or “section” and graphia meaning “picture” or
“describing” that was adopted in 1962 by the international
commission on radiological units and measurements to describe
all forms of body section radiography.
INTRODUCTION

CONVENTIONAL TOMOGRAPHY :
It is a special x ray technique that enables
visualization of a section of the patients anatomy by
blurring regions of the patients anatomy above and below
the section of interest.
INTRODUCTION

Linear tomogram of TMJ
PARASITE LINES
INTRODUCTION

INTRODUCTION
In conventional radiography
all structures of the patient are exposed to X-rays.
Therefore, the image of a particular structure with in a patient is obscured by
overlying and underlying objects.
•To overcome this, the image of the overlying and underlying objects may
be blurred by moving the X-ray tube and film during exposure, about an axis
through the structure of interest.
•.

•The blurring of undesired images by movement of the
X-ray tube and film is refereed to as tomography.
•Tomography refers slice view or sectional imaging
and is usually referred as body section radiography
or linear tomography.
• It is an imaging technique that produces sectional
view of the patient in a plane, parallel to the table top

Parts of linear tomography
•X-ray tube,
•X-ray film
•a rigid connecting rod that rotate about a fixed fulcrum.
•If the tube moves in one orientation, the film moves in the opposite
direction.
•The film is placed in a tray under the X-ray table, so that it is free to
move without disturbing the patient.

•The fulcrum is the only point in the system that
remains stationary.
• The amplitude of the tube travel is measured in
degrees and is called the tomography angle.
•The plane of interest within the patient is positioned
at the level of the fulcrum, and it is the only plane that
remains in sharp focus. All the points above and
below this plane are blurred.

•the point K is above, and point M is below the
focal plane.
•As the X-ray tube moves, only the image of
point L, which is in the focal plane, remains in
sharp focus.
• This is because that L is the only image that
moves exactly the same distance as the film.
•The image of point K moves more than the film,
and the image of point M moves lesser than the
film, hence both images are blurred.
•The thickness of the section that is in focus
depends on the amplitude of the tube travel.
•The longer the amplitude, thinner the section.
•Theamount of blurring depends on the
amplitude of tube travel, distance of the object
from the focal plane and film.

COMPUTED TOMOGRAPHY
•Computed tomography (CT) is a special form
of tomography in which a computer is used to
make a mathematical reconstruction of a
tomographic plane or slice.
•It generates images in transaxial section, i.e,
perpendicular to the axis of rotation of the X-
ray tube.

COMPUTED TOMOGRAPHY (computerized
axial transverse scanning)
It is a radiographic technique that blends the concept of thin
layer radiography (tomography) with computer synthesis.
Alan Cormack in
1960s – image
reconstruction
Godfrey
Hounsfield in
1972 – imaging
technique
120 – 140
kVp
200 – 800
mA
INTRODUCTION

CT HISTORY
First test images 1967
First clinical image 1971
First commercial scanner
1972

Sir Godfrey Newbold Hounsfield
CT brain 1972-73
Noble prize 1979
HISTORY

HISTORY
Siretom scanner

CT
COMPUTERS
Old Mainframe Computers Too Expensive And Bulky
HISTORY

Comparison of CT with Conventional Tomography
Limitations of Conventional
tomography:
1.Image blurring persists
2.Degradation of contrast
due to scatter radiation
3.Problems with
Film/screen combination

Comparison of CT with Conventional Radiography and Tomography
Although spatial resolution is lower in CT, it has
extremely good low contrast resolution, enabling
the detection of very small changes in tissue type
MATTER
LINEAR
ATTENUATION
COEFFICIENT
( µ )
FAT 0.194
WATER 0.222
CSF 0.227
PLASMA 0.230
RED BLOOD CELLS 0.247

–1024 HU to +3071 HU.

FEATURES OF CT IMAGE INCLUDES
(i)images are cross sectional
(ii) eliminates the superimposition of structures
(iii) not influenced by the properties of the
neighbouring region
(iv) subtle differences in X-ray attenuation is 10
times higher than radiographic image, due to
scatter elimination.

•Human body is imagined as a matrix and is divided into number
of columns and rows. In general, 512 or 1024 columns and rows
are used.
• Each matrix element is named as picture element (pixel) in a 2-
dimensional (2D) concept.
•Volume element (voxel) represents a volume of tissue in the
patient and it is a three dimensional (3D) concept.
•Each pixel on the monitor display represents a voxel in the
patient

•The field of view (FOV) is the diameter of the area
being seen by the X-ray at the isocenter.
•The relation between the matrix size, pixel size and
FOV is given below.
• If a CT scans a patient with a FOV of 250 mm and
has a matrix element of 512, then the pixel size is
about 0.5 mm.
•Pixel size=FOV / Matrix size

•Translation is the linear movement of X-ray
tube and detector
• Rotation is the rotary movement of X-ray
tube and detector
•
•Ray refers single transmission
measurement
•projection refers series of rays that passes
through the patient at the same orientation.
•There are two projections, namely, (i)
parallel beam geometry and (ii) fan beam
geometry

SCANNING PRINCIPLE
•The basic principle behind CT is that the internal structure of an
object can be reconstructed from multiple projections of the
object.
•To carry out the reconstruction, the linear attenuation coefficient
of the object is considered as base.
•A X-ray tube emitting a fan beam from a small focus is coupled
to a radiation detector.

•These two are moved together on a carriage, so that a plane of
interest is scanned .The tube potential is about 120–140 kVp
and the X-ray beam is pulsed at the rate of 100 pulses per
second.
•detectors -----solid state crystal or with xenon gas ionization
chambers.
• Each detector is calibrated ---- measures the intensity of the
transmitted X-ray beam.-----measured transmissions are called
projections.

•Each pixel is displayed on the monitor as a level of brightness,
which correspond to a range of CT numbers from -1000 to
+3000.
•The CT number of –1000 represents air, +3000 represents
dense bone and CT number of 0 indicates water.
•The attenuation coefficient of body organs vary with kV and
filtration.
• Conversion of attenuation coefficient into CT number makes the
image, independent of machine parameters.

IMAGE DISPLAY
•The reconstructed image is displayed on a cathode
ray tube monitor, by allotting shades of gray to each
CT number.
•There are 256 shades of gray in the system and each
CT number is allotted one shade of gray.
•The monitor has a matrix size of 512 × 512 and each
pixel represents 12 bits or 4096 gray levels

•Initially, the average CT number of particular tissue is
selected, for example, the abdomen CT number is
20.
•Then, the computer is instructed to assign one shade
of gray to each CT number from -108 to +148, so that
the center CT number is called window level that
determines the brightness level.
• Each pixel brightness is related to average
attenuation coefficient of each tissue voxel.

•The range of CT numbers above and below the
window level (center number) is called window width,
which determines the contrast
•A narrow window width provides higher contrast than
wide window width.
•The window width and window level settings only affect
the displayed image, not the reconstructed image data.

•The typical window level of head, chest (lung) and abdomen (liver) are
40, –500 and 60, respectively.
•The corresponding window widths are 80, 1500 and 150, respectively.
•CT scanning starts with a sinogram (scout image), which is an image of
the raw data acquired by CT before reconstruction. It is obtained by
advancing the patient couch through the gantry with the tube in a fixed
position (0°).
• It is not used for clinical purpose and useful to understand the
tomographic principle. In regular practice, it is used to identify the upper
and lower anatomical borders of the scanning volume.

Windowing- the term used for the method of varying density and contrast.
Window width-range of CT numbers we select for display
Window level-is usually but not always, the central CT number about which the window is chosen

Soft tissue
filter
Bone filter
WL – 50
WW - 200
WL – 50
WW -
400
WL – 600
WW - 1700
WL – 60
WW -
1700

Windowing
•Windowing is the process of using the calculated
Hounsfield units to make an image.
•The various radiodensity amplitudes are mapped to 256
shades of gray. These shades of gray can be distributed
over a wide range of HU values to get an overview of
structures.
•Alternatively, these shades of gray can be distributed over
a narrow range of HU values (called a "narrow window")
centered over the average HU value of a particular
structure to be evaluated. In this way, variations in the
internal makeup of the structure can be discerned. This is a
commonly used image processing technique known as
contrast compression.

• For example, to evaluate the abdomen in order to
find Smalll masses in the liver, one might use liver
windows . Choosing 70 HU as an average HU
value for liver, the shades of gray can be
distributed over a narrow window or range. One
could use 170 HU as the narrow window, with 85
HU above and 85 HU below it, with 70 HU average
value; Therefore the liver window would extend
from -15 HU to +155 HU.

• All the shades of gray for the image would be
distributed in this range of Hounsfield values.
•Any HU value below -15 would be pure black,
and any HU value above 155 HU would be
pure white in this example.
•Using this same logic, bone windows would
use a "wide window" (to evaluate everything
from fat-containing medullary bone that
contains the marrow, to the dense cortical
bone) .

EQUIPMENT FOR
COMPUTED TOMOGRAPHY
• CT scanners --single slice scanner, helical scanner and multislice scanner in the market.
•i) control console,
•ii) computer
•iii) gantry
•iv) couch.
• Recent developments has brought slip ring technology and multidetector array in day-to-day use.
• The Z-axis is the gantry rotation axis, longitudinal, and run along foot to head of the patient
• . The Y axis is perpendicular to the patient in the direction ground to ceiling.
• The X-axis runs sideside of the patient.

