The document details the operation and configuration of CT scanners, explaining the process of image acquisition, reconstruction, and storage. It outlines the essential hardware components, including the system console, CPU, memory, and various x-ray tube features, and discusses advancements in tube design for improved heat capacity and scanning speed. Additionally, it highlights the significance of collimators in managing radiation exposure and maintaining image quality.

1. CT INSTRUMENTATION
AND DETECTOR
CONFIGURATION
Anjan Dangal
B.Sc.MIT 3rd Yr
National Academy of Medical Sciences

2. CT Scanner Operation
CT scanners obtain
individual, cross-
sectional “slices” by
utilizing thinly collimated
x-ray beams taken at
different angles around
the circumference of the
same region and
reconstructing images
through the use of
computer technology.

4. CT Scan Operation is a
Multi level Process
Data Acquisition
Image Reconstruction
Display
Storage

5. Basic Components of CT
Scanning System
May vary but always same
Requires 2 Room:
1. Controller Room ( Free of Radiation )
2. Scanner Room : Patient Undergoes CT Procedure

7. Computer
Performs Varieties of Task
Store and Retrive Highly Accelerated Data
Distinguisable work : Reconstruction Process

8. Computer System have many Hardware that Execute Software
Kept in controlled Temperature and Humidity : Protection
Five Major Hardware components are :
• System console (or input device)
• Central processing unit ([CPU]/image reconstruction system [IRS])
• Internal memory (or hard drive)
• Output device
• External memory (or storage unit)

9. COMPUTER
SYSTEM

10. System Console
• Entire Instruction
• Input Scan Parameter, patient information,
instruction for post processing, filming, archiving
and networking

11. Central Array Processing Unit/
Image Reconstruction system
• Raw data received and reconstruction by algorithm
• Once processed, images are sent to an output device

12. Software
Operating Software: Permanent Software
Application Software: Specific task

13. Liquid Crystal Display
shift from CRT to liquid crystal display (LCD) monitors
because of their low cost, compact size, low power
consumption, good color representation, excellent
image resolution, and luminance (brightness).
Resolution of the monitors can vary from 1200x1600
pixels to 1536x2048 pixels

14. Memory
Internal Memory:
ROM : long term storage of data such as OS
RAM : Temporary Storage such as data acquired from
single CT Scan

15. Block
diagram of
a typical
CT system.
CONSOLE
HOST COMPUTER
SCAN CONTROLLER
DAC
GANTRY CONTROL
PATIENT
HVG
ARRAY
PROCESOR
STORAGE

16. Host Computer
has only the ability to initiate the scan by turning on the x-ray tube
contains software instructions that control data acquisition,
processing, and display
The host computer ultimately controls the functions of all of the CT
components, including the rotation speed of the x-ray tube in the
gantry, radiation dose, scan range, reconstruction and display field
of view, and reconstruction type (algorithm).

17. Scan Controller
The host computer issues instructions to the scan
controller, which in turn regulates the operation and
timing of the patient table, the gantry, and the high-
voltage generator.

18. Digital to Analog
Converter ( DAC )
Before the instructions from the scan controller (in the
form of signals) can be interpreted, the signals must be
converted into a continuous analog waveform signal.
This conversion is performed by the DAC located
between the scan controller and the gantry.

19. High Voltage Generator (HVG)
The high-voltage generator is the
unit that generates the voltage
necessary to produce x-rays.
Depending on the generation of the
scanner, the HVG may be located
either inside or outside the gantry.

20. Transformer
The transformer, another form of HVG, provides the CT
units the necessary power to operate. The supplied voltage
from the power company is usually less than what the CT
scanner needs to operate, so the transformer will usually
be a step-up type (versus a step-down transformer, which
reduces the voltage) that amplifies the voltage, giving the
tube the necessary power it needs to generate x-rays. In
other words, a transformer changes one voltage to
another.
However, a transformer does not change power levels. If
100 watts is supplied to a transformer, 100 watts will come
out the other end

21. Amplifier
The amplifier is where signals go
immediately after leaving the
gantry. These analog signals are
amplified without conversion and
sent on to the sample/hold unit.

