Basics of CT Physics

1. OSR
Dr. Yash Kumar Achantani

2. History
•In April 1972 , British engineer Godfrey N
Hounsfield of EMI laboratories , constructed
this first revolutionary new imaging technique
which he called as “Computerized axial
transverse scanning”.
•This machine scanned the first human head in
1972 at Atkinson morley hospital in London.
•In 1975 Hounsfield build a whole body CT
scanner.
•In 1979 Hounsfield and Cormack were both
awarded with the Nobel prize in physiology
and medicine.
•Computed tomography had many names of
which two of the most popular names are
•Computerized axial tomography (CAT) and
Computerized tomography (CT).

3. Basic Principle
•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 narrow x-ray beam and measuring the
transmitted radiation with a sensitive radiation detector.
•The detector adds up the energy of all the transmitted
photons.
•The numerical data from multiple ray sums are then processed
in computer to reconstruct an image.

5. 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 after passing
through the material and getting
absorbed by the detectors.

6. 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

7. First CT scanner

8. Development of CT
CT scanners have gone through a number of design changes
since the technology was first introduced and it can be
decribed as different generations of CT on the basis of
Scanning motions and No. of detectors used.
1. First generation(translate-rotate , one detector)
2. Second generation( translate-rotate, multiple detectors)
3. Third generation(rotate- rotate)
4. Fourth generation(rotate-fixed)
5. Fifth generation(CVCT scanner)

9. First generation CT
Also known as the original
EMI scanner.

10. Parts:
•X-ray tube. (Oil cooled stationary anode tube)
The x-ray beam employed in this scanner was a pencil like x-ray beam.
•Pair of collimators. (one near the detector and other near the tube)
•Reference detector.(Furnished exposure factor to compensate for
moment to moment variation in x-ray output)
•Water bath.(To facilitate data analysis within the computer)
•Slip rings.(to allow water bath to rotate independently around head)
•Paired Detectors.(Sodium scintillation crystals coupled to
photomultiplier tubes)

11. Motion of Gantry
The Gantry moved through two different types of motions .
1. Linear.
2. Rotary.
The Linear motion was repeated 180 times with each degree of
rotation.
The x-ray beam was on through out the linear motion and off
while off through out the rotary motion.
The transmitted radiation was measured 160 times during each
motion.

12. The total no. of transmission measurement was 160x180=
28,800 times.
Each section simultaneously examined two tomographic
sections one for each of the paired detectors.
The total scan time was 5 minutes for each pair of section.
And as many as 10 sections were required for each patient so
the total scan time was aroud 25-30 minutes.

14. The CT image is then reconstructed and displayed on a 80x80 matrix.
The image is displayed in two different formats
1. A paper printout of CT numbers (proportional to the Linear attenuation
coefficient).
2. A visual image on a cathode ray tube.
Each square in the matrix called as pixel and it represents the tiny
elongated block called voxel.
In the original EMI scanner the beam was collimated to 3x13 mm
(original length was 26 mm) i.e. 13mm for each one of the paired
detectors.
The CT number for each pixel represented the average attenuation
coefficient for all the elements in the block of tissue (voxel) 3x3x13mm in
size.
The size of the pixel is determined by the computer programme, while
the length of the voxel is determined by width of the x-ray beam.

16. Second Generation CT Scanner
•One of the major objective for development of the second and later
generations scanner was the shortening of the scan time for each
tomographic section.
•The increased speed of Scan was accompnished by using
Fan shaped beam.
Multiple detectors.(as many as 30)
•The movement of the x-ray tube-detector array are both linear and
rotary.
•Instead of moving 1 degree after each linear scan the gantry arcs 30
degree.
•So the linear motion has to be repeated only 6 times.
•Each tomographic section can be produced in 10-90 seconds
depending upon the mauufacturer.