The gantry assembly is the largest of these systems. It is made up of all the
equipment related to the patient, including the patient support, the positioning
couch, the mechanical supports, and the scanner housing. It also contains the
heart of the CAT scanner, the x-ray tube, as well as detectors that generate
and detect x rays.

CONTROL CONSOLE
•There are 3 consoles in CT
•one for the technologist to operate the imaging system
• one for the technologist to post process images and
•the other for the physician to view images.
• The operating console is provided with meters, controls for selection of technique
factors,movement of gantry and patient couch, computer commands for image
reconstruction and transfer, selection of kVp, mA and slice thickness.
•Usually, 2 monitors are provided, one to annotate patient data (hospital, patient name, age,
gender), and identification of image (number, technique, couch position), the other for the
operator to view the image.
•The physician control console is used to call up and manipulate the image, optimize the
diagnostic information, contrast and brightness adjustments, magnification techniques,
region of interest (ROI) viewing

COMPUTER
The computer is used to solve more than 2,50,000
equations with the help of microprocessor/array
processor and has primary memory.
The software includes plot of CT numbers, mean and
standard deviation of CT values of ROI, subtraction
techniques, planner and volumetric quantitative analysis
and reconstruction of images in coronal, sagittal and
oblique planes

GANTRY
•CT gantry has the following gadgets:
• (i) X-ray tube,
•(ii) collimation and filtration,
•(iii) detector
•(iv) high voltage generator

Scanning unit (gantry)
GANTRY DIMENSIONS
Height-2-2.5 m
Width-2-3 m
Depth-0.5-1 m
Weight-2000 kg

GANTRY

X-ray Tube
•The X-ray tube uses intense pulse of X-ray and its performance must be stable.
• X-ray intensity must not vary over image acquisition cycle and X-ray spectrum
should be narrow
•Tubes are operated for prolonged exposure time at high mA (e.g. 90 s, 120 kV,
200 mA).
•The Heat capacity of the tube is about 4 MJ and heat exchangers are provided
to cool oil, air, and to maintain gantry at low temperatures
• Usually, scanners are operated at 120 kV, and fixed settings are available
between 80–140 kV.

• The X-ray tube is mounted with its anode-cathode axis, parallel to the axis of
rotation, to reduce heel effect.
•The focal spot determines the amount of information distributed over the
detector array. As focal spot increases the information spread over large number
of detectors, and limit the resolution.
• There are two focal spots (0.6–1.6 mm) and high resolution CT uses small focal
spot size to improve spatial resolution.

•The anode is flat for easy heat dissipation and the angle is smaller than normal.
•The cathode is angled and the focal spot position can be switched magnetically.
•The multislice CT tube is large in size, anode disk is larger in diameter and
thickness.
•The anode heat capacity is > 8 MHU and the anode cooling is about 1 MHU.
• It can be energized up to 60 s continuously, and need high instantaneous
power capacity. It requires high speed rotors for best heat dissipation

Collimation and Filtration
reduces patient dose and improves image contrast, by limiting scatter
radiation.
Single slice scanner uses 2 collimators as pre and post-patient
(predetector) collimation.
Multislice scanner uses only single collimator as prepatient collimation.
.

Gantry
X ray source
Pre Patient Collimation
Post Patient
Collimation
X-ray Detector

Prepatient collimator limits the area of the patient,
determines patient dose (dose profile).
Predetector collimator (i) restricts the X-ray beam seen by
the detector array, (ii) reduces scatter and improves
contrast, and (iii) defines the slice thickness (sensitivity
profile).
The collimator width is about 50 cm at the isocenter, to
cover the full patient and the thickness in the Z-axis is about
1–10 mm

Filters
•The X-ray beam is not monoenergetic and hence filters are
•used to remove low energy photons.
• Aluminium (2.5 mm) + copper (0.4 mm) are used as filters
• modern CT units use 6 mm Al with algorithm help.

Since patient cross section is elliptical, X-ray passes through lesser
tissue at the periphery than at the centre of the body.
Hence, the noise levels may vary, and it is highest at the center
and lowest at the periphery.
As a result, dose is higher at the periphery than at the center. To
solve this issue, bowtie filters of different sizes for head and
body scanning are used

•Absorb low energy xray
•Decrease patient dose
•Provide uniform beam

Detectors
• REQUIREMENT
•small with good resolution
•high detection efficiency,
•fast response
•negligible after glow
•wide dynamic range and
•stable noise free response.
•(i) ionization chamber: Xenon gas filled detectors
•(ii) solid state detector: Scintillation detectors with photo
multipliers or photodiodes

Detectors
Gas Filled Detectors
–Materials Used
•Xenon
•Krypton
•Xenon + Krypton
Since 90% of 50 is 45, the output is same. The overall efficiency
of both the detectors is same.
Gas Filled Detectors Scintillation Detectors
Sensitive face: 100%
Detection Efficiency: 45%
Sensitive face: 50%
Detection Efficiency: 90%

Detectors
•Two types of detectors are used
•Scintillation Detectors
•Gas Filled Detectors
•Scintillation Detectors
–Materials Used
•Sodium Iodide
•Bismuth Germanium Oxide
•Cesium Iodide
•Cadmium Tungstate
Scintillator
Crystal
Photo Multiplier
Detector
Rings

XENON GAS DETECTOR
•high atomic number (54) and its K-shell binding energy is 35
keV.
•High atomic number facilitates photoelectric absorption as main
interaction in the detector.
•kept under high pressure of about 2 MPa

• When X-ray falls on the detector, ionization takes place and
electric charges are produced These charges constitute an
electric signal that is amplified and digitized.
•The digitized electronic signal is proportional to the incident X-
tray intensity.
•The sensitivity is about 50% of the scintillation detector.

Solid State Detector
•Solid state detector consists of a scintillation phosphor coupled
to a PMT or photodiode.
•Size= 1.0 × 15 mm and 1.0 × 1.5 mm in the case of multi
detector array.
•When X-ray falls on the detector light is produced, which is
detected by the photodiode
•The photodiode gives the electric signal that is digitized.
•The digitized electronic signal is proportional to the incident X-
tray

•The phosphors that are used in CT
•Sodium iodide (NaI:Tl)
• bismuth germanate (Bi4Ge3O12),
•cesium iodide + sodium iodide (UV light)
•cadmium tungstate (CdWO4)
•yttrium, and gadolinium ceramics.
•.

•Solid state detectors has better X-ray absorption efficiency (90%) and
large acceptance angle.
•Since they are closely packed, the detection efficiency is high (90%).
•The geometric efficiency is less due to gap between detectors to avoid
cross talk.

Advantage of solid state
detector
• It reduces patient dose
• provide faster imaging rate
• improves image quality by increasing SNR.
•It has negligible afterglow
• the stability of output signal depends
on high voltage supply

•CONTINUATION ---
CT PHYSICS

High Voltage Generator
•The high voltage generator is mounted on the gantry, which takes 0.3 s
for 360°rotation.
•The gantry can tilt up to 30° and weighs about 500 kg.
•The generator is a high frequency generator with capacity of 60 kW.
•It provides stable tube current and the voltage is controlled by the
microprocessor.
•The generator can give a tube current of about 800 mA @125 kV with
pulse duration of 2–4 ms

COUCH
•Made up of carbon fiber.
• It is motor driven, for smooth patient position
•unaffected by patient weight. It moves longitudinally
through gantry aperture, indexed automatically and
the tabletop can be removable.
•In helical CT, the couch motion is defined by the pitch
factor

GENERATIONS
•Data gathering techniques have
developed in stages termed
Generations.
•Scan time reduction is the predominant
reason for introducing new
configurations

FIRST GENERATION
•The axis of rotation passed through the
centre of the patient’s head
•The total number of transmission
measurements = No. of linear measurements
X No. of rotatory steps
•= 160 X 180 = 28,800
•Scan time: 5min
•Rotate-tube /translate-detector
• pencil beam system- parallel ray geometry
•Two side by side X-ray detectors and one
reference detector
•It is translated linearly to acquire 160 rays
across a 24 cm FOV and rotated between
translations to acquire 180 projections at 1°
interval.

•There is a large change in signal due to increased X-ray flux
outside of head and hence patient’s head is pressed into a
flexible membrane surrounded by a water bath.
• The NaI detector signal decayed slowly, affecting
measurements.
•The advantage of the system is the efficient scatter reduction,
best of all scanner generations.
•The disadvantage includes afterglow of NaI

I Generation CT Scanner
•Head kept enclosed in a
water bath
•Two side-by-side detectors
•A reference detector

SECOND GENERATION
•Narrow fan beam(3
0
-10
0
)
•Linear detector array(30)
•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
30
o
•Only 6 times the linear movements got
repeated
•
Scan time~20secs
•Total no of tranmission(600 rays× 540 views =
3,24000).

THIRD GENERATION
•Rotate(tube)-Rotate(detectors)
•Translatory-completely
eliminated
•Pulsed wide fan beam(50
0
-55
0
)
•Arc of detectors(600-900) per
row
•Detectors are perfectly aligned
with the X-Ray tube
•Both Xenon and scintillation
crystal detectors can be used
•Scan time< 5secs

III gen. CT scanners

•The 3rd generation scanners lead to a situation in which each detector
is responsible for the data corresponding to a ring in the image.
•Any drift in the signal levels of the detectors over time affects the t values

that are back projected to produce the CT image, causing ring artifacts

FOURTH GENERATION
•Continuous wide fan beam(50
0
-55
0
)
•Ring of detectors(> 2000) per row
•Rotate(tube)-Fixed(detector)
•X-ray tube rotates in a circle inside the
detector ring
•When the tube is at predescribed
angles, the exposed detectors are
read.
•Scan time< 2 secs

III Vs IV gen. CT scanners

•The fourth generation scanners are designed to overcome the problem
of ring artifacts.
•Since it is rotated continuously, very fast scan time is possible.
• Geometrical misalignment between detector ring radius and X-ray beam origin
may be possible.
•It has inter scan delay times, since the X-ray tube had to return to its starting
position (home).

