Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx

Physical principle of Computed Tomography
(Scanning principle & Data acquisition)
Presented by :
M.Naga Theja
3rd Semester,M.Sc.Radio-Imaging Technology
Reg.No.: 210513003

Table of content
● What is Computed tomography ?
● History of CT
● Components of CT equipment
● Principle of CT
● Data acquisition in CT
● Questions
● Reference

What is Computed tomography ???
● “Computed Tomography,” or CT, refers to a
computerized x-ray imaging procedure in which a
narrow beam of x-rays is aimed at a patient and
quickly rotated around the body, producing signals
that are processed by the machine’s computer to
generate cross-sectional images, or “slices.”
● These slices are called tomographic images and can
give a clinician more detailed information than
conventional x-rays.
● Once a number of successive slices are collected by
the machine’s computer, they can be digitally
“stacked” together to form a three-dimensional (3D)
image of the patient that allows for easier
identification of basic structures as well as possible
tumors or abnormalities.

History of Computed Tomography
● Since the introduction in 1970’s, Ct had undergone
many continuous development improvement in
performance, including increases in acquisition
speed, amount of information in individual slices, and
volume of coverage.
● A graph of these parameters versus time looks
similar to Moore’s Law for computer price-
performance, which observes that computer metrics
(clock speed, cost of random access memory or
magnetic storage, etc.) double every 18 months.
● In the case of CT technology, the doubling period is
approximately 32 months, still an impressive rate. For
example, scan time per slice has decreased from 300
seconds in 1972 to 0.005 seconds in 2005
Evolution of computed tomography scanner performance:
plot of acquisition performance versus time, for computed
tomography scanners. The slope implies a doubling of
performance approximately every 2 years. (Data from
Siemens Medical Systems, www.medical.siemens.com, “CT
History and Technology.”).

A fifth-generation of CT scanners, the
EBCT(Electron Beam CT), uses a stationary X-
ray tube and a stationary detector ring, thereby
avoiding any mechanically moving parts. In
these systems, an electron beam is emitted
from a powerful electron gun and magnetically
deflected to hit a semi-circular anode
surrounding the patient.
5th Generation CT
This is a helical or spiral CT scan or
Volumetric Scanners.The x-ray source and
detectors in 3rd generation is design to freely
moves and attach to the rotating gantry,in this
table with the patient moves smoothly through
the scanner, this derives its name from the
helical path traced out by the x ray beam. It is
also uses a slip ring technology with a
multiplanar reformation excels in 3D.
6th Generation CT

The differences in CT generations can also be clearly highlighted in table form for a quick means
to understand the geometric differences.

Components of CT Equipment
● CT scanners are composed of many different connected parts, with many different
components involved in the process of creating an image.
○ Gantry
○ Slip rings
○ Generator
○ Cooling system
○ X-ray source (CT X-ray tube)
○ Filtration
○ Collimaton
○ Detectors

Gantry
● The gantry is ring shaped part of the CT scanner
having components necessary to produce and
detect x rays. These components are mounted on
a rotating scan frame.
● The range size of aperture is typically 70 to 90
cm, designed to be tilted either forward or
backward according to examination protocols
more or less 15 degrees to 30 degrees is usual.
● Include a laser light that is used to position the
patient within the scanner and Control panels
located on either side of the gantry opening allow
the radiologic technologist to control the
alignment lights, gantry tilt, and movement of the
table.

Slip Rings
● Old model design CT scanner used recoiling system cables to rotate the gantry frame.
This design limited the gantry rotation times.
● Newer systems use electromechanical devices called slip rings which use a brush like
apparatus to provide continuous electrical power and electronic communication across a
rotating surface permit the gantry frame to rotate continuously, eliminating the need to
straighten twisted system cables.
● Slip rings allows the gantry frame to rotate continuously making helical scan modes
possible.

Cooling System
● Cooling mechanisms are included in the
gantry.
● They can take different forms, such as
blowers, filters, or devices that perform oil
to air heat exchange.
● Cooling mechanisms are important
because many components can be
affected by temperature fluctuations.
Generator
● Highly stable 3-phase high frequency
generator is currently used in CT
scanners, designed to be small
enough so that it can be located within
the gantry.
● CT generator produce high kV
generally 120 – 140 kV to increase the
intensity of the beam and thereby
reduce patient dose.