19. Third Generation (Rotate-Rotate)
•Introduced in 1975 by General electric company.
•Used only rotation motion.( linear motion was completely eliminated)
•The scanning geometry came to be known as “Fan beam” geometry.
•Fan shaped beam is used along with multiple detectors that are arranged
along the arc of a circle whose centre in the x-ray tube focal spot and this
point approximately concides with the centre of the patient.
•The original rotate rotate scanners used 288 detectors but the newer ones
use over 700 detectors.
•The fan beam must completely cover the object to be imaged.
•In the original rotate rotate scanner the xray tube was pulsed but now the
tubes used are continuosly on.

20. •As many as thousand projections can be taken in less than one second and
which are altogther computed to produce a single image.
•And each projection is composed of many scan lines which is equal to the
no. Of detectors exposed.

22. Fourth Generation (Rotate-Fixed)
•The detectors do not move and form a ring that completely surrounds the
patient.
•The x-ray tube rotates in a circle inside the detector ring.
•The beam use in fan shaped.
•As many as 2000 detectors are used.
•When x-ray tube is at prescribed angles the detectors are read e.g. The
angular prjection rate of one projection per 1/3 degree will produce 1080
projections per 360 degree rotation(3x360).
•The x-ray tube used for this scanner is continuously on type.

23. •Both rotate-rotate and rotate fixed CT scanners gives excellent results
with no clear advantage of one over the other.
•The advantage of fan beam/multiple detector array is speed.
•The principal disadvantage is increased detection of scattered photons.

25. X-ray tube
•Earlier oil-cooled, fixed anode with
large focal spot was used.
•Now a days, Rotating anode type.
•More heat loading and heat
dissipation capabilities.
•Small focal spot size (0.6mm) to
improve spatial resolution.
•Anode heating capacity of
6.3million Heat units and cooling
rate of 840,000 heat units per min.

26. FILTERS
•To absorb low energy x rays.
•To reduce patient dose.
•To provide a more uniform beam to
reach the film.

27. COLLIMATORS
• To decrease scatter
radiation.
• To reduce patient dose .
• To improve image
quality.
• Collimator width
determines the slice
thickness(voxel length).

29. DETECTORS
The detectors gather information by measuring the x-ray
transmission through the patient.
Two types:
Scintillation crystal detector
Can be used in third and fourth generation scanners.
Xenon gas ionisation chamber
Can be used in third generation scanners only.

30. Scintillation Crystals
•These are the materials that produce light when ionizing radiation
reacts with them.
•The surafces of the crystals are polished to fecilitate extraction of
light so they look like a piece of clear glass or plexiglass.
•The no. of light photons produced is proportional to the energy of
incident x-ray photon.
•Some of the light photons are produced delayed and thus produce
the afterglow.
•All the crystals are matched with the light detectors that convert
the light output to an electrical signal. (Scintillation-Detector)

31. •The first two generations used thallium activated sodium iodide
crystals attached to Photomultiplier tubes.
•Problems with NaI ( hygroscopic, long afterglow).
•Poblem with photomultiplier tube is its large size.
•Photo multiplier tubes are replaced by Silicon Photo-diodes.
•Converts light energy to electric energy which is proportional to the
intensity of light signal received.
•Advantages are Smaller size, Greater Stability and Lower Cost.
•There are several possibilities of replacing NaI , Some of them are CsI,
BGO, CdWO4(most commonly used).
•Advantages: Afterglow is not a problem.
CT stopping power is almost 100%.(prevents Detector cross talk)

32. 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
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).

33. Xenon gas ionization chamber
Parts – Anode, Cathode, Inert gas contained in a tube and Window
for photons to enter.

34. Mechanism of xenon gas detector
 Photon enters detector and interacts with gas atom produces electron-ion
pair.
 Voltage bet cathode and anode moves the e- towards anode and positive ion
towards cathode.
 When e- moves near anode, small current produced which is the output
signal from detector.
 Gas filled detector may operate in one of the three modes which is
determined by the voltage.
1. Ionisation chamber – in low voltage – only e- moving towards anode is
collected.
Current is directly proportional to intensity of incoming radiation.

35. 2. Proportional counter – in high voltage – sufficient current is
there to produce secondary ionization of gas atoms.
Here output signal is proportional to the energy of the
photon.
3. Geiger counter – in very high voltage – secondary ionization is
so large that energy proportionality is lost and all photons
register the same pulse.
Large signals are easily recorded here.
• In CT – energy of photon is not required but x-ray beam
intensity is– so always work in ionization chamber level.