•Third generation fan beam geometry has
the X-ray tube as the apex of the fan.
•In the 4th generation, the individual
detector is the apex
•Though the X-ray tube forms the fan
beam, data are processed for fan beam
reconstruction with each detector as the
vertex of a fan.
•.

•The rays acquired by each detector are fanned out to different positions
of the X-ray source. In the 3rd generation, It and Io are measured at the
center and at the edge of the detector array.
• Hence, the gain of the reference detector and the individual detector
may not be equal.
•In the 4th generation, each detector has its own reference detector,
and hence the gain of the reference and individual detector is equal

FIFTH GENERATION
•stationary/stationary system,
•specifically for cardiac tomography imaging.
•No conventional X-ray tube is used
• instead large arc of tungsten (210°) encircles patient and lies directly opposite
to the detector ring.
•It uses an electron gun that deflects and focuses a fast moving electron beam
along tungsten target ring in the gantry.

•The images are obtained in 50 ms times and can produce fast frame
rate CT movies of the beating heart with minimum motion artifacts.
•The advantage is the speed of data acquisition.
•The whole heart can be acquired in 0.2 s.
•useful in cardiac imaging, pediatric and trauma patients.

Electron Beam CT
•It is a method of improving the temporal
resolution of CT scanners.
• Because the X-ray source has to rotate by
over 180 degrees in order to capture an
image the technique is inherently unable to
capture dynamic events or movements that
are quicker than the rotation time.

•Instead of rotating a conventional X-ray tube around the
patient, the EBCT machine houses a huge vacuum tube in
which an electron beam is electro-magnetically steered
towards an array of tungsten X-ray anodes arranged
circularly around the patient.
•Each anode is hit in turn by the electron beam and emits X-
rays that are collimated and detected as in conventional CT.
• The lack of moving parts allows very quick scanning, with
single slice making the technique ideal for capturing images
of the heart.
• EBCT has found particular use for assessment of coronary
artery calcium, a means of predicting risk of coronary artery
disease.

GENERATIONS
generationconfigurationdetectorbeam Min scan time
first Translate -rotate1-2 Pencil thin2.5min
second Translate -rotate3-52 Narrow fan10sec
Third
Rotate- rotate256-1000Wide fan 0.5sec
fourth Rotate- fixed600-4800Wide fan 1sec
fifth Electron beam1284 Wide fan
electron beam
33ns

•Sequential CT
•Spiral CT

GENERATIONS

1
st
&2
nd
generation
•In the first and second generation designs, the X-ray
beam was not wide enough to cover the entire width
of the 'slice' of interest.
•A mechanical arrangement was required to move the
X-ray source and detector horizontally across the field
of view.
•After a sweep, the source/detector assembly would be
rotated a few degrees, and another sweep performed.
•This process would be repeated until 360 degrees (or
180 degrees) had been covered. The complex motion
placed a limit on the minimum scan time at
approximately 20 seconds per image.

3
rd
&4
th
generation
•In the 3rd and 4th generation designs, the X-
ray beam is able to cover the entire field of
view of the scanner.
•This avoids the need for any horizontal motion;
an entire 'line' can be captured in an instant.
•This allowed simplification of the motion to
rotation of the X-ray source.

•Third and fourth generation designs differ in
the arrangement of the detectors.
•In 3rd generation, the detector array is as
wide as the beam, and must therefore rotate
as the source rotates.
•In 4th generation, an entire ring of stationary
detectors are used.

SIXTH GENERATION
•Third/fourth generation + slip ring technology + helical motion =
Sixthgeneration (1990).
•Slip ring is a circular contact with sliding brushes and allows the gantry
to rotate continuously.
•It eliminates inertial limitations at the end of each slice and has greater
rotational velocities, with shorter scan time.

SIXTH GENERATION OF CT
•Helical CT scanners acquire data while the
table is moving
•Mostly used nowadays

•The total scan time --much shorter
•It allows the use of less contrast agent
•The speed of the couch motion is very important;
hence the term pitch is defined

Helical or Spiral CT
• In older CT scanners, the X-ray source would
move in a circular fashion to acquire a single
'slice'
•once the slice had been completed, the
scanner table would move to position the
patient for the next slice; meanwhile the X-ray
source/detectors would reverse direction to
avoid tangling their cables.

HELICAL COMPUTED TOMOGRAPHY
•Image can be reconstructed at any desired z-axis position along the
patient.
•The slip ring was introduced in 1980, and it has three rings. Each ring
makes connectivity to the X-ray generator, detector, and control
signals. With slip rings arrangement, the X-ray tube can rotate faster (5
s/rot) and move more than 360 degrees.
•Slip rings are electromechanical devices that conduct electricity and
electrical signals through rings and brushes from a rotating surface
onto a fixed surface The power is transmitted through stationary rings.

X-ray Tube and Generator
•requires higher X-ray power, due to continuous large volume data acquisition.
The X-ray tubes are used for longer exposure > 90 s and require good
performance.
• Smaller focal spot is required for scanning thin sections with high resolution. It
is provided with flying focal spot in which rapid deflection for each projection is
possible, to increase the resolution.
• The anode cooling rate must be high and liquid metal bearings are used to with
stand heat.The tube current can be altered during the course of helical scan
sequence, to suit individual body composition.

•Contd…..

• It provides reduction of mean mAs per rotation of the order of 15–55%.
•The beam filtration normally used is 3 mm,
•Al + bow tie filter is usually made up of lowZ material (teflon or copper).
•There is a pre- and post-patient collimator of 100 micrometer thick,
made up of tantalum.
•The gantry mechanical design must be precise to make uniform motion.

Detector
•solid state scintillation or xenon gas ion chambers
•Currently, ceramic scintillation phosphors are in use, which gives an
•absorption efficiency of about 99%.
•Few commonly used ceramic scintillation phosphors are:
•(i) lutetium orthosilicate (Lu2O:Ce),
•(ii) gadolinium orthosilicate (Gd2O:Ce),
•(iii) yttrium aluminum perovskite (YALO3:Ce).

•The table movement in a helical scan is a variable one and it can
•be moved either fast or slow. Hence, to identify the nature of table
•motion, the term pitch is used and it is defined as follows:
Tabletop movement per rotation
•Pitch = ——————————————————
Slice thickness
•If a table moves 10 mm in one rotation of a gantry in 1 s, to make
a 10 mm slice thickness of a patient, then
10 mm/s
•Pitch = —————— = 1
10 mm

Interpolation
•CT reconstruction algorithm assumes that the X-ray source path
is circular, not helical around the patient.
• But, in a helical scan, the X-ray beam path is helical in nature
and gives only helical data set. Hence, before the actual CT
reconstruction, the helical data set has to be interpolated into a
series of planar image sets.
•Then, with interpolated data set, CT images can be
reconstructed at any position along the length of the scan

•Interpolation is a weighted average of the data from either side of
the reconstruction plane.
•The advantages of a helical scan is the scan speed and patient
throughput: e.g. a chest scan with 10 mm slice can be done in single
breath hold in 15–20 s (couch motion 10 mm/s, pitch 1.5) and avoids
slice misregistration.
• Use of higher pitch reduces patient dose andexposure time.

Multislice/multidetector/multirow
CT
multidetector array (MDA)
•Though it offers flexibility of CT acquisition protocol, the number of parameters
have increased. It has better efficiency for patient imaging and detector pitch needs
to be defined

Multidetector /Multislice /Multirow CT
•Most recent advancement,
introduced in 1998.
•This uses usually 64-128 adjacent
multiple detector arrays in
conjunction with a helical CT
scanner , the collimator spacing is
wider and more of the x-rays that
are produced by the tube are
used in producing image data.
•Scanning time: 0.25 second

IMAGE RECONSTRUCTION
The photon recorded by the detectors
represent a composite of absorption
characteristics of all elements of the patient in
the path of x-ray beam. Computer algorithms
use these photon count to construct one or
more digital cross-sectional image.
The X-ray beam attenuation is collected in a
grid like pattern called matrix.
These cross sectional image of the body is
divided into tiny blocks called as VOXEL
(VOlume Element){3D}.
Each square of image matrix is called as
Pixel{2D}.
Image are typically 512 x 512 or 1024 x 1024
pixels.

Multislice CT
third generation CT + helical scanning +low voltage slip rings+multidetector array
•faster rotation subsecond times (0.5–0.8 s), that reduces the examination time.
•image quality is similar to that of single slice scanners. But, it is different in dose,
pitch, image artifacts, and method of image reconstruction.
.
•Compared to 1 s single slice scanner, MSCT perform 0.5 s rotation with
simultaneous acquisition of 4 slices. Thus, it gives 8 times higher performance than
single section CT, for same scanning time. Recent advances has brought scanners
with 4, 8,16, 64 ,128 and 256 slices in practice.