22. Sample/ Hold
The sample/hold is located between the amplifier and
the analog-to-digital converter (ADC).
This is where the area of interest is sampled to
differentiate among structures with various densities
and then assigned various shades of gray to the pixels
to represent those structures.
The S/H determines the relative attenuation of the
beam by the tissues that were scanned.

23. Analog to Digital Converter
The ADC is the unit that takes the analog signal output by the
scanning equipment and converts it into a digital signal that can
be understood and analyzed by the computer.
The unit has three major components to perform three separate
operations: sampling, quantization, and coding.
In sampling, the system takes portions, or samples, of the
continuous analog signal.
The quantizing component processes the sample data into
discrete-time, discrete-value signals.
Next, the digital signal is coded with a specific binary bit
sequence correlating to each sample of output from the
quantizer.

24. Array Processor
The array processor is a specialized high-speed computer that takes
hundreds of detector measurements from hundreds of different
projections and pieces them back together at a very high rate of speed.
The array processor is also responsible for retrospective reconstruction
and post processing of image data.
The number of array processors affects the image reconstruction time,
which is the time it takes to reconstruct the images in the dataset.

25. Scanner Room
Gantry
Table

26. Patient Table / Couch

27. support and transport device for the patient throughout the
scanning procedure
generally curved; flat tables for radiotherapy planning
couches allow for scanning from the head to the mid-thigh with
movement in 1mm increments.
maximum weight recommendation of 400–485 lbs ( 181- 220 KG );
bariatric tables can hold up to 660 lbs

28. EXTERNAL APPEARANCE
1.GANTRY APPERTURE
2.MICROPHONE
3.SAGITTAL LASER LIGHT
ALIGNMENT
4.PATIENT GUIDE TABLE
5.X-RAY EXPOSURE INDICATOR
LIGHT
6.EMERGENCY STOP BUTTOM
7.GANTRY CONTROL PANNEL
8.EXTERNAL LASER ALIGNMENT
LIGHT
9.PATIENT COUCH
10.ECG GATING MONITOR

29. INTERNAL APPEARANCE1.X-RAY TUBE
2.FILTER,COLLIMATOR,REFERENCE DETECTOR
3.INTERNAL PROJECTOR
4.OIL COOLER
5.HVG (0-75 KV)
6DIRECT DRIVE GANTRY MOTOR
7.ROTATION CONTROL UNIT
8.DAS
9.DETECTOR
10.SLIP RING
11.DETECTOR COTROL UNIT
12.HVG (75-150 KV)
13.POWER UNIT
14.LINE NOISE FILTER

30. Gantry
All of the components for
generating x-rays, moving
the tube around the
patient, collimating to the
appropriate protocol-driven
slice thickness, and the
detectors are housed in a
unit called the gantry

31. Feature
• Round Aperture of about 50-85 cm ( 70 cm )
• allow room for insertion of both the patient and the table
• may need to be tilted to protect, for example, the patient’s eyes
on a head scan.
• tilt to up to a ± 30° angle to adjust positioning
• Newer CT scan gantry tilt may or may not be available
• Today there are systems that utilize an adjustable head holder,
tilting various degrees to compensate for gantry angulation

32. Positioning laser
mounted to the
gantry are arranged
in sets of three to
help the
technologist align
the patient properly.

33. Plane Specification
• x and y axes are positional coordinates that involve centering the
patient within the gantry correctly
• x-axis is the lateral or horizontal plane
• y-axis is the vertical or AP/PA plane
• Ideally, the anatomy of interest should be centered correctly for
imaging and technique purposes. In terms of imaging, the anatomy
should appear in the middle of the display field of view (DFOV); in
terms of technique, the dose modulation will be correctly calculated
when the area of interest is in the center of the scan field of view
(SFOV)

34. zY
X

35. X ray TubeVaccum Tube that produces x ray.
Composed of Cathode ( filament ) and anode ( target ).
Cathode is negatively charged and incorporated a wound filament that produces
electron when heated .
The rotating anodes are usually made of rhenium, tungsten, gadolinium, and a
molybdenum alloy, utilizing a small target angle (focal spot) of about 7° and a rotation
speed of 3,600 to 10,000 rotations per minute (rpm). The target angle is fixed
Anode disc is supported on long stem that is supported by ball bearing within the
tube.
Anode can be rotated by electromagnetic induction from the series of stator windings
located outside tube.
it is impractical to pass a rotating shaft through a high-vacuum seal, all of the rotating
parts of the tube are located and sealed inside the vacuum tube envelope. The
rotating parts consist of the anode target connected to a molybdenum shaft that is
surrounded by a rotor. A bearing is also located inside the vacuum so that the anode,
shaft, and rotor can rotate freely inside the tube envelope. A stator is placed outside
the envelope to provide an alternating magnetic field that causes the rotator
assembly to rotate.