36. 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.

37. 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.
 In III gen CT, the X-ray tube and
detectors maintain a fixed
relationship, so the beam is always
aligned with the long axis of each
detector.
 In IV gen CT, angle at which X-rays hit
the detector changes constantly.
 Obliquely entering X-rays would pass
through only a short distance of gas
before they hit the wall of the detector.
 In such a case, the X-rays are absorbed
in the detector walls and the information
they carry in lost for all time.

38. Other Scan Configurations
The rotate rotate and rotate fixed scanners could not achieve scan time
less than one second due to machanical problem.
The desire for production of faster scan was due to the desire to image the
moving structures e.g. Walls in the heart, or contrast material in the
blood vessels.
This could be achieved by one of the following methods.
a. Use of multiple x-ray tubes e.g. Dynamic spatial reconstructor.
b. Eliminate all the motion of x-ray tube and detector e.g. CVCT
scanner.

39. 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 15 RPM.
•Data for an image acquired in about
16 ms.
•Reconstruct 250 C.S. images from
each scan data.
•Disadvantage - High Cost and
mechanical motion is not
eliminated

40. DYNAMIC SPATIAL RECONSTRUCTOR
•The DSR consists of a gantry weighing approximately 17 US tons with
a length of 20.5 feet and a diameter of 15 feet.
•The images produced on the fluorescent screen by the firing of the x-
ray guns are recorded.
•To improve the image quality, the video imaging chains have been
converted from image isocon cameras to charged coupled device
(CCD) cameras

41. DYNAMIC SPATIAL RECONSTRUCTOR

42. DYNAMIC SPATIAL RECONSTRUCTOR

43. Electron Beam Computed
Tomography(EBCT)
Also known as CVCT/ Ultrafast CT/ Milisecond CT/
Cine CT.

44. EBCT / CVCT
•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.

45. EBCT / Ultrafast CT
•Motion of the parts of the machine is completely eliminated.
•Electron gun (320cm long, 130keV).
•Focusing and deflecting coils.
•Four 180cm diameter tungsten target rings
•2 Rings of detectors – 432 detectors each.
•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 50 ms
•Without moving the patient, 8 continuous tomographic images
can be obtained.

47. 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.
•The vast amounts of data captured in a single breath-hold are
transferred at speeds of 200 Mbps to 1 Gbps 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 and virtual
colonoscopies .

48. Radiation dose with EBCT
•Unlike conventional scanners it doesn’t expose the entire
circumference of the body to the X-ray beam.
•EBCT 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.

50. Helical / Spiral CT
Pt table translates through the gantry as 3rd or 4th gen x-ray tube
rotates continuously.
This acquires volume of data for reconstruction purposes.
Advantages – Minimizes motion artifacts.
Reduced pt dose.
Improved resolution.
Multi planar reconstruction.
3D reconstruction.

51. Terms used in helical CT
•Acquisition – entire volume of data collected during a
continuous spiral scan.
•Revolution – no. of 360 rotation during single acquisition.
•Pitch – distance the couch travels during a 360⁰ rotation of x-
ray tube.
•Pitch factor – pitch divided by collimated slice
thickness/width of the beam.
•Interpolation – recon method- permits realignment of spiral
data of an axial slice.

52. •Helical scan reduce the pt dose and scan time which can be taken in
a single breath.
•The same slice thickness can be done in 50% greater table speed and
33% less radiation than in conventional CT using a pitch of 1.5.
•Continuous data acquisition improves spatial resolution of z-axis.
•MPR and 3D recon can be done.

53. Multi Detector CT
It uses 3rd gen mechanism only with added multiple rows of
detector arcs.
Equal width or variable width detectors are placed back to
back in z-axis according to manufacturer.
Advantages - It enhances z-axis resolution, speed of
anatomic coverage under scanning as large area can be
covered during each revolution of the gantry.