Multidetector Array
•. In a single slice scanner, the detectors are wide (15 mm) and collimator
determines (adjustment) slice thickness of 1–13 mm.
•In a MSCT, the individual detector elements along the z-axis are summed, to get
several slice thickness. Thus, the slice width is determined by the detector not
by the collimator.
•2 commercial designs, namely,
•adaptive array detector)
• linear or matrix detector
•In the adaptive array design, the detector width is unequal, e.g. it may have
detector width as 1.0, 1.5, 2.5 and 5 mm from the center to edge.
• In the linear array, the width is equal, e.g. it may have 1.25 mm throughout the
dimension with 16 detector module.

•Usually, MSCT with multidetector array employs third generation CT
with 16 detectors.
•Whereas 4th generation CT require much more detector elements to
cover the entire detector ring of 360°.
•MSCT can be used for conventional axial scanning and helical
scanning.

Pitch
•In MSCT, the pitch influences radiation dose, image quality and scan
•time. The pitches are (i) collimator pitch and (ii) detector pitch. The collimator
pitch is given by the relation:
Table movement per 360° gantry rotation
•Collimator pitch = ——————————————————————
Collimator width at isocenter
•Pitch = 1, refers normal scanning, pitch = 0.75 refers over scanning,
and the table motion is slow. This gives increased image quality with increased
radiation dose.
If the pitch = 1.5, it is said to be under scanning and the table motion is faster with
lesser patient motion.It consumes smaller volume of contrast agent and good for
pediatric patients. Higher pitch is always advocated for pediatric CT protocol.

The detector pitch is defined as follows:
Table movement per 360° rotation of gantry
•Detector pitch = ———————————————————————
Detector width
•Collimator pitch = Detector pitch –: N
•where, N is the number of detector arrays in the multislice CT.
•The MSCT scanners with 4 detector arrays uses 3–6 as detector pitch values,
•e.g. a pitch of 6 corresponds to 1.5 for conventional scanner, since N = 4.

advantages of MSCT
•increased speed
•shorter acquisition times with improved temporal resolution
(lesser motion artifacts
•high axial resolution with thinner slices in the longitudinal (z)
axis
•retrospective creation of thinner or thicker sections from raw
data
•accurate anatomical 3D reconstruction especially in
angiography and virtual endoscopy with lesser helical artifacts

advantages of MSCT
•increased volume coverage per unit time
•reduced partial volume artifacts and noise
•detailed multi-planar reconstruction images,
• use of lesser contrast materials and delivery of contrast
material at faster rate, thus increasing contrast enhancement in
images

•The major benefit of multi-slice CT is the
increased speed of volume coverage. This
allows large volumes to be scanned at the
optimal time .
•The ability of multi-slice scanners to achieve
isotropic resolution even on routine studies
means that maximum image quality is not
restricted to images in the axial plane - and
studies can be freely viewed in any desired
plane.

Three dimensional (3D) Image
Reconstruction
Because contemporary CT scanners offer
isotropic, or near isotropic resolution, display
of images does not need to be restricted to
the conventional axial images.
Instead, it is possible for a software program
to build a volume by 'stacking' the individual
slices one on top of the other. The program
may then display the volume in an alternative
manner.

•IMAGE RECONSTRUCTION

IMAGE RECONSTRUCTION
•When source-detector makes one sweep across the patient, the
internal structures of the body attenuate the X-ray beam according their
mass density and effective Z.
• The intensity of radiation detected varies according to this attenuation
pattern and an intensity profile or projection is obtained.
•These projections are not displaced visually, stored in digital form, in
the computer.

•The computer processes the projections that involve super position of
each projection to reconstruct an image of the anatomic structures
within that slice.
•The individual value of the matrix elements (-mu) are obtained by
solving the simultaneous equations.
•The matrix of values that are obtained represents cross sectional
anatomy.
•Dedicated array processor is used to do calculation and instantaneous
image display

image reconstruction
algorithms
•iterative technique,
•back projection
• filtered back projection (FBP).
•They are basically a mathematical algorithm that takes the projection
data and reconstruct cross-sectional CT image.

•The reconstruction involves millions of data points that
may be performed in seconds.
•Thus, the reconstruction creates an image, which is a
map of X-ray attenuation/CT number of the tissue in
the plane of interest.
•Most modern scanners use filtered back projection
image reconstruction.

ITERATIVE METHOD
•Iterative method is the original method used by G.
Hounsfield
• It uses an exact mathematical solution for
reconstruction
•To obtain a solution, it starts with an assumption that
all the pixels have the same value.
• These assumed values are compared with
measured or collected data.

•Then, corrections are made in assumed values so
that the assumed and collected data come closer.
• This is repeated until all the pixel values are
equal to the collected data with reasonable
accuracy.
• It is slow and takes longer computer time and
gives imprecise CT values due to rounding errors
(e.g. 0.95 1.0).


BACK PROJECTION
•In this method, required projections of an object are obtained by
multiple scans.
•Then, the projections are back projected to produce the image of that
object.
•All points in the back projected image receive density contribution from
neighboring structures and creates noise. Hence, the image quality is
very poor and large number of projection are required to improve the
image quality.
•The disadvantage of the method is the blurred image of the object

Mathematical methods of image reconstruction
1.Back projection
Also called summation method. It Is the oldest means of image reconstruction.
Its principle demonstrates when a ray from two projection is superimosed, or back projected
They produce a crude reproduction of original object.
Filtered back-projection is one of this method. modification of these methods, called the Feldkamp
reconstruction, is used for MDCT and cone-beam reconstructions to account for the diverging x-ray
beam.
2.Iterative methods
It start with assumption that all point in matrix have same value and it was compared with
measured value and make correction until Values come with in acceptable range.
It is used instead of filtered back-projection to reduce noise from images. This technique allows the
use of low-dose protocols yet still produces images with comparable or better image qualit

Reconstruction algorithms
1.Backprojection: The process of
converting the data from the attenuation
profile to a matrix is known as
backprojection.

Reconstruction algorithms
2.Filtered backprojection:
•Simple backprojection technique results in
characterstic star-like artifact.
•To eliminate this artifact, a mathematical
filter is used in scanned data before
backprojection, and hence k/a filtered
backprojection.
•The process of applying a filter function to
a scanned data is called convolution.

Multiplanar Reformatted Imaging
•Data from a single CT imaging procedure, consisting of either
multiple contiguous or one helical scan, can be viewed
as
images in the axial, coronal, or sagittal planes or in any
arbitrary plane depending on the diagnostic task; this is referred
to as

multiplanar reformatted imaging
•Have the capability of viewing normal anatomy or pathologic
processes simultaneously in three orthogonal planes often
facilitates radiographic interpretation

•These are two-dimensional and require a certain degree of
mental integration by the viewer for interpretation.
•This limitation is overcomed by computer programs that
reformat data acquired from axial CT scans into three-
dimensional images.
•Three-dimensional reformatting requires that each original
voxel, shaped as a rectangular solid, be dimensionally altered
into multiple cuboidal voxels. This process, called

interpolation,
creates sets of evenly spaced cuboidal voxels (cuberilles) that
occupy the same volume as the original voxel.

Fig: Three-dimensional rendering. Three-dimensional images can be reconstructed from the cuberilles,
thresholded for bone

(left)
or soft tissue
(right), oriented in any arbitrary direction, and made to appear
to have depth by highlighting structures near the front and shadowing structures near the back. This
patient has hemifacial microsomia and demonstrates incomplete development of the left frontal,
sphenoid, temporal maxillary, zygomatic, and mandibular bones. Note also the reduced size of the left
orbit, depression of the tip of the nose, missing and incompletely erupted left maxillary teeth, deviation
of the right mandible to the left, sunken left midface, and malformation of the left ear.

MULTIPLANAR
RECONSTRUCTION
•used in multislice spiral CT.
•The MPR algorithms include
•(i) maximum intensity projection (MIP)
•, (ii) shaded surface display (SSD),
• (iii) shaded volume display (SVD).
•The MIP reconstructs an image by selecting the highest value of the pixels along
any arbitrary line through the data set and exhibits only those pixels. It is the
simplest form of 3D imaging and widely used in CT angiogram.
• It differentiates vasculature from surrounding tissue, but lacks vessel depth..
•Small vessels passing obliquely through the voxel may not be imaged, due to
partial volume averaging.

Multiplanar reconstruction
•Multiplanar reconstruction (MPR) is the
simplest method of reconstruction.
• A volume is built by stacking the axial slices.
The software then cuts slices through the
volume in a different plane (usually
orthogonal).
• Optionally, a special projection method, such
as maximum-intensity projection (MIP) or
minimum-intensity projection (mIP), can be
used to build the reconstructed slices.

Fig.showing 1 3D and 3 MPR views

•MPR is frequently used for examining the spine. Axial
images through the spine will only show one vertebral
body at a time and cannot reliably show the intervertebral
discs.
•By reformatting the volume, it becomes much easier to
visualise the position of one vertebral body in relation to
the others.
•MIP reconstructions enhance areas of high radiodensity,
and so are useful for angiographic studies.
• mIP reconstructions tend to enhance air spaces so are
useful for assessing lung structure.