36. it is impractical to pass a rotating shaft through a high-vacuum seal, all of the
rotating parts of the tube are located and sealed inside the vacuum tube envelope.
The rotating parts consist of the anode target connected to a molybdenum shaft
that is surrounded by a rotor. A bearing is also located inside the vacuum so that
the anode, shaft, and rotor can rotate freely inside the tube envelope. A stator is
placed outside the envelope to provide an alternating magnetic field that causes
the rotator assembly to rotate.
X ray tube is enclosed in a housing unit called insulating oil .
This Oil provides electrical insulation from tube voltage and transmit the heat
generated in the housing unit to the unit’s surface .
The exterior of housing unit is cooled with a fan and insulating oil is cooled by
passing it through a heat exchanger.
use of the CT tube over the period of scanning results in severe heating. Tube must
posses high heat capacity: the amount of heat that tube can store without
operation damage to tube.
modern CT tube requires high anode heat storage capacity of 2–5 million heat units
with rapid dissipation (400,000 heat units/m

37. CT Tube utilize bigger filament than conventional
Radiography = Increased focal spot size
CT can compensate for loss of resolution due to larger
focal spot sizes by employing resolution enhancement
algorithm such as bone or sharp algorithm, targeting
techniques and decrease section thickness
Modern tubes will last for 150,000–200,000 slices.
Current systems utilize a “moving” focal spot, allowing the
accumulation of twice the information obtained in a single
rotation

38. Early Days = Early days Pulsed X ray tube used i.e X rays produced in short
duration of time
Pulse Time varies between 1 and 4 ms.
Non Operating time was 10 – 14ms as no x-ray photons were emitted
because x-ray detectors could not take measurements while the signals
were sampled
For high-speed scanning, it is necessary to have continuous flux of x rays

39. Design:dates backto 1913 when Coolidge builtthefirst high-voltage tube
The glass frame is a composite
of several types of glass.
The main section is a
borosilicate glass with good
thermal and electrical
insulation properties.
The thickness of the glass is
typically between 0.18 and
0.30 mm

40. In more advanced tube designs, the glass
frames are replaced with metal frames
advantage :
being able to operate at or near ground
potential to improve the efficiency of the
motor that drives the anode assembly.
Another advantage of the metal frame is
the reduced spacing between the frame
and anode, which accommodates a
larger sized anode without significantly
increasing the size of the tube envelope.
The metal case can also collect off-target
backscattered electrons.

41. Anode angle 7 degree
For CT scanners with a small z coverage, this effect can be
safely ignored because the coverage along the patient long axis
(perpendicular to
the CT gantry plane) is relatively smaller. As the volume coverage increases,
however, this issue can no longer be ignored and must be properly
addressed.

42. Traditionally, the tube target is made of a molybdenum alloy and the
focal track consists of a layer of tungsten-rhenium. The advantage of this
design is the quick heat transfer from the focal track to the bulk of the
target.
introduction of helical/spiral CT and faster scan speeds, this design no
longer offers the required heat capacity. The newer design uses brazed
graphite in which the metal target (similar to the traditional design) is
brazed to a graphite body to increase the heat storage per unit weight.
This design combines the good focal track characteristics of the metal
tube with the heat storage capability of graphite
An alternative design uses chemical vapor deposition todeposit a thin
layer of tungsten-rhenium on a high-purity graphite target. advantages
of this approach are the light target weight and high heat
storagecapacity. The disadvantages are higher cost, increased potential
for particles, and limited focal spot loading.

