Basic principle of ct and ct generations, a concise presentation to provide relevant information.

1. Supervising Faculty:
Dr. B. P. Baruah
Mentor:
Dr. Sushma Rani
Presenter:
Dr. Tarun Goyal

2. A conventional X-ray image
is basically a shadow.
Shadows give you an
incomplete picture of an
object's shape.
This is the basic idea of
computer aided
tomography. In a CT scan
machine, the X- ray beam
moves all around the
patient, scanning from
hundreds of different
angles.

3. 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.
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.
Godfrey N. Hounsfield
Allan MacLeod Cormack

4. Year Event
1917 Austrian mathematician Johann Radon proved that a 2D/3D
object could be reproduced from an infinite set of all its
projections
1956 Bracewell, working in radioastronomy, constructed a solar map
from ray projections.
1961 and 1963 Oldendorf and Cormack understood the concept of computed
tomography and built laboratory models.
1968 Kuhl and Edwards built a successful mechanical scanner for
nuclear imaging, but did not extend their work into diagnostic
radiology
1969 G.N. Hounsfield developed first clinically useful CT head
scanner
1971 First clinically useful CT head scanner was installed at Atkinson-
Morley Hospital (England)
1972 First paper on CT presented to British Institute of Radiology by
Hounsfield and Dr. Ambrose

5. Computed Tomography, CT for short (also referred
to as CAT, for Computed Axial Tomography), utilizes
X-ray technology and sophisticated computers to
create images of cross-section “slices” through the
body.
Tomography is a term that refers to the ability to
view an anatomic section or slice through the body.
Anatomic cross sections are most commonly refers
to transverse axial tomography.

6. Basics Principle
The basic principle behind CT is that the internal structure of an
object can be reconstructed from multiple projections of the object.
The ray projections are formed by scanning a thin cross section of
the body with a narrow x-ray beam and measuring the transmitted
radiation with a sensitive radiation detector.
CT scanning is a systematic collection and representation of
projection data

7. Parts of CT Scan Machine
The basic technology employed in the CT scanner is
designed to provide a source of x-rays to be transmitted
through the patient and then get detected by the
detectors.
The CT system consists of:
1. a computer work station for operation of the scanner,
2. image processing computers,
3. electronic cabinets,
4. the gantry
5. and the patient imaging table.

8. Composition Of Gantry
1. The gantry houses the key components of the scanner.
2. Production of the x-ray beam and detection and acquisition of
the beam must be located within the rotating portion of the
gantry.
3. The fan-beam x-ray tube sits opposite the detector array within
the rotating gantry.
4. The three phase power generator is also within the gantry
module.
5. The x-ray tube in a CT scanner is designed to produce a fan beam
of x-rays that is approximately as wide as the body.
6. Tissue attenuation is measured over a large region from one
position of the x-ray tube.
7. On the opposite side of the patient is the detector array that
measures the strength of the x-ray beam at various points
laterally across the body.

9. A look inside
of the Gantry
T: X-ray tube
D: X-ray detectors
X: X-ray beam
R: Gantry rotation

10. X Ray Tubes
The radiation source for CT would supply a monochromatic x-ray beam
(i.e., one made up of photons all having the same wavelength) with
advantage that image reconstruction is simpler and more accurate.
Earlier models used oil-cooled, fixed-anode, relatively large (2 X 16 mm)
focal spot tubes at energies of about 120 kV (constant potential) and 30
mA.
The beam was heavily filtered to remove low energy photons and to
increase the mean energy of the radiation.
New fan beam units have a diagnostic type x-ray tube with a rotating
anode and a much smaller focal spot around (0.6 mm). These tubes have
large heat loading and heat dissipation capabilities to withstand the very
high heat loads generated when multiple slices are acquired in rapid
sequence

11. X-ray beam collimated at two points, one close to the x-
ray tube and the other at the detector(s) with perfect
alignment. Each detector has its own collimator.
Collimator at the detector controls scatter radiation.
The collimators also regulate the thickness of the
tomographic slice (i.e., the voxel length).
Pixel size is determined by the computer program and not
by the collimator.
COLLIMATORS