3D rendering techniques
Surface rendering
•A threshold value of radiodensity is chosen by the
operator (e.g. a level that corresponds to bone). A
threshold level is set, using edge detection image
processing algorithms.
•From this, a 3-dimensional model can be constructed
and displayed on screen.
•Multiple models can be constructed from various
different thresholds, allowing different colors to
represent each anatomical component such as bone,
muscle, and cartilage.
–However, the interior structure of each element is not visible
in this mode of operation

Volume rendering
•Surface rendering is limited in that it will only display
surfaces which meet a threshold density, and will
only display the surface that is closest to the
imaginary viewer.
• In volume rendering, transparency and colors are
used to allow a better representation of the volume to
be shown in a single image - e.g. the bones of the
pelvis could be displayed as semi-transparent, so
that even at an oblique angle, one part of the image
does not conceal another.

3D rendering software
•Some examples of CT 3D surface
rendering software include
Mimics, 3D doctor, Amira....etc
•Some examples of CT 3D volume
rendering software include 3D
doctor, ScanDoc-3D....etc

Image segmentation
Segmentation (image processing)
•Where different structures have similar
radiodensity, it can become impossible to
separate them simply by adjusting volume
rendering parameters. The solution is called
segmentation, a manual or automatic
procedure that can remove the unwanted
structures from the image.

A volume rendering of this volume
clearly shows the high density bones
•Bone reconstructed in 3D

Using a segmentation tool to remove
the bone to show brain vessels
•Brain vessels reconstructed in 3D after bone has been removed by
segmentation

•The SSD is a computer-aided technique, --used for bone imaging, and
virtual colonoscopy.
•The SVD makes the surface boundaries very distinct and provides an
image that appears exact 3D.

CT FLUOROSCOPY
•CT fluoroscopy provides an image sequence over the same region of tissue
•. It is a pseudo-real-time tomography images, and there is no table movement.
Images are reconstructed nearly real time during continuous rotation of the X-ray
tube.
• CT images are constantly updated to include the latest projection data at the
rate of 6 frames per second. Hence, 6 images are obtained in 1 s for a 360°
rotation. The time taken for 1 mage = 1/6 s = 167 ms and the angle is 60°.
•After 1 s, the CT scans 60° arc (167 ms), creates a subframe and the old is
discarded. Thus, new subframe is added to the old 5 subframes accounting 17%
new information and 83% old information

• Most recent 6 subframes are summed to produce the CT
display.
•CT fluoroscopy images has excellent temporal resolution,
motion at the image level can be followed in real time. The X-ray
tube is operated with a current of 20–50 mA, whereas regular
CT uses 150–400 mA.
•This procedure is commonly used for taking needle biopsies or
to drain fluids.

Dual Source CT
•Siemens introduced a CT model with dual X-ray tube
and dual array of 64 slice detectors, at the 2005
Radiological Society of North America (RSNA)
medical meeting.
• Dual sources increase the temporal resolution by
reducing the rotation angle required to acquire a
complete image, thus permitting cardiac studies
without the use of heart rate lowering medication, as
well as permitting imaging of the heart in systole.
•The use of two x-ray units makes possible the use of
dual energy imaging.

•Thank you

CT Data Acquisition Components

DATA ACQUISITION
The scanning process begins with
data acquisition.
Data Acquisition refers to a method
by which the patient is systematically
scanned by the X ray tube and
detectors to collect enough
information for image reconstruction.
A basic data acquisition scheme
consists of
• X ray tube
• Filters
• Collimators
• Detectors

CT Gantry

CT gantry internal components
1.X-ray tube & collimator
2.Detector assembly
3.Tube controller
4.High freq. generator
5.Onboard computer
6.Stationary computer

CT gantry internal components

CT Patient Couch

CT

X-RAY TUBE
•Rotating anode type
•More heat loading
and heat dissipation
capabilities
•Small focal spot size
(0.6mm) to improve
spatial resolution

FILTERS
Compensation filter is being used
•To absorb low energy x rays
•To reduce patient dose
•To provide a more uniform beam

COLLIMATORS
•To decrease scatter
radiation
•To reduce patient dose
•To improve image quality
•Collimator width determines
the slice thickness

DETECTORS
•The detectors gather information by measuring the x-
ray transmission through the patient.
•Two types:
Scintillation crystal detector
(Cadmium tungstate+ Si Photodiode)
Can be used in third and fourth generation scanners
Xenon gas ionisation chamber
Can be used in third generation scanners only

Scintillation crystal detector used in I & II gen. CT scanners

Scintillation crystal detector used in III and IV gen. CT scanners

Detector Cross-talk
•Detector cross talk occurs when
a photon strikes a detector, is
partially absorbed and then
enters the adjacent detector and
is detected again.

•Crosstalk produces two weak
and signals coming from two
different detectors.

•Crosstalk is bad because it
decreases resolution.

•Crosstalk is minimized by using
a crystal that is highly efficient in
absorbing X-rays (high stopping
power).

Xenon gas ionization chamber

Gas filled detector’s efficiency
Gas filled detectors are less efficient than solid state detectors.
The problem can be partially overcome by the following 3 ways.
• By using Xenon (z=54), the heaviest of the inert gases
• By compressing the Xenon 8 to 11 atmospheres to increase its density
• By using a long chamber to increase the number of atoms along the
path of the beam.

Why are Xenon gas detectors not used
in IV generation CT scanners?
•Typical size of a chamber is
1-2 mm wide, 10mm high
and 8-10cm deep
•These 10mm long side
plates are the reason why
Xenon detectors are not in
IV generation CT scanners.

Disadvantage of Xenon gas detector
Efficiency - 50 to 60%
This low efficiency is caused by two factors.
•Low density of the absorbing material
•Absorption of X-rays by the front window, which
is needed to contain the high pressure gas

OTHER SCAN
CONFIGURATIONS
Interest in faster scan times evolves from a desire
to image moving structures such as the wall of the
heart and contrast material in blood vessel and heart
chambers and to overcome motion artifacts due to
cardiac rhythm and patient breathing .
•Dynamic Spatial Reconstructor(DSR)
•Electron beam computed tomography

DYNAMIC SPATIAL RECONSTRUCTOR
•28 X-ray tubes
•X-ray tubes are aligned with 28
light amplifiers and TV cameras
that are placed behind a single
curved fluorescent screen
•The gantry rotates about the
patient at a rate of 50 RPM
•Data for an image acquired in
about 16 ms.
•Reconstruct 250 C.S. images
from each scan data

DSR

Disadvantages of DSR
•High Cost
•Mechanical motion is not eliminated

Electron Beam Computed
Tomography
•Electron gun
•Large Arcs of tungsten targets
•Detector ring
•17 slices per second

•What is the radiation dose with EBT?
•An advantage of the EBT scanner over
conventional scanners is instead of exposing the
entire circumference of the body to the X-ray
beam, the EBT X-ray beam enters from the back.
Thus, anterior structures such as the breast and
thyroid are subjected to a lesser dose of radiation
(17% of the entrance skin dose). EBT scanning is
usually 1/5th to 1/10th the radiation exposure as
Spiral CT scanning.

DSR
• The Dynamic Spatial Reconstructor
•
The seminal scanner for dynamic volumetric imaging is the x-ray CT scanner known as the Dynamic Spatial Reconstructor (DSR)
designed and installed at the Mayo Clinic.[1, 2] The DSR has the ability to obtain up to 240 contiguous 0.9mm thick sections in a
short a time period as 1/60 second and to repeat this acquisition rate 60 times per second. In practice, the data rate is somewhat
reduced from these numbers. The DSR consists of a gantry weighing approximately 17 US tons with a length of 20.5 feet and a
diameter of 15 feet. 14 x-ray guns reside in a hemicylindrical configuration and aim at a juxtaposed hemicylindrical fluorescent
screen. The images produced on the fluorescent screen by the firing of the x-ray guns are recorded by a bank of 14 television
cameras which, until recently, were image isocons which sent analogue signals to be recorded on a bank of 8 video disc
recorders. Since american television is comprised of 240 usable lines, and video rates are 60 per second, it is possible to
reconstruct a cross sectional image of the body by digitizing each of the 240 television line for each of the 14 cameras and
produce 240 cross sections representing 1/60 second resolution. Each stop action stack of slices in actuality represents
approximately 0.1second which is the time in which all 14 x-ray guns are sequentially pulsed on. Because there were only 8 video
disc recorders, every two television line were averaged to reduce the lines per camera to 120, and the data from two cameras
were recorded interlaced on a single video channel. In addition, as the images are produced the gantry rotates at 15 rotations per
minute. Thus, in 1/60 second, the gantry moves a degree and a half such that if the organ system of interest moves slow enough
to allow for 2/60 second of scanning, it is possible to generate 28 angles of view to use in the reconstruction process. Up to a
point, the more angles of view used in the reconstruction process, the better the images. Typically, at least 4/60 seconds of
scanning are used to generate a good quality reconstruction. This can not only be accomplished by utilizing contiguous 1/60
second data sets, but it is possible to retrospectively gate the data together by selecting the same time point from within several
physiologic cycles such as the cardiac or respiratory cycle. Physiologic signals are recorded in with the video signals to allow for
this retrospective gating process. The designers of the DSR, to achieve the ability to obtain dynamic, volumetric image data sets
made compromises in the image resolution such that grey scale resolution was sacrificed. To improve the image quality, the
video imaging chains have been converted from image isocon cameras to charged coupled device (CCD) cameras. [76] Much
larger lenses were ground by Old-Delf and tapered fiberoptics were pulled to take the images from the lens to a microchannel
plate intensifier which then transmits the image to the CCD chips. The process of pulling the tapered fiberoptics introduces some
twisting of the fiberoptics and therefor custom warping algorithms had to be developed for each camera. The images are digitized
on each, camera, the images are unwarped, and the data is sent now to digital tape running at video rates. Although the DSR has
remained a one of a kind system and represents a true tour de force, much of the current image manipulation and display
associated with the massive data sets generated have served as the vanguard for data handling of images coming off of the
currently commercially available scanners.