43. Straton
Tube

44. Straton is the name of the Novel, directly cooled x ray tube proprietary to
Siemens
Gantry Rotation time is Below 4sec and allows virtually unlimited volume
coverage at maximum speed without compromise in resolution and
image quality
Directly Anode cooling enables extremely high cooling rate and
eliminates need for high Anode heat storage capacity
Cooling delays eliminated even for large patients
Robust Design even at high G forces
Anode and tube assembly much smaller than conventional design

45. Straton : Revolutionary Tube technology

46. Conventional Tube : Entire Anode Including the bearing is inside vaccum so
cannot be reached effectively by cooling fluid
So cooling rate is rather slow
Anode Heat storage of upto 8MHU available but ……………………………….
Overheated = penalty= 5 to 10 min to cool
New Straton x ray tube allows directly cooling of anode with bearing located
outside the vaccum
Electron Beam is shaped and deflected by electromagnetic field
Results is cooling of anode at rate of 4.7 MHU/min
Eliminated the need of anode heat storage capacity ? 0 MHU. However, a
small amount is still needed for the anode to act as a heat spreader from
thearea of the focal spot to the large area of the cooling surface.
Max Load = 20 sec only to cool

47. the rotation of the anode serves a dual purpose:
• inside of the tube the rotation maintains a tolerable focal spot
temperature rise,
• outside of the tube the rotation causes turbulent oil flow to maintain
large heat transfer coefficients.

48. Electron Beam Dynamics
• Electron Beam reflection and focusing
• achieve an x-ray focal spot fixed in space while the whole tube is
rotating, a stationary beam deflection system is necessary
• this is achieved by a dipole magnet system deflecting the electron
beam generated on the tube axis such that the beam hits the
anode disk exactly at the focal spot position of the tube housing
assembly
• Used Kvp : 40 – 150 kvp , elctrons speed 50% of light

49. iMRC X ray Tube

50. WHY It developed ?Increase of gantry Speed has made need of High Power Density in
Focal Spot
Requires Enhanced Track speed of rotating Anode
Anode has to made it condition under high thermal condition and
centrifugal forces upto 32 G
Electron Emission capacity should also rise but must have longer life
Light Weight design is appreciated to reduce mechanical distortion

51. Tube Design
A novel X-ray tube is presented, which combines the high focusing
capability of a double quadrupole magnetic cathode system with
high heat conduction capability of the
anode, high mechanical integrity and long life of an segmented
anode which spins on a film of liquid metal in a hydrodynamic
bearing.

52. Novel Philips iMRC X-ray tube for computed tomography

53. The cathode consists of a flat thermo-ionic electron emitter and
a magnetic quadrupole focusing system with a high focusing
factor, which delivers a very high focal spot current density even
at low tube voltage.
By collecting all the scattered electrons, an electron trap pulls
40% of the primary energy off the rotating anode.
The anode disc is segmented to withstand thermal stress and
supported by a dual suspended hydrodynamic bearing. The
bearing takes care of a high conductive heat dissipation and is
wear-free even at high rotor speed and 32 g centrifugal
acceleration.

54. • Greatly improved electron gun: Comparatively large and flat 7 mm x 7 mm
tungsten foil electron emitter, which replaces a conventional W-coil
Adapted grain structure in the emitting foil for stability against centrifugal
forces
• Paired quadrupole focusing system: Focusing factor enhanced from ca. 3
with conventional electrostatic focusing to ca. 50 (area ratio of emitting
surface to focal spot)
• Current density of more than 1000 mA/mm2 in the focal spot: Enhanced
focal spot power density, 120 kW for a 1.3 mm (width) x 1,4 mm (length)
focal spot with 8° anode angle
• Reduced space charge effects. Tube current in medical Computed
Tomography almost independent of the tube voltage (80 kV up to 140 kV,
high tube current of 1000 mA even at low tube voltages.
• Direct cooled all metal segmented high speed anode with enhanced focal
track speed (ca. 100 m/s). Almost complete heat removal between
patients.
• Noiseless and improved heat conducting hydrodynamic bearing. No wear
during operation, even at large centrifugal forces

55.  Unipolarhighvoltage supply,therefore:
1. Almost no off focal X-raysusinga scattered electron trapfor
improved image noise
2. 40% less anodeheating comparedwitha glass tube
3. Reducedenvironmental impact,less weight,
4. No lead, noberyllium
5. Improved tubelife time

56. FILTARTION
oAbsorbs soft, low energy x-rays. Make uniformity
of x-ray spectrum
oReduces scatter, reduce patient dose
oImprove image quality
o3 mm Al equivalent thickness
oflat: copper or aluminum
oComb shaped/ bow-tie: Teflon (a material with
low atomic number and high density)

57. Collimatorlimit the amount of radiation exposure
in CT scanning serves the dual function of reducing
scatter radiation and improving image contrast
main purpose is to ensure that a consistent beam
width is received by the detectors.