12. DETECTORS
Two types of detectors are used
1. Scintillation Detectors
2. Gas Filled Detectors
Scintillation Detectors
These are the materials that will produce light (scintillate) when
ionizing radiation reacts with them. The combination of a scintillation
crystal and the light detector is called a scintillation detector.
Material Used:
1. Sodium Iodide (Nal)
2. Cesium Iodide(Csl),
3. Bismuth Germinate (BGO) and
4. Cadmium Tungstate (CdW04

13. Xenon Gas Ionization Chambers
Those rotate-rotate CT units that do not use scintillating crystals use
xenon gas ionization chambers.
Regardless of the shape or size,
all gas-filled detectors use
ionization of the gas by the
incoming radiation to produce a
signal.
Photon interacts with gas atom by ionizing the atom into an electron-ion
pair. Voltage between the anode and cathode will cause the electron to
move toward the anode, and the positive ion to move toward the
cathode.
When the electrons reach the anode, they produce a small current in the
anode. This small current is the output signal from the detector.

14. Data Accumulation
Data-gathering techniques have developed by stages. These stages
have been called "generations.“
The number of ray projections increases from 28,800 in the
original scanner to more than one million in newer scanners.
Image quality is related to the number of ray projections used to
reconstruct each CT scan image
Eliminates the superimposition of structures
Sensitive to subtle differences in x-ray attenuation

15. 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
What are we measuring?

16. 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
CT NUMBER & HOUNSFIELD UNIT

17. Values of CT Image Reconstruction
• CT numbers (Hounsfield units) are the pixel
values assigned in the image
• They are computed by calculating the relative
difference between the linear attenuation
coefficient of tissue and that of water

18. Hounsfield Units (HU) or CT numbers
Calculation
• For tissue and that of water the equation is:
CTTissue = (µtissue - µwater)/µwater x 1000
• The CT number for water has a value of 0 because:
(µtissue - µwater)/µwater = 0

19. CT numbers for various Tissues in
Hounsfield units (HU)
Air - 1000
Lung - 830 to – 200
Fat -30 to –250
Water 0
Heart 10 to 60
Brain 20 to 40
Blood 20 to 80
Liver 20 to 80
Muscle 35 to 50
Spleen 40 to 60
Bone 150 to 500
Bone (dense) 350 to 1000
Metal >2000

20. • CT image is displayed on TV monitor for immediate viewing and
recorded on film for storage
• Display matrix has on average 512x512 some may have 1024x1024
• But one display has only 256 shades of gray
• Thus, we are going to image CT no from -1000 to 1000 with 256
shades of gray
• Pixel size on average 0.1 mm with scan field view of 40cm
• With pixel size of 0.1 mm ,40 cm scan field contain 4000 pixels
• These 4000 pixels displayed on 512x 512 TV monitor will result in 8
pixel per matrix element
IMAGE DISPLAY

21. Now the challenge is how to display CT no from -1000 to 1000 with
only 256 shades of gray
1. Thus, assign 8 CT no.s to the same shade of gray and display entire
range of information in compressed scale.
2. And select a CT no that will be average of body examined
Computer instructed to assign one shade of gray to each of 128
CT no below and 128 CT no above baseline.
Centre CT no is ‘window level’
Range of CT no's above and below are called ‘window width’
This leads to the concept of Windowing, also known as grey-
level mapping, contrast stretching, histogram modification
or contrast enhancement

22. The values below are written as width and level (W:x L:y)
in Hounsfield units (HU).
head and neck
brain W:80 L:40
subdural W:130-300 L:50-100
stroke W:8 L:32 or W:40 L:40 3
temporal bones W:2800 L:600
soft tissues: W:350–400 L:20–60 4
chest
lungs W:1500 L:-600
mediastinum W:350 L:50
abdomen
soft tissues W:400 L:50
liver W:150 L:30
spine
soft tissues W:250 L:50
bone W:1800 L:400

24. Image Quality
Image quality is the visibility of diagnostically
important structures in the CT image.
The factors that affect CT image quality are
1. Quantum mottle (noise)
2. Resolution (Spatial and contrast)
3. Patient exposure.

25. • 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
Quantum Mottle (NOISE)

26. RESOLUTION
Spatial Resolution Contrast Resolution
Spatial resolution is the
ability of the CT scanner
to display separate images
of two objects placed
close together
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.

27. 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.