Dynamic Spatial
Reconstructor
•The Dynamic Spatial Reconstructor (DSR) is a high-
temporal resolution, three-dimensional (3-D) X-ray
scanning device based on computed tomography (CT)
principles. It was designed for investigation of some
problems inherent in current diagnostic imaging
techniques, and to allow quantitative studies of
cardiovascular structure and function. One of the
research protocols in which DSR is currently used
involves studying selected pediatric patients with
complex congenital heart disease. Initial results show
that 3-D dynamic images can be obtained from these
patients with minimal invasiveness and that these
images may provide useful diagnostic information.

Dynamic Spatial Reconstructor
•Dynamic Spatial Reconstructor
•The first three-dimensional volume scanning (simultaneous acquisition
of multiple contiguous slices) CT scanner with high temporal resolution
(scan repetition rate of up to 60 times/sec), called the dynamic spatial
reconstructor (DSR), was presented in 1980 [8].
•This machine allowed to examine the renovascular anatomy, detecting
arterioles as small as 1 mm in diameter [9] and providing detailed
dilution curves that showed the transit of contrast in the four zones of
the renal cortex (superficial, middle, inner and juxtamedullary) and in
two different areas (outer and inner) of the renal medulla. These dilution
curves allowed a precise calculation of intrarenal RBF. Despite the
great potential of the DSR, it was not extensively used because of its
limited availability, and its high operating and maintenance costs.
However, further studies on intrarenal hemodynamics were made
possible when Imatron, a California company, marketed the first
commercially available EBCT, which is described in the next section.

Dynamic scanning

•the acquisition of the same physical image or set of images in rapid succession
such that time dependent changes (e.g. contrast enhancement, motion) can be
studied. The term was originally used and most often refers to computed
tomography CT scanning. Very rapid dynamic CT acquisitions have been
accomplished by cine CT, which has scan times as short as 50 msec. Recently,
the development of slip ring technology has resulted in sub-second scan times on
conventional X-ray tube based CT scanners. With these faster scan times, many
dynamic processes can be monitored. Ultimately, the number of images that can
be obtained during a dynamic image study is limited by the heat loading of the X-
ray tube.
•There are a variety of applications for dynamic CT scanning. These include dual-
phase abdominal studies after contrast administration, cardiac imaging studies,
studies of perfusion using iodinated contrast media, and measurement of
cerebral blood flow using administered xenon as a diffusible agent (see xenon CT
scanning and perfusion measurements). Sometimes, the term dynamic scanning
is used for rapid scanning of a volume even if only a single time point is sampled.
This is in some respects a misuse of the term since the images do not portray
dynamic events. It derives from the increased scanning rates made available by
the advent of dynamic scanning (see incremented dynamic scanning).

Dynamic Spatial
Reconstructor
•The Dynamic Spatial Reconstructor (DSR) is an experimental
apparatus that deserves
•mention, being the only method besides RT3D ultrasound capable
of real time 3D cardiac
•imaging. It was constructed for research (and recently
decommissioned) at the Mayo Clinic
•and was too expensive for general clinical use. The DSR
incorporated aspects of fluoroscopy
•and CT, using multiple X-ray sources and fluoroscopic screens to
gathered 3D data in real-time
•(Robb 1983). During its many years of operation, the DSR
gathered unique and valuable data
•for research in cardiac dynamics and for validation of other
methods of measurement.

•Cardiovascular Computed Tomography (CVCT)
•Also called as ultrafast CT / Electron beam CT (EBCT)
•Motion of the parts of the machine is completely eliminated
•Electron gun (320cm long, 130keV)
•Focusing and deflecting coils
•Four 180cm diameter tungsten target arcs
•2 Rings of detectors
•Electron beam scans the large target, X-rays are produced and
collimated into a 2cm wide fan beam by a set of circular collimators
•The X-ray beam passes through the patient and is detected by an array
of luminescent crystals
•Both the tungsten targets and the detector array cover an arc of 2100
•One scan can be obtained in 50Mts
•Without moving the patient, 8 contiguous tomographic images can be
obtained.

Electron Beam Computerized
Tomography (EBCT)
•This instrument represents a novel concept in the use of x-ray to obtain fast tomographic
scanning. In contrast to the DSR and conventional CT, EBCT has no mechanical parts (x-
ray tubes and/or TV cameras) moving around the patients, resulting in lower heat
production and enabling fast scanning. An electron beam, originating from an electron gun
located behind the patient is magnetically deflected sequentially onto four tungsten target
rings, producing eight fan beams (two from each target ring) of x-ray radiation that pass
through the patient. Eight almost simultaneous renal tomographic sections can thereby be
obtained, that are thicker (8 mm) than those produced by the DSR. Alternatively,
consecutive 1.5, 3, or 6 mm thick tomographic slices can be obtained by using a single
target ring and moving the patient table at pre-determined increments. Although its
temporal resolution is lower than that offered by the DSR (50 or 100 msec/image), it is
nonetheless sufficient to obtain adequate evaluation of renal function. Furthermore,
because of the slightly longer scan duration and lower image noise compared to the DSR,
its spatial resolution is superior [10].
•Jaschke, et al. [11, 12] were the first to demonstrate the potential of the EBCT in measuring
RBF, and establish the basic principles for that calculation which correlated highly with
measurements obtained with radioactive microspheres. Subsequent validation studies
demonstrated the accuracy of EBCT-derived measurements of renal, cortical and medullary
(compared to their in vitro) volumes [13] and perfusion (compared to electromagnetic
flowmetry) within a wide range of RBF values [14], as well as changes in blood flow
distribution

•The Imatron Electron Beam Tomography (EBT) Scanner
•Imatron's Electron Beam Tomography (EBT) scanner combines
advanced science and technology to create an innovative diagnostic
imaging system. Dramatically different from conventional (mechanical)
CT scanners, Imatron's patented electron beam technology and unique
design offer diagnosticians scan times as fast as 50 and 100
milliseconds. In addition to routine cross sectional imaging of all body
organs, EBT is able to evaluate physiology and blood flow and to
perform any other examination where speed is essential.
Utilizing proprietary EBT technology, a powerful electron beam is
generated and then focused onto one of four tungsten target rings
positioned beneath the patient. Each 210 degree sweep of the electron
beam produces a continuous 30 degree fan beam of x-rays that pass
through the patient to a stationary array of detectors which generates
cross-sectional images.

•Engineering
•Imatron's engineering department is an accomplished group of scientists
and engineers working to create the world's most advanced computed
tomography (CT) scanner. Together, they harness and shape a powerful
(650 mA at 130 kV) electron beam, then guide it through a high-vacuum
chamber and around multiple tungsten-coated target rings.
These patented image acquisition techniques are at the heart of the
Electron Beam Tomography (EBT) scanner, creating a scanner so fast
that it can capture stop-action images of the human heart.
The vast amounts of data captured in a single breath-hold are
transferred at speeds of 200 Mbs to 1 Gbs over a DICOM network to our
near real-time reconstruction systems. Sophisticated programming
techniques enable us to create 2D and 3D images, including virtual
angiograms (fly-throughs of the arteries of the heart) and virtual
colonoscopies (fly-throughs of the colon).

Radiation dose from EBT scans
compared to other sources of
radiation
•EBT Coronary Calcium
Scan
Non-invasive Coronary
Angiogram
EBT Low-dose Lung Scan
EBT Abdominal/Pelvis
Scan
Background Sunshine
Radiation in 1 year
Cross country airplane trip
Standard Chest X-ray
Standard Abdominal X-ray
Standard Spine X-ray
series
Standard Coronary
Angiogram
Standard Lower G.I. X-ray
series
Spiral CT Whole body scan
50 to 70 mrem
80 to 120 mrem
100 to 150 mrem
100 to 300 mrem
300 mrem
2 mrem
8 to 10 mrem
48 mrem
300 mrem
500 to 1000 mrem
600 mrem
600 to 1000 mrem
0.5 to 0.70 mSv
0.8 to 1.2 mSv
0.75 to 1.5 mSv
0.75 to 3.0 mSv
3.0 mSv
0.02 mSv
0.08 to 0.1 mSv
0.48 mSv
3.0 mSv
6.0 to 10.0 mSv
6.0 mSv
6.0 to 10.0 mSv
Mrem and mSv exposure varies with body size.
Recommended safety limits for radiation
exposure is less than 5,000 mrem per year.

Electron Beam CT
•The latest foray of technology in CT is the electron beam
CT (EBCT) scanner. In EBCT an electron beam is electro-
magnetically steered towards an array of tungsten X-ray
anodes that are positioned circularly around the patient.
The anode that was hit emits X-rays that are collimated
and detected as in conventional CT. The use of an electron
beam allows for very quick scanning because there are no
moving parts. An entire scan can be completed in 50 to
100 milliseconds. This quick scan time makes this the only
CT method which can scan the beating heart. At the
present time, these machines are installed in only a few
sites world-wide.