58. • Set at pre-patient and post patient level
• Pre patient collimators are located
adjacent to the x-ray beam as it exits the
tube to control its shape.
• Post patient (or detector) collimators
reduce the amount of scatter radiation
occurring after the beam has passed
through the patient.

59. Both the size of the collimators and the geometry
between them define the width of the slice of the scan
Geometry of collimators/detectors used to determine scan slice width. The
presence of a detector collimator will positively influence the slice profile, shown
in solid lines. The dose profile, shown in dotted lines, will remain wide, however,
and exceed the slice profile.

60. Detectors
analog signals must be converted to digital signals
detectors recognize and measure the ionizing output from incidental x-
rays that pass through the patient and transform them into digital
electrical signals
An air-reference detector, also located in the gantry, The measurement
value of the intensity is as a reference point for reconstruction of the
image detectors must be very small and uniform for placement tight
alignment is very important for the prevention of artifacts

61. DETECTOR EFFICIENCY
Capture efficiency
the efficiency with which the detectors gather the photons exiting from the patient
Absorption efficiency
the number of photons absorbed by the detectors
Stability
the consistency of detector’s response
Response time
the speed at which the detector records an event and recovers for the next event
Dynamic range
the accuracy in the response to both low and high levels of transmitted radiation
Reproducibility
the consistency of responses to similar levels of transmitted radiation

62. DETCTOR SYSTEM
GAS ION DETECTOR
SCINTILLATOR DETECTOR

63. Solid State Scintillator Detector

64. utilize a solid state scintillation crystal mounted beside a
photomultiplier tube benefit of is their increased sensitivity.
need less frequent calibration than do gas-filled detectors
The cost is higher than for gas, but there are advantages: increased
efficiency reduces the need for tube loading and is associated with
lower patient dose, reduced noise on the image, and a longer tube life
sodium iodide scintillation crystals are nearly perfect in their x-ray
efficiency, they produce a lag or afterglow,
bismuth germanate, cesium iodine, or cadmium tungstate because
these significantly reduce afterglow.
crystal lattice of the rare earth compound gadolinium oxysulfide (GOS)
has been produced that has a large x-ray absorption coefficient and
fast decay so, reacts very rapidly to changes in x-ray intensity.

65. Xenon gas ionization
chamber

66. xenon gas detectors measure ionization in the
air by collecting electrons into individual
chambers separated by tungsten plates
As x-rays fall into the individual gas-filled chambers, they release ions
that are either positively or negatively charged that are then attracted to
the oppositely charged plates.

67. WHY XENON GAS ?
heaviest of inert gases, which improves the efficiency of gas ionization chambers
constancy of the pressure of xenon gas in the chambers contributes to the
improved sensitivity of the detector
Xenon has a fast rate of decay, reducing the potential for afterglow and thereby
reducing the chance for artifacts
Diasadv: less efficient than solid state or scintillation detectors. Xenon gas atoms
are widely spread and therefore must be contained in a highly pressurized, long,
narrow air-tight chamber for x-rays to collide with the gas atoms. It is the length of
the chambers that limit use.
QDE is only 50 – 60 percent.
Can be used for 3rd generation scanner only.