28. Scanning Motions
CT scanners have gone through a number of design changes
since the technology was first introduced in 1971. Scan time
reduction is the predominant reason for introducing new
configurations.
We will discuss geometry of design in five categories:
1. First generation (translate-rotate, one detector)
2. Second generation (translate-rotate, multiple detectors)
3. Rotate-rotate (third generation)
4. Rotate-fixed (fourth generation)
5. Other geometries

29. Original EMI Scanner / 1st Generation CT
Original EMI unit was a first-generation
scanner
Employed a pencil-like x-ray beam and a
single detector; that is, one detector per
tomographic section
Five-view study of the head took about
25 to 30 minutes
Was designed specifically for evaluation of
the brain
Gantry moved through two different types
of motion, one linear and the other rotary

30. linear motion was repeated over
and over 180 times.
Between each of these 180 linear
movements, the gantry rotated 1°.
Thus, the total rotary motion
encompassed a 180° semicircle.
The axis of rotation passed through
the center of the patient's head
The x-ray beam was "on“ throughout the linear movement and "off “ during the
rotary movements
The total number of transmission measurements was the product of the number
of linear measurements (160) and rotary steps (180), which was 28,800 in the
original EMI scanner.
The transmission readings represented a composite of the absorption
characteristics of all the elements in the path of the beam

31. In the original EMI scanner, the CT image was reconstructed and
then displayed on an 80 X 80 matrix in two different formats:
a paper printout of CT numbers and
a visual image on a cathode ray tube.
CT numbers were proportional to the linear attenuation coefficient.
Each square in the image matrix was called a pixel, and it
represented a tiny elongated block of tissue called a voxel.
An oil-cooled stationary anode tube was used in the original EMI
scanner. Its nominal focal spot size was 2.25 X 12 mm. The tube
was operated at 120 kVp and 33 mA. The x-ray beam was heavily
filtered with a half-value layer of 6 mm of aluminum.
Beam size was restricted by a pair of slit like collimators, one near
the tube and the other near the detectors.

32. This is the first
CT image taken by
first generation
CT.

33. • 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)

34. 2nd Generation CT
The main objective for its development was to shorten the scanning
time for each tomographic section
This was achieved by abandoning the single detector and pencil beam
of the original EMI scanner and adopting a fan-shaped beam and
multiple detectors.
The movements of the x-ray tube-
detector array are both linear and
rotary, just like a first generation
scanner, but the rotary steps are
larger.
Instead of moving 1°at the end of
each linear scan, the gantry rotates
through a greater arc, up to 30°.

35. With 30 detectors, the linear movements only have to be repeated
six times rather than the 180 linear movements of the original EMI
unit.
Second-generation scanners produced a tomographic section in
between 10 and 90 sec, depending on the manufacturer.
The other advantage of 2nd
Gen CT other than reducing
imaging time is that thorax
can also be imagined.
By adding detectors angularly
displaced, several projections
could be obtained in a single
translation.

36. 3rd Generation (Rotate/Rotate type)
Introduced by General
Electric Company in
1975 in which complete
translation motion was
eliminated.
Only rotation motion
was required, with
both the x-ray tube and
detectors rotating
around the patient

37. This scanning geometry came to be known as "fan beam" geometry, or
"third-generation" geometry, and the original unit could produce a scan in
4.9 sec.
Multiple detectors are aligned along the arc of a circle whose center is the
x-ray tube focal spot
Both the x-ray tube and detectors rotate about the patient (rotate-rotate
geometry) in concentric circles whose centers approximately coincide with
the center of the patient.
Detectors vary in no. from 288 from the original to 700 in modern
equipments.
The fan beam must completely cover the object to be imaged.
In the original rotate-rotate scanners the x-ray tube was pulsed. Each pulse
of the tube produced one projection, and the number of scan lines in each
projection was equal to the number of detectors.

38. In some rotate- rotate scanners the x-ray tube is continuously on, and
individual projections are obtained by reading the output of the
detectors at rapid intervals.
Let us not confuse ourselveswiththe terms“CT Image, Projections and Line”.
The CT image is the image of one CT slice that is viewed on the TV
monitor or on film. The image is computed from many projections
A projection is produced each time the detectors are read. Therefore,
the data contained in a projection is a set of readings composed of
one signal from each detector that has been exposed to the x-ray
beam.
We describe the data (signal) collected from one exposed detector as
a line, or scan line.

