Computed Tomography
An overview of Radio-Physics.
Dr. Syed Mohammad Faizal
JR-1, Department of Radiodiagnosis
ModeratorDr. Swasti Pathak
FacultyProf. Dr. Rajnikant Yadav
Principle:
Internal structure of the object can be
reconstructed from multiple projections of the
object.
Tomography = Tomos ( slice) + graphein ( to
write)
Slit scan projection of
Patient’s body
Digitizing Image
received by computer
Reconstructing image using
Mathematical Algorithms
History
 Godfrey N Hounsfield(1970-71), Research eng.
with Electro-Musical Instrument Ltd.(EMI):
Invention of Computed Tomography
Godfrey Hounsfield & Cormack…1979 Nobel
prize for Medicine…For Discovery of CT.
CT Generations
• Time reduction is predominant reason for introducing
newer generations.
 Changes down the Generations:
Fan shaped beam
Increasing No of detectors
Ring of detectors.
1st Gen CT Scanner
Original EMI unit
• Linear scan and Rotate
1°rotation and 180 linear scans
i.e. one linear scan
Gantry rotation by 1°
Again linear scan
• Pencil like X-ray beam.
• Paired Detectors.
2nd Generation
 Fan shaped beam
 Linear scan and Rotate
i.e. 30⁰rotate and 6 scans.
i.e. one linear scan
Gantry rotation by 30°
Again linear scan
 Multiple Detectors…
up to 30 detectors
…decreased no. of rotatory
steps from 180 to 6.
3rd Gen CT Scanner
• Rotate- Rotate scan
• Fan Shaped Beam
• Detector array…up to
300 detectors
4th Gen. CT Scanner:
• Rotate- Fixed scan
• 360⁰ ring Stationary
Detectors
...decreased calibration
requirements.
Generations
Projection
X R Tube-Detector
movement
Detectors
Scan time
First (EMI)
Head Only
Pencil Like beam
Translate (linear)
and rotate 180
scans,1⁰rotation
Paired
4.5-5 min.
Second
Single Projection
Fan Shape Beam
Translate (linear)
Up to 30 in
and rotate 6 scans , linear array
30⁰ rotations
10-90 sec
Third
Single Projection
Fan Shape Beam
Rotate-Rotate
Both @ 360⁰
2-10 sec
Fourth
Single Projection
Fan Shape Beam
Rotate-Fixed
600-2000
Tube 360⁰ Rotation placed in 360⁰
ring
Helical
MULTIPLE proj.
Fan Shape Beam
Rotate-Fixed
placed in 360⁰
Tube 360⁰ Rotation ring
Up to 300 in
curved array
2-10 sec
Spiral (Helical) CT:
Advances in SLIP RING
TECHNOLOGY made possible
invention of spiral CT.
Slip Ring Tech. Consists of
brushes that fit into grooves
to permit current and
voltage to the X ray tube to
be supplied while the tube is
in continuous rotation
around the gantry.
Spiral CT
Spiral is a misnomer…
Helical is the proper term…
since tube rotates in
helical path with
constant radius.
( spiral path would indicate
progressive decreasing radius)
Spiral CT
The patient table is also
moved slowly in the gantry
while tube is in motion.
Thus data comprising of
continuous helical scan of
the patient is acquired.
Pitch = Table increment mm/sec
Section thickness mm
Pitch <1.0 imply data oversampling
> 1.0 imply some data is being
missed.
Two general advantages of increasing pitch are:
1. faster scanning
2. reduced dose (the radiation is less concentrated)
The major limiting factor associated with increased pitch is
reduced image detail in the direction the body is moved.
Spiral CT
ADVANTAGES:
• Much shorter total scan time…less contrast
media.
• Scan can be completed in One breath
hold….decrease motion artifacts.
• With adjusting pitch, area of interest can be
oversampled.
Conventional CT Scanner
•
•
•
•
•
•
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Parts- Gantry, X ray Tube, Detectors, Computer
Image Reconstruction
Image Display- Window level, Window width.
Image Quality- Quantum Noise, Resolution
Artefacts
Patient Exposure.
RECENT ADVANCES in CT.
PARTS Of CT Unit
Gantry:
• Movable form of CT unit
containing X Ray tube and
detectors.
• Gantry frame maintains alignment of tube and
detectors.
• Gantry aperture for movement of patient for
scanning.
Table:
Made up of Carbon graphite
to decrease the beam
attenuation.
 Tabletop has certain weight
limits.
 Top is motor driven to allow pt movement through
gantry aperture and also vertical movement.
X Ray Tube…
Earlier-
• stationary anode
• oil cooled
• 2x16mm focal spot
• 120 kVp and 30mA
• 80x80 matrix size
Now-
• Rotating anode
• high heat loading and
dissipating capabilities
• 0.6-1.2 focal spot
• 120 kVp and Up to
1000mA
• with 512x512 matrix
generally.
