CT generations and image reconstruction

1. COMPUTED
TOMOGRAPHY

2. PRINCIPLE
• Internal structure of an object can be
reconstructed from multiple
projections of the object

3. HOW CT WORKS?
• A thin cross section of the body is scanned with
a narrow beam of Xrays and the transmitted
radiation is measured with a sensitive radiation
detector
• The radiation detector adds up the energy of all
the transmitted photons
• A numerical data is obtained which is then
computer processed to reconstruct an image
• A series of projection data is obtained, these
data are used to reconstruct cross-sectional
images

4. FIRST GENERATION CT
• For head imaging only
• Single narrow pencil
shaped beam
• Single detector
• Translate-rotate
motion
• Scan time: 5 mins for
a pair of tomographic
section

5. • Starting at a particular angle , the x-ray tube and detector
system translated linearly across the FOV acquiring 160
parallel rays per view .
• After end of translation , tube and detector assembly rotated
around the subject by 1 degree.
• This procedure was repeated until 180 projection.
• A total of 180 x 160 = 28,800 rays were measured.
• This combination of linear translation followed by incremental
rotation is called translate – rotate motion .
FIRST GENERATION CT

7. FIRST GENERATION CT

8. • Early detector system couldn’t accommodate large change in
signal so patient head was recessed via a rubber membrane
into a water filled box / water bath .
• It acted to bolus the x-rays so that the intensity outside the
head is similar to the intensity inside head .
• Though water bath cannot be used for body scanning , it was
used because it allowed Hounsfield to maximize accuracy of
attenuation coefficient measurement (limitation of dynamic
range , beam hardening correction)
FIRST GENERATION CT

9. • ADVANTAGE
- Less scatter radiation
• DISADVANTAGE
-The major drawback was nearly 5 minutes was required to
complete a single image.
-Contrast resolution of internal structures
was unprecedented , images had poor
spatial resolution .

10. SECOND GENERATION CT
• Full body scanner
• Single Fan shaped
beam
• Multiple detectors
• Translate-rotate
motion
• Scan time: 10-90 sec
•

11. • Since more detectors are used, only few linear
movements are needed
• So the gantry rotated through a greater arc,
upto 30 degree
• The number of repetitions depends on the
number of detectors used

12. SECOND GENERATION CT

13. • DISADVANTAGE
• Even small deviations(because of vibration or other
misalignment)of scanner hardware position relative to
reconstruction voxels would cause data to be back projected
through the wrong voxels, creating severe artifacts.
• disadvantage of fan beam is the increased radiation
intensity towards the edge . But it is compensated with the
use of bow-tie filter (limits the range of intensity reaching
detector and hardens beam)
SECOND GENERATION CT

14. THIRD GENERATION CT
• Fan shaped beam
• Curvilinear detector
array
• Rotate-rotate motion
• Scan time: 1 sec or
less
• Ring artifacts are
produced

15. THIRD GENERATION CT

16. DISADVANTAGE
• Any error or drift in the
calibration of detectors
relative to other detectors is
back projected along these
ray path and reinforced
along a ring where they
cross .
• The result is the ring artifact.
THIRD GENERATION CT

17. FOURTH GENERATION CT
• Fan shaped beam
• Multiple individual
detectors used
• The xray source is rotated
around a fixed detector
array
• As the fan beam passes
across each detector,
image projection is
acquired
• Scan time: 1 sec or less

18. • DRAWBACKS
 Size and geometric dose
inefficiency
Because tube rotated inside
the detector ring large ring
diameter was needed .
 Scatter could not be removed
FOURTH GENERATION CT

20. FIFTH GENERATION CT
• specifically for
cardiac imaging
• Scan time: 10-20ms.
• large bell shaped x-
ray tube.

21. • A large arc of tungsten encircles the patient and lies directly
opposite to the detector ring.
• X-rays are produced from a focal track as a high energy
electron beam strikes the tungsten.
• There are no moving parts in the gantry.
• Electron beam is produced in cone like structures behind the
gantry and is electronically steered around the patient so that
it strikes the annular target.
• Wherever it strikes – produces x-rays.
• The concept is known as EBCT(Electron Beam CT)
FIFTH GENERATION CT

23. • Helical or Spiral CT
• As the table is smoothly moved through the rotating gantry ,
the tube and detector moves in a helical or spiral path in
relation to the patient
• Three technological developments were required
-slip ring technology
-high power x-ray tubes
-interpolation algorithms
SIXTH GENERATION CT