X ray source - CT x ray
tube
● A standard rotating anode tube, Tungsten
is often used for the anode target material
because it produces a higher intensity xray
beam.
● CT scan tubes contain more than one size
of focal spot; 0.5 and 1.0 mm are the
common size of focal spot.
● Just like as in standard xray tubes,
because of reduced penumbra small focal
spot in Computed tomography tubes
produce sharper images like better spatial
resolution.
● A CT scan tube must be designed to
handle such stress as they expose for a
long time.
Collimators
● Collimation restrict the xray beam to a
specific area, as a result it helps reduce
scatter radiation.
● In MDCT systems, slice thickness is
also influenced by the detector element
configuration. Scanner vary in the
choices of slice thickness available.
Choices range from 0.5 to 10 mm.
● Some CT scan systems also use
predictor collimation. This is located
below the patient and above the
detector array(postpatient collimation)..
● The primary functions of predetector
collimations are to ensure the beam is
the proper width as it enters the
detector and to prevent scatter radiation
from reaching the detector.

Filtration
● Used to shape the xray beam & reduce the radiation dose to the patient and help to minimize
image artifact.
● Different filters are used when scanning the body then when scanning the head. Human body
anatomy having distinctive quantities has a round cross section that is thicker in the middle
than in the outer area.
● Hence, body scanning filters are used to reduce the beam intensity at the periphery of the
beam, corresponding to the thinner areas of a patient’s anatomy. Because of their shape they
are often referred to as bow tie areas.

Detectors
The detectors is a component of CT scan machine which collect information regarding the degree
to which each anatomic structure attenuated the beam.
1. Xenon gas detectors
● Pressurized xenon gas fills hollow
chamber to produce detectors that
absorb nearly 60% to 87% of the
photons that reach them.
● Xenon gas is used as its stable under
pressure, significantly less expensive
compared with the solid – state variety.
● a photon enters the channel, it ionizes
the xenon gas & accelerated and
amplified by the electric field.
● The collection charge produces an
electric current. This current is then
processed as raw data.
2. Solid state crystal detectors
● Also called scintillation detectors because
they use a crystal that fluoresces when
struck by a xray photon.
● A photodiode is attached to the crystal
and transforms the light energy into
electrical (analog) energy. The individual
detector elements are affixed to a circuit
board.
● These solids have high atomic numbers
and high density in comparison to gases,
solid state detectors have higher
absorption coefficients nearly 100% of the
photons that reach them.

Xenon gas detectors
Solid state crystal detectors
( cadmium tungstate, bismuth germinate, cesium
iodide, and ceramic rare earth compounds such as
gadolinium or yttrium)

Principle of CT
● CT images are the explanation of relative
attenuation of x ray passing through the body
of different densities.
● A tissue’s CT attenuating ability is related to
its density and represents the likelihood that
an x ray photon will pass through the tissue
to be recorded by the detectors rather than
interacting with tissue’s atoms (absorption of
the x rays into the tissue) which prevents the
photon from reaching the detector at all.
● A particular tissue’s x-ray attenuating ability
is expressed by its attenuation coefficient,
μ.
● The higher the μ value, the lower the number
of photons that reach the detector,The higher
the μ value, the lower the number of photons
that reach the detector.
The attenuation coefficient of a tissue is not
constant and may be altered by the tissue
thickness and the energy of the x ray photon

Cotd.,
● The μ value is directly related to the tissue’s density, that is, the higher the tissue density, the
higher its μ value.
● The CT detector is a photon flux counter where a scintillation detector measures, records,
and converts to light the incident x ray photons exiting the patient.
● This light is then transformed to an electrical signal by the photodiode and the electrical
signal is then converted to a digital signal by the computer.
● The photon flux measured by the detector is represented by the term I where I is a function of
the photon flux emitted by the x ray tube (I 0 ) and the attenuation coefficient (μ) of the tissue.
I = I 0 × e −μ . As μ increases, the fraction e−μ decreases such that as μ increases, the
fraction of the incident photons leaving the x ray tube that are detected by the detector
decreases.