39. 4th Generation CT scanners (Rotate-Fixed type)
Detectors form a ring that completely surrounds the patient and they do
not move. (Stationary detector ring and rotating X-ray tube)
The x-ray tube rotates in a circle inside the detector ring, and the x-ray
beam is collimated to form a fan beam.
When the x-ray tube is at prescribed angles, the exposed detectors are
read.
One CT scan will be made up of many projections, each projection taken at
a slightly different angle and that is why continuous-on x-ray tubes are
generally used today.

40. The detectors in a
fan beam array have
their own individual
collimators. Most
scattered photons
are absorbed by
these collimators.
The advantage of a fan-beam/multiple detector array
is speed (reduced to few seconds). One principal
disadvantage of the fan beam is an increased amount
of scattered radiation.

41. Other Scan Configurations
This include but is not limited to:
 5th generation / CINE CT / CVCT
 6th generation / Spiral CT/ HELICAL CT
 7th generation / cone beam CT
 MultiSlice CT

42. CineCT /Ultrafast CT
● Nomechanicalscanning motion
● X-raydetector andtube anodeare stationary. It incorporates a
semicircular tungsten x-ray target into the gantry.
● Anode,isaverylargesemicircularringthat formsanarc aroundthe patient
scancircle.
● Source of X-raysismoved around the samepath asafourth generation
CTscannerbysteeringanelectron beamaroundthe X-ray anode
● Fifty millisecond multilevel scanning at rates of 17 scans per
second allows quantization of left ventricular (LV) function at
each tomographic level during a cardiac cycle.

43. Capable of scanning entire airway from nasopharynx to larynx
i.e distance of ~8cm in 0.24 seconds.
It allow analysis of entire airway during inspiration, expiration
and apenic episodes.
Cine CT systems although have higher noise level and lower
spatial resolution but are ideal for cardiac imaging , lung
imaging and pediatric studies.
A very fast scanner collecting data for
1 slice in 50 to 100ms.
Sweeps an intense electron beam
across large stationary anode target
which surrounds the patient.

44. 5th Gen CT / IMATRON / CVCT / CINE CT / Ultrafast CT

45. SPIRAL CT / HELICAL CT
● Design:x-raytuberotates aspatientismoved smoothlyintox-rayscan
field
● Simultaneoussourcerotation, tabletranslation anddata acquisition
● Producesonecontinuousvolumesetofdatafor entire region
● Dataformultipleslicesfrompatientacquiredat 1sec/slice
Threetechnological developments:
1. Slip-ringgantry designs
2. Veryhighpowerx-ray tubes
3. Interpolationalgorithmsto
handleprojection data

46. Continuous rotation of the x-ray tube and detectors and simultaneous
translation of the patient through the gantry opening.
q
Slip rings are circular electrical conductive
rings and brushes which transmits electrical
energy across moving interface.
It allows scan frame to rotate continuously
with no need to stop between rotations to
rewind system cables.
Interpolation methods developed by
Kalender are used to generate projections
in a single plane.
The overlapping sections generated by
math, not beam improves Z axis with no
increase in dose which helps in improving
image quality.

47. In helical CT, pitch is the ratio of the patient’s movement through the
gantry during one 360 degree rotation relative to the beam collimation.
For single slice CT, Pitch is equal to the table movement per rotation
divided by slice collimation.
On most CT and PET/CT systems the pitch is entered as a parameter that
is set in defining the procedure for the acquisition of patient data.
When the pitch is greater than 1.0 mm, there are interspaces created and
some portion of the body is being missed.
PITCH

48. The increment is the distance between the slices.
In CT data are acquired as projection information along
the helical path.
Slices may be reconstructed at various positions along the
helix.
One important factor in defining the slices and their
positional relationship to one another is the distance
between the reconstructed axial slices in the ‘z’ direction.
INCREMENT

49. Advantages of Spiral CT
• Faster image acquisition
• Quicker response to contrast media
• Fewer motion artifacts
• Improved two-axis resolution
• Physiological imaging
• Improved coronal, sagittal, and 3D imaging
• Less partial volume artefact

50. 7th Generation CT
It is a new technology
to overcome the
limitations of single
row detectors; hence it
has multiple detector
array.
The collimator spacing
is wider and more of
the x rays produced by
x ray tube are used in
producing image data.