68. The relationship between the
arrangement of the tube, the beam
shape, and the detectors is referred
to as the detector geometry

70. SLIP RING TECHNOLOGY
• Electromechanical devices consisting of circular electrical conductive
rings and brushes transmit electrical energy across a rotating interface
• Allows spiral/helical scanning
• Conducts electrical energy across a rotating cylindrical surface
• Utilize brushes made of a conductive material such as silver graphite alloy
that slide along grooves in the ring to maintain conduction
• Gantry rotates continuously, accelerating data collection
• Dynamic CT of moving organs

71. TWO DESIGNS
DISC DESIGN CYLINDRICAL DESIGN

72. DISC DESIGN
oConductive rings forms concentric circles in the plane of rotation
Conductive rings carries voltage to components such as generator,
x-ray tube and collimator
CYLINDRICAL DESIGN
oConductive rings positioned along the x axis of rotation to form a
cylinder
oThe brushes that transmit electrical power to the CT components glide
in contact grooves on the stationary slip rings

73. HIGH VOLTAGE SLIP RINGS
LOW VOLTAGE SLIP RINGS
POWER SUPPLY

74. LOW VOLTAGE SLIP RING
oIn a low voltage slip ring system AC power and x-ray control
signal are transmitted to slip rings by means of low voltage
brushes that glide in contact groove on the stationary slip ring
oSlip rings then provide power to the high voltage transformer
which subsequently transmit to x-ray tube
AC SLIP RING
HV
GENERATOR
X-RAY
TUBE

75. HIGH VOLTAGE SLIP RING
oIn high voltage slip ring supply AC delivers power to the
high voltage generator which subsequently supply high
voltage to the slip ring
oHigh voltage from slip ring transmitted to x-ray tube. In
this , high voltage generator does not rotate with the x-ray
tube
AC
HV
GENERATOR
SLIP RING X-RAY
TUBE

77.  Most Manufacturers share common detector design
 3 essential layer :
• Scintillator ( Converts X rays to light )
• Photodiode ( Light to current )
• Substrate ( Mechanical and Electrical Infrastructure )

78. Schematic description of a CT detector layers. The X-ray illuminates a scintillator layer that converts X-ray into optical
photons. The optical photons are converted into electric current in a Photodiode layer. The substrate keeps the structure flat
and move the signals to the analog electronics to amplify. The amplified signal is digitized and transmitted from the detector
system to acquisition circuitry to generate images
ELECTRICAL SIGNAL
Scintillator Layer
Photodiode
Substrate

79. Detectors span roughly one meter with varying axial
coverage in order to image most of the population
Philips 8-cm data measurement system. The system comprises of 42 modules. Each module is built out of detector array
(photo courtesy of Philips Healthcare)

80. A 64-slice scintillator array. The scintillator (yellowish
material) is segmented by a reflector (white strips). The reflector’s
primary function is to keep the light generated in a detector element
within that element to minimize cross-talk

81. Multi-slice segmented photodiode

82. Scintillators for CT

83. Scintillator
materials emitting visible light when energy is deposited in the material, e.g.
by X-rays or energetic charged particles are known as scintillators
For efficient detection Inorganic compunds such as cadmium tungstate
(CdWO4) have been used the typical
thickness of a scintillator can range from 0.3 to 2.5 mm
the fast gantry rotation and the large number of projections demand signal
integration times as low as 100 μs.
cadmium tungstate (CdWO4) or gadolinium oxysulfide (GOS, Gd2O2S) are
Chosen . CdWO4 has a light yield of 20 photons per keV, whereas GOS,
depending on the doping, reaches 35-60 photons per keV

84. Thescintillatorcrystalsaremachinedintosmallpiecesofabout2-5 mm³,whicharethen
mountednexttoeachotherwithreflectivematerialbetweenthemtoavoidlateral
cross-talk ofthescintillationlightbetweentheindividualcrystals

85. Requirement
high light output (accounting for X-ray conversion efficiency and optical
transparency),
high X-ray stopping power,
good spectral match with the photo-detector,
short primary decay time (up to tens of ls),
low afterglow,
radiation damage resistance,
light-output stability (time, temperature),
compact
packaging, and
easy machining.