54. MDCT

56. Formation of CT images

57. Artifacts in CT
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 co-efficients 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.
Artifacts can seriously degrade the quality of computed tomographic (CT)
images, sometimes to the point of making them diagnostically unusable.
To optimize image quality, it is necessary to understand why artifacts occur and
how they can be prevented or suppressed.

58. There are mainly Four varieties of CT artefacts.
1. Physics-based artifacts.( result from the physical processes
involved in the acquisition of CT data)
2. Patient-based artifacts .(caused by such factors as patient
movement or the presence of metallic materials in or on the
patient)
3. Scanner-based artifacts.( result from imperfections in scanner
function)
4. Helical and multisection technique artifacts.(produced by the
image reconstruction process)

59. Physics-based Artifacts
Beam Hardening
I. Cupping Artifacts.
II. Streaks and Dark Bands.
Partial Volume
Photon Starvation
Undersampling

60. Beam Hardening
An x-ray beam is composed of individual photons with a range
of energies (polychromatic).
As the beam passes through an object, it becomes “harder”
that is to say its mean energy increases, because the lower
energy photons are absorbed more rapidly than the higher-
energy photons .

61. Cupping Artifacts

62. Streaks and Dark Bands
•In very heterogeneous cross sections,
dark bands or streaks can appear
between two dense objects in an image.
•Occur because the portion of the beam
that passes through one of the objects at
certain tube positions is hardened less
than when it passes through both
objects at other tube positions.
•Can occur both in bony regions of the
body and in scans where a contrast
medium has been used.

63. Measures For Minimizing Beam
Hardening
1. Filtration: Using “bowtie” filter.
2. Calibration correction. (Manufacturers calibrate their
scanners using phantoms in a range of sizes. This allows the
detectors to be calibrated with compensation tailored for the
beam hardening effects of different parts of the patient)
3. Beam hardening correction software. (An iterative
correction algorithm may be applied when images of bony
regions are being reconstructed)
4. Avoidance of Beam Hardening by the Operator. (Either by
means of patient positioning or by tilting the gantry)

65. Partial Volume

66. Photon Starvation
A potential source of serious streaking artifacts is photon starvation, which
can occur in highly attenuating areas such as the shoulders .
When the x-ray beam is travelling horizontally, the attenuation is greatest and
insufficient photons reach the detectors.
The result is that very noisy
projections are produced at these tube
angulations.
The reconstruction process has the
effect of greatly magnifying the noise,
resulting in horizontal streaks in the
image.

67. If the tube current is increased for the duration of the scan, the problem of
photon starvation will be overcome, but the patient will receive an
unnecessary dose when the beam is passing through less attenuating parts.
Therefore, manufacturers have developed techniques for minimizing
photon starvation.
1. Automatic Tube Current Modulation. The tube current is
automatically varied during the course of each rotation, a process
known as “milliamperage modulation”.
This allows sufficient photons to pass through the widest parts of the
patient without unnecessary dose to the narrower parts)
2. Adaptive Filtration. (Software correction smoothens the attenuation
profile in areas of high attenuation before the image is reconstructed)

69. Undersampling
View aliasing can be minimized by
acquiring the largest possible number of
projections per rotation. On some
scanners, this can be achieved only by
using a slower rotation speed, while on
others the number of projections is
independent of rotation speed.
Ray aliasing can be reduced by using
specialized high-resolution techniques,
which manufacturers employ to
increase the number of samples within
a projection.

70. Patient-based Artifacts
Metallic Materials. The presence of metal objects in the scan field
can lead to severe streaking artifacts. They occur because the
density of the metal is beyond the normal range that can be handled
by the computer, resulting in incomplete attenuation profiles.
Additional artifacts due to beam hardening, partial volume, and
aliasing are likely to compound the problem when scanning very
dense objects.
Patient Motion. Patient motion can cause misregistration artifacts,
which usually appear as shading or streaking in the reconstructed
image .

71. Metallic Materials
Special
software
correction
along with
correction of
beam
hardening
must be used
to overcome
this artefact.

72. Patient Motion
Avoidance of Motion Artifacts by the
Operator.
1. Sedation.
2. Using as short a scan time as possible.
3.Hold breath for the duration of the
scan.
Built-in Features for Minimizing
Motion Artifacts.
1. overscan and underscan modes.
2. software correction.
3. cardiac gating.