Detectors
• Measure the transmitted radiation.
• Requirements:
High Stabilty.
Fast Response Time.
Wide Dynamic Range=
Two Types:
1. Scintillation Detectors
2. Ionization Chambers
Measurable largest signal
Measurable smallest signal
Scintillation Crystals and
Photomultiplier Tubes:
•
Convert X Ray Photon energy into Electrical Signals.
•Crystals function just like an Intensifying screen.
• Crystals material: Sodium Iodide, Calcium Fluoride
• Photomultiplier tubes replaced by Silicon
Photo iodides - smaller, more stable, lower cost.
XENON Gas filled Ionization Chambers:
Photon entering detector ionizes gas atom into +ve and
-ve ions. -ve ion(electron) moves to anode, producing
current (output signal) in anode.
•Xenon- heaviest inert gas therefore more density.
compressed to increase density.
Scintillation
Ionization
Crystals
Chambers
• Near 100% efficiency
• Large size
Less efficiency -60% due to low
density of absorbing material.
Small size
• Long after glow of NaI crystals
Negligible lag time.
• Detector Crosstalk: when photon
strikes detector, gets partially
absorbed and then enters adjacent
detector and is detected again.
Leads to decreased resolution.
Can be used in 4th gen scanner
COMPUTER:
CT console provides access to
software program that controls
data acquisition, processing and
display.
Image
Reconstruction:
• Cross sectional layer of the
body is divided into many
tiny blocks called VOXEL…
… a 3D element
Image Reconstruction:
Each Voxel has been traversed by
numerous X ray photons and
the intensity of the transmitted
radiation is measured by detectors.
The Degree of Attenuation of X-ray beam depends on
• Composition of tissue in the path
• Thickness of section
• Quality of the X-ray beam
Image Reconstruction Algorithm
use following formula to
calculate Attenuation Coefficient
for each Tissue block or Voxel.
•
•
•
•
•
N= No. Of Transmitted Photons
N₀= No. of Initial Photons
e= Constant (2.718)
µ = Attenuation Coefficient
X= Tissue thickness.
Algorithms for Image reconstruction:
After solving Thousands of equations determine linear
attenuation coefficient of each voxel is determined.
Algorithm is a mathematical method to solve these
equations.
• Back Projection Method
• Iterative Methods
• Analytical Methods:
Two Dimensional Fourier Transformation
Filtered Back projection
CT number or Hounsfield No.
These numbers are calculated by comparing linear attenuation
coefficient of each pixel to the linear attenuation coefficient of
water.
CT No. Of Various Body Tissues:
Bone average
Bone Cortical
Liver
Cerebellum
Blood
CSF
Water
Fat
Lungs
Air
+1000
+80
+40-70
+30
+13-18
+15
0
-100
-150-400
-1000
Image Display
• The viewed image is then
reconstructed as a
corresponding matrix of
picture elements as PIXELS...
a 2D element as shades of
grey color.
Display Matrix:
256x256 ,
512x512 or
a Voxel
a Pixel
1024x1024 pixel sizes
but generally 512x512 pixels is used.
Window Width and Window Level
• Human eye can recognize 256 shades of Gray. So we
have to image -1000 to +1000 CT numbers i.e. 2000
numbers in 256 shades of gray.
• How this is possible???
We usually select a CT number that will be about the
average CT Number of the tissue being examined.
We usually select a CT number that will be
about the average CT Number of the
tissue being examined.
128
• i.e.+200 HU for bone and computer is
+200 HU for Bone
programmed to assign one shade gray
to each of the 128 numbers above and
128
128 numbers below this Baseline CT number .
• Here +200 is WINDOW LEVEL and Range above and
below i.e. 128 is called WINDOW WIDTH.
• So in practice Different window level and
window width is employed to obtain
maximum information for different body
tissues.
i. e . Bone window
Lung window etc.
Quantum Mottle (Noise):
• Precision is measure of background or matrix uniformity.
• Deviation from this uniformity represents Quantum Mottle
or Statistical fluctuation.
• It is function of how much photons are absorbed effectively
by each voxel of tissue.
Factors affecting Noise:
• Radiation Dose.
More mA= More the Photons absorbed=Less Noise
• Voxel size
Small voxel size = capture less photons = more noise.
 Decreasing Slice Thickness (To Improve Detail)
Small Voxel size
More Noise
INCREASED RADIATION
DECREASE noise .
Contrast
• Most CT images can demonstrate contrast
difference of as little as 0.4% as compared to
minimum of 10% in routine Xray.
Resolution
Spatial Resolution
Contrast Resolution
• Spatial resolution is
• Contrast Resolution is
ability of the CT scanner
ability of system to
display as distinct image
to display separate
of areas that differ in
images of two objects
density
by
small
amount.
placed close together.