Cotd.,
● The tube voltage (photon energy) is the energy possessed by each individual x ray leaving
the x ray generator and its unit is the kiloelectron volt (KeV).
● The tube current or photon (x ray) flux is the frequency with which x rays leave the x ray tube
and its unit is the milliampere (mA). Increasing the KeV will have the following effects.
○ The tissue penetration will increase, the μ value will decrease
○ the number of photons detected by the detector will increase,
○ the image noise will decrease,
○ the radiation dose will increase and the visualized tissue contrast will decrease.
● Therefore, increasing KeV will decrease the CT number of the iodinated contrast and result in
less opacified coronary arteries. The opposite is also true. Decreasing the KeV will result in
higher CT numbers and brighter contrast within the coronary arteries. Increasing mA serves
to decrease image noise, to increase the radiation dose and to increase the visualized tissue
contrast.

Cotd.,
● The attenuation coefficient of a particular tissue is not
measured directly but rather is calculated using the
count data (I and I0) by manipulating the equation
I = I0 × e−μ.
● The Hounsfield unit behind the image is known as the
CT number.
● It is computed from the calculated attenuation
coefficient using the equation CT number = [(μ tissue
− μ water )/μ water ] × 1,000
● The CT number is inversely related to μ such that the
greater the μ value, the higher the CT number.
● he CT number is directly related to the brightness of
the image where higher CT numbers appear brighter
on the screen.
● The CT number or Hounsfield unit (H.U) is not an
absolute number. It is a representation of the density
of the particular tissue relative to the density of water

Data Acquisition
Generation of X rays :
● A rotatory anode x ray tube is the source of x ray production in CT.
● The x-rays from the target are spread over a wide solid angle. To minimize radiation dose
and generation of background scatter, the x-ray beam is collimated by an aperture into a
thin fan beam.
● For CT scanners, the beam is typically a few millimeters thick in the patient, angleing a fan
of about 45 degrees.
● Additionally, because human anatomy typically has a round cross-section that is thicker in
the middle than in the periphery, more x-ray flux reaches detectors in the center than on
the edges. This means that patients receive more dose than is necessary on the periphery
of their anatomy.
● To compensate for this effect, a bowtie-shape filter is placed in the beam, which is
tapered such that its center is thinner than its edges, to equalize the flux reaching the
detectors and minimize patient dose.

Cotd.,
● The inefficiency in conversion of electron current into x-rays has been a significant
practical limitation in the operation of x-ray imaging equipment.
● The tube is quickly heated to high temperatures, which must be limited to avoid damage.
Anode targets have been designed to rotate on bearings, spreading out the area that is
heated by the beam. Heat sinks are used to remove heat from the system by convection or
water-assisted cooling.
● In typical clinical operation, an x-ray tube delivers on the order of 2 × 10 power of 11 x-
rays per second to the patient, providing a high signal-to-noise ratio for measurements.

Cotd.,
Detection of X-Rays:
● Detection of x-rays is accomplished by the use of special materials that convert the high
energies (tens of keV) of the x-ray quantum into lower energy forms, such as optical
photons or electron-hole pairs, which have energies of a few electron volts.
● The detector materials, such as phosphors, scintillating ceramics, or pressurized xenon gas,
ultimately produce an electrical current or voltage.
● Electronic amplifiers condition this signal, and an analog-to-digital converter converts it
into a digital number. The range of signals produced in tomography is large, varying from a
scan of air (no attenuation, or 100% transmission) to that of a large patient with metal
implants (possible attenuation of 0.0006%), a factor of almost 10 to power of 5.
● Furthermore, even at the lowest signal levels, the analog-to-digital converter must be able
to detect modulations of a few percent. Thus the overall range approaches a factor of one
million, specifying the equivalent of a 20-bit analog-to-digital converter.

Cotd.,
Gantry electromechanics :
● To obtain required measurements at different angles, all the electrical components must
be rotated around the patient.
● In modern scanners, this puts tremendous requirements on mechanical precision and
stability. The gantry can weigh 400 to 1,000 kg, span a diameter of 1.5 m, and rotate 3
revolutions per second. While rotating, it may not wobble more that 0.05 mm.
● Originally, the gantry was connected by cables to the outside environment and had to
change rotation direction at the end of each revolution. A major breakthrough in scanning
operation occurred with the invention of slip-ring technology, which used brush contacts to
provide continuous electrical power and electronic communication, allowing continuous
rotation.