74. Scanner-based Artifacts
Ring Artifacts
If one of the detectors is out of calibration on a third-generation
(rotating x-ray tube and detector assembly) scanner, the detector will
give a consistently erroneous reading at each angular position, resulting
in a circular artifact .
A scanner with solid-state detectors, where all the detectors are separate
entities, is in principle more susceptible to ring artifacts than a scanner
with gas detectors, in which the detector array consists of a single xenon-
filled chamber subdivided by electrodes.

75. Ring Artifacts
•Problem can be solved by recalibration or repair of the detector.

76. Helical and MultisectionCT Artifacts
Helical Artifacts in the Axial Plane: Single-Section Scanning.
Helical Artifacts in Multisection Scanning.
Cone Beam Effect.
Multiplanar and Three dimensional Reformation.
1. Stair Step Artifacts
2. Zebra Artifacts

77. Helical Artifacts in the Axial Plane:
Single-Section Scanning
In general, the same artifacts are seen in helical scanning as in sequential
scanning. However, there are additional artifacts that can occur in helical
scanning due to the helical interpolation and reconstruction process. The
artifacts occur when anatomic structures change rapidly in the z direction
(eg, at the top of the skull) and are worse for higher pitches.
To keep helical artifacts to a minimum, steps must be taken to reduce the
effects of variation along the z axis. This means using, a low pitch, and thin
acquisition sections rather than thick. Sometimes, it is still preferable to
use axial rather than helical imaging to avoid helical artifacts (eg, in brain
scanning).

79. Helical Artifacts in
Multisection Scanning
The helical interpolation process leads to a more complicated form of axial
image distortion on multisection scanners than is seen on single-section
scanners.
The typical windmill-like appearance of such artifacts is due to the fact
that several rows of detectors intersect the plane of reconstruction during
the course of each rotation.
As helical pitch increases, the number of detector rows intersecting the
image plane per rotation increases and the number of “vanes” in the
windmill artifact increases.

81. Cone Beam Effect
As the number of sections acquired per rotation increases, a wider collimation is
required and the x-ray beam becomes cone-shaped rather than fan shaped.
As the tube and detectors rotate around the patient , the data collected by each
detector correspond to a volume contained between two cones, instead of the
ideal flat plane. This leads to artifacts similar to those caused by partial volume
around off-axis objects.
The artifacts are more pronounced for the outer detector rows than for the inner
ones , where the data collected correspond more closely to a plane.
Cone beam effects get worse for increasing numbers of detector rows. Thus, 16-
section scanners should potentially be more badly affected by artifacts than four-
section scanners

82. Manufacturers have addressed the problem by employing various
forms of cone beam reconstruction instead of the standard
reconstruction techniques used on four-section scanners.

83. Multiplanar and Three dimensional
Reformation
1. Stair Step Artifacts.
2. Zebra Artifacts.
Major improvements in multiplanar and three-dimensional reformation
have come about since the introduction of helical scanning and ,to an even
greater extent,with multisection scanning. The faster speed with which the
required volume can be scanned means that the effects of patient motion
are much reduced,and the use of narrower acquisition sections and
overlapping reconstructed sections leads to sharper edge definition on
reformatted images.

84. Stair Step Artifacts
Stair step artifacts appear around the edges of structures in multiplanar
and three-dimensional reformatted images when wide collimations and
non overlapping reconstruction intervals are used.
They are less severe with helical scanning, which permits reconstruction of
overlapping sections without the extra dose to the patient that would occur
if overlapping axial scans were obtained.
Stairstep artifacts are virtually eliminated in multiplanar and three-
dimensional reformatted images from thin-section data obtained with
today’s multi sections canners.

86. Zebra Artifacts
Faint stripes may be apparent in multiplanar and three-dimensional
reformatted images from helical data because the helical interpolation
process gives rise to a degree of noise inhomogeneity along the z axis.
This “zebra” effect becomes more pronounced away from the axis of
rotation because the noise inhomogeneity is worse off-axis.