Ultrafast / Electron Beam CT
Scan
•What is an ultrafast/electron beam CT (computed tomography) scan?
•In conventional x-rays, a beam of energy is aimed at the body part being studied.
A plate behind the body part captures the variations of the energy beam after it
passes through skin, bone, muscle, and other tissue. While much information can
be obtained from a regular x-ray, specific detail about internal organs and other
structures is not available.
With computed tomography (also called CT or CAT
scan), the x-ray beam moves in a circle around the body. This allows many
different views of the same organ or structure, and provides much greater detail.
The x-ray information is sent to a computer which interprets the x-ray data and
displays it in two-dimensional form on a monitor.
A new technology called ultrafast
CT (also known as electron-beam tomography, or EBT) is now used, in some
cases, to diagnose heart disease. Ultrafast CT can take multiple images of the
heart within the time of a single heartbeat, thus, providing more detail about the
heart's function and structures, and also greatly decreasing the amount of time
required for a study.
•A three-dimensional (3D) version of ultrafast CT may be used to assess the
pulmonary arteries and veins in the lungs.
•Ultrafast CT scan may also be used to evaluate selected heart defects after birth.

Electron beam computed
tomography (CT)
•Exam Overview
•Electron beam computed tomography (CT) scanning is a
new test that can be used to detect calcium buildup in
the lining of arteries.
•Electron beam CT scanning is much faster than standard
CT scanning. Electron beam CT scanning can produce
an image in a fraction of a second and can take an
accurate picture of an artery even while the heart is
beating. Standard CT scanning is not fast enough to take
pictures of a pumping heart. In standard CT, several
pictures, or "slices," of the heart are taken from different
angles. These pictures are then analyzed using a
computer to create a three-dimensional view of the heart.

EBCT
•Why Is It Done?
•This test is used to identify calcium
buildup in heart arteries, which can be a
risk factor for coronary artery disease
(CAD). It may be used as a screening
tool to detect hardening of the arteries
in people who are at high risk of
developing atherosclerosis.

EBCT
•Results
•Fat and calcium buildup may be seen in the arteries during an
electron beam CT scan. Aggressive treatment for
atherosclerosis may be appropriate.
•If electron beam CT scanning does not show the presence of
calcium buildup in the arteries, then the chances of having CAD
are low. 1 Most people who have a negative angiography test
result also have a negative electron beam CT scan test result. 1
•A high calcium score on an electron beam CT scan indicates a
greater risk of having cardiovascular problems within the next 2
to 5 years, especially when a person also has multiple risk
factors for developing coronary artery disease. 1 However, the
scan does have a fairly high rate of false-positive results.

Electron Beam CT
•Advanced Body Scan of Newport is a physician owned
facility that opened in June 2001 with a mission to provide
the community with cutting edge diagnostic and screening
medical imaging procedures. Electron Beam Tomography
(EBT) is an innovative outgrowth of Computed
Tomography (CT) technology specifically developed to
acquire images fast enough to freeze the beating heart.
This 'Ultrafast' cat scanner is open and non-
claustrophobic, and the X-ray radiation used to acquire
the body images is only about 20% of what you would
receive from a conventional CT scanner.
The full body
EBT scan delivers not much more radiation than your
total annual radiation exposure from natural causes.

Electron Beam CT
•When you visit us at Advanced Body Scan of Newport, you can
be assured that you will be made to feel welcome and nurtured
from your initial greeting to your final medical consultation. You
stay fully clothed for the exam which takes only a few minutes to
complete. No shots or needles are involved.
You simply lie
down on your back, hold your breath for a few seconds while
the images are taken, and within minutes, you can see the
results yourself on the screen.
•Advanced Body Scan of Newport offers discount scan packages
to corporate customers. Our sales staff will be happy to answer
any questions and customize any multiple scan package to
meet your needs.
•Only GE-Imatron™ Electron Beam CT Scanners have FDA
approval for Coronary Artery Calcium Scoring and CT Coronary
Angiography.

Electron Beam CT
•Heart Scan: Coronary Artery Calcium Score) the EBT Heart Scan is
a breakthrough in Early Detection and Cardiac Risk Assessment.
Coronary artery calcium is a marker for plaque in a blood vessel.
These calcium deposits form years before the first symptoms of heart
disease, such as chest pain or shortness of breath, appear.
•Full Body Scan: The Body Scan includes the Heart Scan, the Lung
Scan, a CT scan of your Abdomen and Pelvis and a Bone Scan,
allowing us to detect liver tumors, gallstones, kidney stones, kidney
tumors, aneurysms and abnormalities of the prostate or ovaries.

Before you leave you will have had a personal consultation with our
health professional.
•Electron Beam CT Coronary Angiography (EBA):
Approved by
the FDA in November 1999, the Electron Beam CT scanner
provides an opportunity to examine your coronary arteries
without the risks of conventional angiography.
Coronary
angiography done on the Electron Beam CT Scanner takes only
about half an hour and is reimbursed by insurance plans

Electron Beam CT
•Lung Scan: Lung Cancer strikes 150,000 Americans each year.
Despite treatment advances, five-year survival for lung cancer
patients remains only 14%. Only CT scans can find lung cancer at
an earlier and potentially more curable stage. All current or former
smokers or those exposed to carcinogens such as asbestos could
benefit from this test.
•3D QCT Bone Scan: Bone density provides the critical information
you need to identify osteoporosis, a
debilitating disease that can
lead to bone fractures. An essential exam for all post-menopausal
women, 3D QCT Bone Densitometry is more accurate than
conventional DEXA studies.
•Virtual Colonoscopy: Colorectal Cancer is the second leading
cause of cancer death. This test is a new technique for visualizing
colon polyps that could become cancerous. This alternative to
standard Colonoscopy does not require a hospital or outpatient stay,
or anaesthesia. Utilizing air and advanced image rendering, a virtual
voyage through your colon is generated from the scan images.

CVCT
•Cardiovascular CT
•
•Integrating knowledge from Philips' leadership in cardiac imaging and monitoring
systems, Brilliance CT is uniquely designed to adapt to your patients and
overcome the many challenges that unpredictable heartbeat rhythms can present.
Philips-patented Rate Responsive™ image acquisition technology adapts to your
patients - rather than the other way around. The many intelligent technologies built
into the Brilliance CT configurations not only improve departmental workflow, but
the early detection of cardiac disease as well.
•
•
•Features and Benefits
•Redefining patient care through automatic adjustment to variations in patient heart
rate and rhythm during the scan and subsequent image reconstruction
•Least invasive, most reliable solution for assessment through accurate
determination of the most stable cardiac phase for each heart region
•Achieve temporal resolutions as low as 53ms to visualize coronary anatomy and
determine stenosis as well as plaque constituents.

Toshiba CVCT
•Expanding the Limits of Cardiovascular CT
Delivering 16 0.5mm isotropic slices with 400msec rotation
times, the Aquilion™ 16 CFX images the smallest vessels with
confidence in the widest range of patients.
•Temporal resolution as low as 400 msec for clear, detailed
cardiac evaluation
•Flexible scan speeds and SURECardio software ensure optimal
imaging for variable and high heart rates
•Industry’s longest couch supports whole-body CTA without
patient repositioning
•Ideal for use in cardiology, trauma, neurology and oncology
•A suite of applications, called SURETechnologies, specifically
designed to provide the highest productivity and best image
quality at the lowest possible dose

Cine ct
•Cine ct,
•a very fast computed tomography CT scanner which can
acquire images from a single or small number of image
locations in a small fraction of a second. The scanner, also
named millisecond CT, ultrafast CT or electron beam CT,
requires no mechanical motion to acquire the projection data.
Instead of rotating an X ray tube around the object, the cine CT
system sweeps an intense electron beam across a large,
stationary anode target which surrounds the patient. X-rays are
emitted from the point where the electrons strike the target. The
X-rays transmitted through the object are measured by a
stationary array of detectors. The time needed for collecting
data for a single slice is 50-100 ms, although a longer scan time
can be used to reduce image noise.

Cine CT
•In some scanning modes, two detector rings allow the imaging of two
sections in a single scan of the X-ray target. Also, multiple target rings,
scanned sequentially, allow a volume to be imaged rapidly. The system is
very well suited to dynamic studies or studies where motion artefacts might
be a problem (e.g. cardiac applications), and for imaging of uncooperative
patients (e.g. children).
•Ideally, the X-ray target would completely surround the object, thereby
allowing the source of X-rays to rotate through 360. In the ideal system, the
detectors would also be on a complete ring. Mechanical constraints do not
allow this arrangement. As a result, the source and detectors are not
actually on the same plane, introducing complications to the reconstruction
process and the possibility of artefacts. Another limitation in image quality
results from the limit on the electron beam current. Although the current is
high (~ 1 000 mA), the very short imaging time leads to a low mAs. Cine CT
systems generally have higher noise level and lower spatial resolution than
conventional CT systems, but are ideal when their high scanning speed is
well suited to the clinical application. Among the possible uses are cardiac
imaging with and without the use of contrast agents, lung imaging, and
paediatric studies.

Cine CT
•The cine CT system has no mechanical scanning motion. In this
system both the X-ray detector and the X-ray tube anode are
stationary. The anode, however, is a very large semicircular ring
that forms an arc around the patient scan circle, and is part of a
very large, non-conventional X-ray tube. The source of X-rays is
moved around the same path as a fourth generation CT
scanner by steering an electron beam around the X-ray anode.
Because the electron beam can be moved very rapidly, this
scanner can attain very rapid image acquisition rates. In the
literature, this system has been referred to variably as fifth
generation and sixth generation. It has also been described as a
stationary-stationary scanner. The terms millisecond CT,
ultrafast CT and electron beam CT have also been used,
although the latter can be confusing since the term suggests
that the patient is exposed to an electron beam.