86. These demanding CT requirements make single crystals
and polycrystalline ceramics the most suitable
• CdWO4 ,
• Gd2O2S:Pr,Ce (GOS) ,
• (Y,Gd)2O3:Eu ,
• recently the GE Gemstone

87. Scintillators for multi-slice CT geometry are made in two-dimensional (2D) arrays,
with a typical pixel size of ~ 1 mm .
The arrays packaging also includes a reflective material matrix, typically consisting of
a mixture of a high reflectance pigment (e.g., TiO2) and a binder (e.g., optical epoxy),
or a certain multi-layer structure (e.g., sputtered silver on a polymer).
very promising group of materials is the garnet of the type (Lu,Gd,Y, Tb)3
(Ga,Al)5O12.available both in single crystal and polycrystalline-ceramics forms, offer
superior transparency, increased light yield, very short decay times,
The GE GemstoneTM has been the first garnet-scintillator introduced commercially
for CT detection.
evaluated for the Philips dual-layer detectors, are low-Z scintillators such as ZnSe:Te,
used for detecting the low-energy part X-ray spectra. Additional materials with
potential implementation in CT are halide ‘‘super-bright’’ scintillators, for example
SrI2:Eu, the light yield of which is reported to exceed 90,000 photons/MeV

88. Antiscatter Grid
only a small portion of the incident X-ray radiation is directly
absorbed by the photo-electric, majority is Compton and coherent
effect
main contributor to the scattered radiation-related artifacts and
image quality degradation is multiple Compton scattering effect
Provided that the ratio of scattered to direct photons is sufficiently
low, the image quality is not much affected. With increasing scatter to
primary ratio (SPR), image artifacts emerge, mainly in the form of
cupping, streaks and degredation in Image quality, low SNR and CT
Number Shifts .

89. a key solution for effectively reducing scattered radiation are anti-
scatter grids (ASGs) used as collimators in front of detectors , enabling
scatter reduction by over a factor of 10 . Both 1D and 2D ASGs are used
in CT scanners ; 2D ASGs generally reduce more effectively scattering
especially for scanners with a large axial collimation.
Example of a 1D AS grid, b molybdenum-based 2D ASG
and , c tungsten-based 2D ASG connected to a Philips tile-based detector. All images are courtesy of Philips
Healthcare

90. Detectors Configuration
Increase Number of Slices
Increased Speed of Acquisition
Dose Reduction

91. • detector refers to a single element or a single type of detector
• detector array =entire collection of detectors
• X ray Quanta = Electrical Signals

92. Single Detector Row System
In third generation systems approximately 700 detector elements were
arranged in an arc;
fourth-generation systems used as many as 4,800 detectors in a single
row arranged in a complete ring.
detector element is quite wide in the z direction ( approx. 15mm)

93. opening or closing
the collimator
controls the slice
thickness by
controlling the
portion of the
detector’s width
that is exposed to
the incoming x-rays

94. Multi detector Row System
provides longer and faster z axis coverage
per gantry rotation as it could produce multiple slices in single rotation
Speed of gantry rotation increases volume coverage per unit time
Slice thickness is determined by a combination of the x-ray beam width
(controlled by the collimators) and the detector configuration
Depending on manufacturer they are of different types

95. Parallel Rows of Equal Size is referred as
Uniform Array
Isotropic having
Equal dimension

96. Parallel Rows with variable width
detector are called Adaptive Array
Anisotropic

97. Thinner Rows Centrally and Wider Rows
Peripherally: Hybrid Array

98. Slice thickness of an MDCT scanner is not
determined solely by the degree of
physical collimation of the x-ray beam as
is the case with SDCT, but is also
impacted by the width of the detectors
in the slice thickness (z axis) dimension.

103. Dual Layer
Detector
through two attached
scintillator layers, optically
separated, and read by a side-
looking, edge-on, silicon
photodiode, thin enough to
maintain the same detector
pitch and geometrical efficiency
as a conventional CT detector

104. The top scintillator layer’s atomic number and thickness
have been optimized to maximize energy separation at 140
kVp, while maintaining high enough signal statistics
for the low-energy raw data even for a large patient. ZnSe
advantage in light yield [39] (*70 % better than GOS)
contributes to a high SNR in the top (low-energy) layer
detector, enabling it to function at very low dose without
causing artifacts, typical to electronic-noise dominant

105. Stellar Detector- Fully Integrated
Detector with Reduced Electronic
Noise and High Dynamic Range
First fully Integrated detector : Photodiode and ADC have
been integrated next to each other reducing the path of
signal . Transfer of Digital Signal is done without any losses
and the electronic noise produced by the detector is
reduced by the factor of two

106. References
CT For Technologist
Physics of Medical Imaging , Bushberg