51. There is formation of cone beam and multiple parallel rows of
detectors.
Widened ( Z-direction) X ray beam and detector array to acquire
multiple slices
This gives advantage of reducing scan time with increased
Z- Resolution but it is very expensive.

52. MultiSlice CT ● Relatesto the techniqueofdoubleor
triple rotationofthe tube and
detectors aroundthe sameaxial
plane
● Providesdoubleoftriple the volume
per slice,uponwhich the finalimage
canbe derived
● Inpracticeeachrotationproducesits
ownbankof rawdata,
● Multiscanningtherefore
reducesmotionartifactsand
consequentlyimprovesimage
quality
● Development insoftwareand
computer capacityleadto
processingand reconstruction
inashorttime

53. This picture shows the role of collimator in MDCT

54. Multiple detector array illustration
Image showing comparison of the
different geometrical generations

55. Generation Source Source Collimation Detector
1st Single X-ray Tube Pencil Beam Single
2nd Single X-ray Tube Fan Beam (not enough to cover FOV) Multiple
3rd Single X-ray Tube Fan Beam (enough to cover FOV) Many
4th Single X-ray Tube Fan Beam covers FOV
Stationary
Ring of
Detectors
5th
Many tungsten
anodes in single
large tube Fan Beam
Stationary
Ring of
Detectors
6th 3G/4G 3G/4G 3G/4G
7th Single X-ray Tube Cone Beam
Multiple
array of
detectors

56. Let us know our department CT machine Setup..
Name: GE Optima 660 – 128 slice Computed
Tomographic Scanner with slice thickness of
0.35mm with Advantage Workstation (AW)
Volume Share workstation.
It has got the Performix 40 tube, backed by a powerful 72-kW generator,
delivers peak mA capability of up to 600 mA.
The 40-mm wide V-Res detector (HiLight scintillator) acquires data at
0.35-mm microVoxel spatial resolution through GE innovations.
The Optima CT660 image chain is powered by the Volara XT Data
Acquisition System (DAS).

57. Performix 40 X ray Tube
Cone-beam filtered back projection utilizes two sets of counter-opposed
projections to provide 128 distinct projection measurements per rotation for axial
and a helical acquisition mode.
For cardiac acquisitions, fast rotation speed provides excellent temporal
resolution (44 msec).
The table allows patients as heavy as 500 lbs (226.796 kgs) to be imaged through a
long scannable range.

58. In computed tomography (CT), the term artifact is
applied to any systematic discrepancy between the CT
numbers in the reconstructed image and the true
attenuation coefficients of the object.
CT images are inherently more prone to artifacts than
conventional radiographs because the image is
reconstructed from something on the order of a
million independent detector measurements.
CT Scanner Artifacts

59. Types of artifact
1. Streaking, which is generally due to an inconsistency in a single
measurement
2. Shading, which is due to a group of channels or views deviating
gradually from the true measurement
3. Rings, which are due to errors in an individual detector
calibration
4. Distortion which is due to helical reconstruction.

60. ON BASIS OF ORIGIN
1. Physics-based artifacts, which result from the physical
processes involved in the acquisition of CT data
2. Patient-based artifacts (motion artifacts) which are caused
by such factors as patient movement or the presence of
metallic materials in or on the patient
3. Scanner-based artifacts, which result from imperfections in
scanner function
4. Helical and multi-section artifacts, which are produced by
the image reconstruction process.

61. BEAM HARDENING ARTIFACT
• can give a "cupped appearance" when grayscale is
visualized as height
• As heterogeneous/polychromatic X ray beam passes through
patient, low energy photons are absorbed
• the mean energy of the spectrum increases when
passing the object, often described as getting "harder"
• Linear attenuation coefficient is directly related to energy
• Linear attenuation coefficient of tissue near beam entry is
higher than that on exit side
• leads to an effect increasingly underestimating material
thickness

62. • Cause: presence of tissues with highly varying
absorbtion properties in a voxel.
• Rectification : Usage of Thinner CT slices
Partial Volume Averaging Artifact
Thick slice Thin slice

63. Motion Artifact
Streak Artifact
Ring Artifact