Cotd.,
Helical/Spiral scanning :
● One of the primary goals of CT manufacturers has been to provide faster scan times and
larger scan coverage. With the advent of slip-ring technology and continuous gantry
rotation, the main limitation to scanning speed was the stepping of the patient bed to
position sequential slices.
● In the late 1980s continuous motion of the patient table was introduced, which allowed
faster scan times but required different data handling for image reconstruction.
● In helical scans the gantry is at continuously different table positions throughout each
rotation. A good mathematical value for each gantry position is to interpolate a
reconstruction plane from corresponding neighboring gantry positions. This approach
provided adequate image quality, and in fact had the added benefit that slices could be
reconstructed retrospectively for arbitrary table positions.
● Furthermore, analysis revealed that on average the spatial resolution was better with
helical scans rather than sequential scans. A drawback was that the interpolation process
could create stair-step artifacts on the boundaries of extended high-contrast objects.

Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx

Cotd.,
Detector configuration :
● One problem quickly encountered with single slice (single detector row) helical scanning
(SSCT) was excess stress on the x ray tube. That is, the x ray tube would heat to extreme
temperatures as very high energy was deposited onto the anode. This problem limited the
ability to perform thin slice imaging necessary for acceptable coronary imaging. Alternatively,
if thin slice imaging were performed, tube current was limited to 100 mAs, which in many
instances was not satisfactory and produced noisy images. Thus, multislice CT scanners
(MSCT) were created. MSCT is also known as multidetector CT (MDCT) and multirow CT.

Cotd.,
● The primary difference between MSCT and SSCT is
the detector arrangement.
● SSCT uses a one dimensional detector arrangement
where many individual detector elements are
arranged in a single row across the irradiated slice
that receives the x ray signals.
● In MSCT, each detector in a single row is long
enough in the slice thickness direction (z axis) to
intercept the entire x ray beam width including the
penumbra.
● Each individual detector in each row is then divided
into multiple detector elements forming a two-
dimensional array. In MDCT, there are multiple rows
of detectors. By increasing the number of detector
rows, the z axis coverage slab thickness increases
thereby decreasing the number of gantry rotations
necessary to image the selected field of view (scan
length), theoretically reducing the strain on the x ray
tube

Cotd.,
● For example, 1.25 mm long detectors and the scanner had 16 rows of detectors, the z axis
coverage (slab thickness) per gantry rotation would total 20 mm. However, a typical scan
would acquire over 1,000 views per gantry rotation and collect data along over 800 detectors
in each row, thus generating huge amounts of data.
● Due to early limitations in acquiring, handling and processing such large quantities of data,
MSCT was limited to four detector rows only.
● Subsequent MSCT scanners possessed increasing numbers of detector rows starting at 16
rows and moving to 64, 156 and 320 rows.

Cotd.,
● Depending on the manufacturers multi
detector row systems are of many types
:
○ Parallel rows of equal sizes are
referred as uniform or fixed array.
○ Parallel rows with variable width
detectors are adaptive array.
○ Thinner rows centrally, wider rows
peripherally are hybrid array.
A. Slice width
B. Fixer/Uniform array
C. Adaptive array
D. Hybrid array
● Slice thickness of MDCT, is not
determined by physical collimation of x
ray as in case with SDCT, but also
impact by the width of detector in the
slice thickness dimension.

Questions…?
1. Use of CT number ?
2. What is attenuation coefficient ?
3. Detectors with high efficiency ?
4. Wobbling of gantry should not be more than ?
5. Advantage of helical scanning ?

References
● Computed tomography (CT). (n.d.). Nih.gov. Retrieved August 13, 2022, from
https://www.nibib.nih.gov/science-education/science-topics/computed-tomography-ct
● Themes, U. F. O. (2016, July 15). Basic Principles of Computed Tomography Physics and
technical considerations. Radiology Key. https://radiologykey.com/basic-principles-of-
computed-tomography-physics-and-technical-considerations/
● CT Scan Components. (n.d.). Radtechonduty.com. Retrieved August 15, 2022, from
http://www.radtechonduty.com/2017/03/ct-scan-components.html

Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx

More Related Content

What's hot (20)

Similar to Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx (20)

Recently uploaded (20)

Physical principle of Computed Tomography (Scanning principle & Data acquisition).pptx