To increase Resolution, Patient Radiation Dose should have
to be Increased.
Spatial Resolution
• Expressed in mm or Line pairs per cm depending on
manufacturer.
• i.e. Resolution of 0.5 mm= 10 lines pair/ cm= 10 lines
of 0.5mm separated by 10 spaces of 0.5mm
• Modern scanner have resolution up to 15 lines pair
per cm.
Spatial Resolution
Depends on:
• Scanner design- Xray tube focal spot size, Detector size,
Magnification.
• Computer Reconstruction
• Display matrix
i.e. increasing the matrix size or
decreasing individual pixel size
Improve Spatial
Resolution
Contrast Resolution
• It is ability of system to display as distinct
image of areas that differ in density by small
amount.
background noise =
Contrast Resolution
ARTIFACTS:
Artifacts is the discrepancy between the CT
Numbers in reconstructed image and true attenuation
coefficient of the object.
Motion Artifacts:
•Image displays an object in
motion as streak in
direction of motion.
i.e. motion of gas in stomach, metallic object.
• To Prevent:
Remove metallic objects.
Gantry angulation i.e. to avoid dental filling.
Ring Artifacts
• Seen Rotate Rotate geometry
scanner.
• Result of the miscalibration of
single detector.
• With sum effect of multiple
projection in 360 degree results
in annular or ring artifacts.
Partial volume
Effect:
• Since the data from entire
section (voxel width) is
averaged together to form
the image, any object in
the section with width less
than the section thickness may get
obscured.
• To prevent: can be overcome by obtaining slightly
overlapping section.
Beam Hardening:
• It results of the attenuation of the beam as it
passes through the patient since the low
energy photons are rapidly absorbed.
• As a result CT number of posterior structures
may be much different than similar density
anterior structures.
• Seen as Dark bands or Cupping artifacts if
lower CT numbers than anticipated.
Beam Hardening.
In CT head in 360 tube rotation,
Centre of brain will always get
partially attenuated Xray beam
as compared to periphery.
PreventGenerally reconstruction programs anticipate and correct for
this variation, but it is not precise.
Capping artifacts if reconstruction algorithm overcompensates.
CONE BEAM Artefacts:
 Imp in Multislice CT as
detector width, All the
Xray beam pass through
the patient not exactly
parellel but instead at an
Cone Angle at the
Periphery of detectors.
 Cone angle greatest at
periphery of detector array.
Cone beam artefacts
• Seen in 8 slice CT onwards.
• To prevent this
Cone beam Reconstruction
Algorithm is used.
Patient Exposure
•
•
•
•
X-ray Chest…0.02 mSv
CT Head…….2.0 mSv………100 CxR equivalent
CT Chest…….8.8 mSv……...400 CxR equivalent
CT Abd/Pelvis…10 mSv……500 CxR equivalent
Indices:
 CTDI- Computed Tomography Dose Index
 MSAD- Multiple scan Average Dose.
(Dev.By Centre for Devices and Radiological Health,
FDA-US)
Recent Advances In CT
 Multi Slice (Multi Detector) CT
Spiral CT
Dual Source CT
Electron Beam CT
Single slice CT
Single row of detectors
Multi Slice CT
Multiple Rows of detector rings
Multislice CT
• Multiple Rings of detectors are used.
• 2 Slice CT images 2 body slices in 1 Tube
Rotation.
• 2, 4, 8, 16, 32, 40, 64 Slice CT are in use.
• The major benefit of multi-slice CT is the
increased speed of volume coverage.
• This has particularly benefited CT angiography
techniques - which rely heavily on precise
Dual Source CT
 Two X-ray tubes oriented at
90 degrees to one another.
 Dual Detector System.
 Scan in half the time of
a standard CT.
 Scan Time < 1 sec
 Acquires cardiac images from
single heartbeats.
 Contrast Dose required is less.
Electron Beam CT
Electron beam CT
Vs
X-ray Tube in Conv. CT
 X-ray tube -large , stationary, partially surrounding
imaging circle.
 In EBCT an electron beam is electro-magnetically
steered towards an array of tungsten X-ray anodes that
are positioned circularly around the patient.
EBCT
• This motion can be very fast as compared to mechanical
rotation of the Xray tube in Conventional CT Scanner.
• Total scan time-50 to 100 milliseconds
• Scan the beating heart. Adv for CARDIAC CT.
• The Coronary Calcium Score relates to the extent of
coronary plaque disease- substantial diagnostic and
prognostic implications.
Inverse Geometry CT
• IGCT reverses the shapes of the detector and X-ray
sources.
• An array of X-ray sources
• A point detector.
 Advantages :
• Avoid Cone beam artefacts.
• Less Scatter Radiation.
 Still in Experimental stage
Thank You...