25. Slip ring
• A slip ring passes electrical power to the rotating components
without fixed connections.
• It allows the complete elimination of interscan delays except
for the time required to move the table to next slice position.
SIXTH GENERATION CT

26. Pitch
• It is defined as the table movement per rotation divided by
beam width.
Interpolation
• Helical CT scanning produces a data set in which CT images
can be reconstructed at any position along the length of
scan.
• It allows the production of additional overlapping images
with no additional dose to the patient.
SIXTH GENERATION CT

27. Advantages
• Fast scan times and large volume of data collected.
• Minimizes motion artifacts.
• Reduced patient dose.
• Improved spatial resolution.
• Enhanced multiplaner or 3D renderings.
• Improved temporal resolution.
SIXTH GENERATION CT

28. SEVENTH GENERATION CT
MULTI DETECTOR CT
• A body section can be scanned faster with a multiple row of
detectors system with multiple fan beams scanning
simultaneously.
• Crucial for covering a large body section with thin beams for
producing thin , high-detail slice images or 3-D images.

29. • When multiple detector is used , the collimation spacing is
wider therefore more of x-rays that are produced by x-ray
tubes are used in producing image data.
• With conventional single detector array scanners , opening up
the collimator increases slice thickness which is good for
utilization of x-ray but reduces spatial resolution in the slice
thickness dimension.
• With introduction of multiple detector arrays , the slice
thickness is determined by the detector size and not by the
collimator.
SEVENTH GENERATION CT

30. Pitch
•Pitch is equal to the table
rotation per gantry rotation
divided by width of
detector.
SEVENTH GENERATION CT

31. MULTIPLE ARRAY DESIGNS

33. COMPONENTS
XRAY TUBE: -Rotating anode with a smaller focal
spot
-Have large heat loading and heat
dissipating capacities
COLLIMATORS: -used at 2 places
-detector collimator controls
scatter
-control the thickness of ct slice

34. DETECTORS
SCINTILLATION DETECTOR
- combination of scintillation crystal and light
detector
• With interaction with crystal, energy of Xray photon
is converted into light photon. This light output is
converted into electric signal using a light detector
• Initially thallium activated NaI crystals with
photomultiplier tubes were used
•

35. LIMITATIONS:
• NaI : -hygroscopic and
requires a air tight
container
-long afterglow
NaI is replaced by
CsI, BGO, CdWO4
• Photomultiplier tubes are
replaced by silicon
photodiodes

36. XENON GAS IONISATION CHAMBERS:
• Photon interacts with a gas atom by ionisation
of atom into a electron-ion pair
• Voltage between anode and cathode drives
negative ion(electrons) to anode which in turn
produces a small current in the anode

37. • Current produced is directly proportional to
the intensity of incoming radiation
LIMITATIONS:
-This type of detector cannot be used in rotate-
fixed CT
-Reduced efficiency

38. PIXEL AND VOXEL
• Each square in the
image matrix is
called pixel
• The 3D volume
element of pixel is
called voxel

39. CT NUMBER- HOUNSFIELD
UNITS
• Numerical values for each pixel
• It is directly related to the Xray linear
attenuation coefficient for the tissue contained
in the voxel

41. IMAGE RECONSTRUCTION

42. BACK PROJECTION
• Depending on the amount of
radiation passed through the
tissue, a grey scale density is
assigned to each projection
• When different projections
are superimposed or back-
projected, they produce a
crude reproduction of
original object

44. ITERATIVE RECONSTRUCTION
• It starts with an assumption and compares this
assumption with measured values, makes corrections
to bring the two into agreement, and repeats the
process over and over until the assumed and
measured values are the same or within acceptable
limits
• 3 variations
-simultaneous reconstruction
-ray by ray correction
-point by point correction

47. ANALYTIC METHODS
• Currently used in CT
• Fournier analysis
• Filtered back projection

48. FOURNIER ANALYSIS
• Any function of time or space can be
represented by sum of various frequencies and
amplitudes of sine and cosine waves
• In a wave, height corresponds to amplitude
and length corresponds to frequency

50. FILTERED BACK PROJECTION
• Similar to back projection except that the
projected image is modified by filtration to
eliminate the frequencies responsible for
blurring .

52. THANK YOU