CVCT

Applications of CVCT
•CardioVascular CT is an effective, non-invasive imaging
technique for patients with congenital and acquired
heart disease. Various forms of aortic disease -
including aortic malpositions, dissections, and coronary
aneurysms - are well suited for investigation by CV CT.
The ability of CV CT to acquire volumetric axial images
in a short breath-hold enables clinicians to build
accurate 3D anatomical models of the entire heart and
vascular structures. In addition, CV CT can be used to
screen pericardial lesions, and accurately show the
extent and location of intracardiac masses and tumors
adjacent to the heart.

Formation of CT images

CT Sinogram
The data acquired for one CT
slice can be displayed before
reconstruction. This type of
display is called a Sinogram.

What are we measuring?
The average linear attenuation
coefficient (µ), between tube
and detectors
Attenuation coefficient reflects
the degree to which the X-ray
intensity is reduced by a
material

PIXEL & VOXEL
•Cross sectional layer of
the body is represented
as an image matrix.
•Each square of the
image matrix is called
pixel(picture element)
and it represents tiny
block of tissue called
voxel (volume element)

•The linear attenuation coefficient () of
each pixel is determined by :
1. Composition of the voxel
2. Thickness of the voxel
3. Quality of the radiation beam
Linear attenuation coefficient
µ

Algorithms for image
reconstruction
•An algorithm is a mathematical method for
solving a problem
•The linear attenuation coefficients of all the
pixels in the image matrix should be
determined by solving thousands of
equations.
•All algorithms attempt to solve the equations
as rapidly as possible without compromising
accuracy.

IMAGE RECONSTRUCTION
Process of generating an image from the raw data or set of
unprocessed measurements made by the imaging system.
Image reconstruction algorithms
•Back projection
•Iterative methods
•Analytic methods : 2D Fourier analysis
Filtered Back
Projection

Simple back projection method
•The image profiles look like steps. The height of the steps is
proportional to the amount of radiation that passed through the
block. The steps are then assigned to a gray scale density that
is proportional to their height. When the rays from the two
projections are super imposed, or back projected, they produce
a crude reproduction of the original object. In practice, many
more projections would be added to improve image quality.
•All points in the back projected image receive density
contributions from neighboring structures.
•The radio dense material severely attenuates the beam and
produces localized spikes in the image profiles. Back projection
of the rays from these spikes produces a star pattern. A great
number of projections will obscure the star pattern, but the back
ground density remains as noise to deteriorate the quality of the
CT image.

Simple back projection method

Iterative method
•Assumption (for eg. all points in the matrix
have same value)
•Comparison (with the measured values)
•Correction (to bring the two into agreement)
•Repetition (of the process until the assumed
and measured values are the same or within
acceptable limits)

Iterative method
Simultaneous reconstruction
•All projections for the entire matrix are calculated at the
beginning of the iteration, and all corrections are made
simultaneously for each iteration.
Ray by Ray correction
•One ray sum is calculated and corrected and these corrections
are incorporated into future ray sums, with the process being
repeated for every ray in each iteration.
Point – by – point correction
•The calculations and corrections are made for all rays passing
through one point, and these corrections are used in ensuring
calculations, again with the process being repeated for every
point.

Two – dimensional Fourier
analysis:
•Basis: Any function of time [f(t)] or
space [f(x)] can be represented by the
sum of various frequencies and
amplitudes of sine and cosine waves.

2D FOURIER ANALYSIS

Filtered back projection
•The image is filtered or modified to counter balance
the effect of sudden density changes, which causes
blurring (star – pattern). Those frequencies
responsible fro blurring are eliminated to enhance
more desirable frequencies.
•The density of the projected rays is adjusted that is,
the inside margins of dense areas are enhanced
while the centers and immediately adjacent areas are
repressed. The net effect is an image more closely
resembling the original object.

FILTERED BACK PROJECTION

Comparison of Mathematical methods
•In terms of speed, Analytical methods are
faster than iterative methods
•But with incomplete data, iterative methods
are faster than analytical methods. Analytical
methods does time – consuming
interpolations to fill in missing data, whereas
iterative methods simply average adjacent
points.

CT NUMBER & HOUNSFIELD UNIT
The computer calculates a relationship between the
linear attenuation coefficients of the pixel and water
which is given as CT number.
To image materials with  higher than dense bone
CT number larger than 1000 should be available
CT numbers based on a magnification constant of
1000 are Hounsfield units

Relationship between CT numbers & Gray scale

WINDOWING
•Windowing is the process in which the gray level
can be manipulated using the CT numbers to provide
optimum demonstration of different structures seen
on the image.
•Window Width(WW) is the range of CT numbers for
the gray scale & WW control alters the image
contrast
•Window Level(WL) is the centre of the gray-scale
image & WL control alters the image density/CT
number of the tissue to be displayed

Graphic illustration of the effect of different
WW & WL settings on the CT image

Image Quality in CT
Image quality is the visibility of diagnostically
important structures in the CT image.
The factors that affect CT image quality are
• Quantum mottle (noise)
• Resolution : Spatial and contrast
• Patient exposure.
The factors are all interrelated

Quantum mottle (Noise)
•Quantum mottle is the statistical fluctuations
of X-photons absorbed by the detector
•The only way to decrease noise is to increase
the number of photons absorbed by the
detector.
•The way to increase the number of photons
absorbed is to increase x-ray dose to the
patient.
•Mottle becomes more visible as the accuracy
of the reconstruction improves.

RESOLUTION
i) Spatial resolution
•Spatial resolution is the ability of the CT
scanner to display separate images of
two objects placed close together.

ii) Contrast resolution
•Contrast resolution is the ability of the
CT scanner to display an image of a
relatively large (2 or 3mm) object that is
only slightly different in density from its
surroundings.

RADIATION DOSE
•Even distribution of radiation dose to the
tissues as exposures are from almost all
angles.
•No overlapping of scan fields takes
place.
•Exposure factors used are higher to
improve spatial and contrast resolutions
and to reduce noise.

Comparison of CT with Conventional Radiography

CT ARTIFACTS
Artifacts are distortions or errors in the image
that are unrelated to the object scanned .
Most common artifacts in CT are
•Motion artifacts
•Streak artifacts
•Beam hardening artifacts
•Partial volume averaging artifacts
•Ring artifacts

EFFECTS OF ARTIFACTS
•DETERIORATE IMAGE QUALITY
•SUBJECT INFORMATION IS
LOST
•PATHOLOGICAL DETAILS ARE
LOST

MOTION ARTIFACTS
Cause : Patient movement
Appearance: Blurred / streaks / ghost images
Rectification:
•reduction in scan time
•Clear and concise instruction to the patient
•proper patient immobilization
•if needed,administration of
sedatives/antiperistaltic drugs

Motion artifact

STREAK ARTIFACTS
Cause: Presence and movements of
objects of very high density(contrast
media, metallic implants,surgical
clips)
Appearance: Streaks
REMEDY:-
•Remove the offending object
if
possible. Use a smoothing
algorithm. e.g. Standard algorithm.

Streaking
Streaking occurs due to inconsistency in a
small group of readings
•Partial volume
•Photon starvation
•Metal artifacts
•Patient movement

DENTURES
PRODUCING
STREAK ARTIFACT

SURGICAL
CLIP IN HEART
PRODUCING
STREAK ARTIFACT

BEAM HARDENING
ARTIFACTS
Cause :
Polyenergetic X ray spectrum(25-120kV)
APPEARANCE:-
Wide dark streak
Rectification :
•Beam hardening correction algorithm

PARTIAL VOLUME
AVERAGING ARTIFACTS
•Cause: presence of tissues with highly
varying absorbtion properties in a voxel.
•Rectification : Usage of Thinner CT
slices

OUT OF FIELD ARTIFACT
CAUSE:-
Scan FOV not covering
the entire anatomy
APPEARANCE:-
Shading/streaks
REMEDY:-
Ensure that scan field of
view is larger than the
object
to be scanned

RING ARTIFACTS
•CAUSE : Detector failure or miscalibration
of a detector
•APPEARANCE:-
Ring
•Rectification : regular quality assurance
checks

RING APPEARANCE

ct physics final jhlgkuyfkytfukyguktfkytfkytfiyt

About This Presentation

Slide Content

Tags

Categories

Download

Quick Actions

Statistics

Related Slideshows

ct physics final jhlgkuyfkytfukyguktfkytfkytfiyt

About This Presentation

Slide Content

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15

Slide 16

Slide 17

Slide 18

Slide 19

Slide 20

Slide 21

Slide 22

Slide 23

Slide 24

Slide 25

Slide 26

Slide 27

Slide 28

Slide 29

Slide 30

Slide 32

Slide 33

Slide 38

Slide 39

Slide 41

Slide 42

Slide 43

Slide 45

Slide 47

Slide 48

Slide 49

Slide 50

Slide 51

Slide 52

Slide 53

Slide 54

Slide 55

Slide 56

Slide 57

Slide 58

Slide 59

Slide 60

Slide 61

Slide 62

Slide 63

Slide 64

Slide 65

Slide 66

Slide 67

Slide 68

Slide 69

Slide 70

Slide 71

Slide 73

Slide 74

Slide 75

Slide 76

Slide 78

Slide 79

Slide 80

Slide 82

Slide 83

Slide 84

Slide 85

Slide 87

Slide 88

Slide 89

Slide 90