2. COMPOSITION OF MATTER:
Matter is anything that has mass and
occupies space.
It occurs in three states: Solid, Liquid
and Gas
ATOM is the fundamental unit of
matter that cannot be subdivided by
chemical methods.
3. BOHR RUTHERFORD MODEL:
In atomic physics, the Bohr model, introduced by Niels Bohr in 1913,
depicts the atom as small, positively charged nucleus surrounded by
electrons that travel in circular orbits around the nucleus—similar in
structure to the solar system, but with attraction provided by electrostatic
forces.
5. The quantum mechanical model is based on quantum theory, which says
matter also has properties associated with waves. According to quantum
theory, it’s impossible to know the exact position and momentum of an electron
at the same time. This is known as the Uncertainty Principle.
The quantum mechanical model of the atom uses complex shapes of
orbitals (sometimes called electron clouds), volumes of space in which there is
likely to be an electron. So, this model is based on probability rather than
certainty.
Four numbers, called quantum numbers, were introduced to describe the
characteristics of electrons and their orbitals:
Principal quantum number: n
Angular momentum quantum number: l
Magnetic quantum number: m1
Spin quantum number: ms
8. The number of protons only in a nucleus is called the atomic number (
the Z number).
The atomic mass (A) is the total number of protons and neutrons in
the nucleus of an atom.
When the number of protons equals the number of electrons that
atom is known to be in a stable or neutral state
The electrons in the orbit are maintained by the electrostatic force
between the positively charged nucleus and the negatively charged
electrons on the one hand, balanced by the centrifugal force of the
revolving electrons.
12. ELECTRON BINDING ENERGY:
The amount of energy required to remove an electron from a given
shell must exceed the electrostatic force of attraction between it and the
nucleus. This is called electron binding energy of the
electron/ionization energy
It is specific for each shell of each element.
The electron shell closest to the nucleus will have the greatest binding
energy and it will decrease successively in each shell
13. For an electron to move from a specific orbit to another
orbit farther from the nucleus, energy must be supplied in
an amount equal to the difference in the binding energies
between the two orbits.
In contrast, in moving an electron from an outer orbit to
the closer to the nucleus, energy must be lost and this
energy is the difference between the binding energies of
the two orbits. This lost energy is given up in the form of
electromagnetic radiation.
14. IONIZATION:
When the number of orbiting electrons
in an atom is equal to the number of
protons in its nucleus, the atom is
electrically neutral.
If an electrically neutral atom loses an
electron, it becomes a positive ion and the
free electron is a negative ion. This
process of forming an ion pair is termed
ionization.
15. NATURE OF RADIATION:
DEFINITION: Radiation is the transmission of energy through
space and matter.
PARTICULATE
RADIATION
ELECTROMAGNETIC
16. PARTICULATE/CORPUSCULAR RADIATION
Particulate radiation consists of atomic nuclei or subatomic particles
moving at high velocity.
e.g. alpha particles, beta particles, and cathode rays are examples of
particulate radiation.
17. The capacity of particulate radiation to ionize atoms
depends on its mass, velocity, and charge. The rate of
loss of energy from a particle as it moves along its track
through matter (tissue) is its linear energy transfer (LET) .
A particle loses kinetic energy each time it ionizes adjacent matter; the
greater its physical size and charge and the lower its velocity, the greater
is its LET.
For example, alpha particles, with their high charge and low velocity,
lose kinetic energy rapidly and have short path lengths (are densely
ionizing); thus they have a high LET. Beta particles are much less densely
ionizing because of their lighter mass and lower charge and thus have a
lower LET. They penetrate through tissue more readily than
do alpha particles.
18. ELECTROMAGNETIC RADIATION:
Electromagnetic radiation is the movement of energy
through space as a combination of electric and magnetic fields.
It is generated when the velocity of an electrically charged particle is
altered.
E.g. Gamma rays, x rays, ultraviolet rays, visible light, infrared
radiation(heat), microwaves, and radio waves.
19. Some of the properties of electromagnetic radiation are best expressed
by wave theory, whereas others are most successfully described by
quantum theory.
Wave theory:
Electromagnetic wave
Wavelength
Electric field
Electric Field
Magnetic
Magnetic
Field Field
Direction
Direction
•All electromagnetic waves travel at the velocity of light (3.0 x 108 m/s )
• Waves of all kinds exhibit the properties of wavelength (λ) and frequency (v).
• λ x v = c = 3 X 108 meters/second
22. According to wavelengths radiation can differ in properties:
Short wavelength
OR
Long wavelength
23. The short wavelength
Increased frequency
increased energy accompanied with it
increase the
power of penetration. They are termed as Hard Radiation which is
characterised by low absorption and low ionisation potential.
The long wavelength
decreased frequency
decreased energy accompanied with it
less power of penetration. They are termed as Soft Radiation which
is characterised by high absorption and high ionisation potential.
24. HISTORY:
X-RAYS:
X-Rays were first discovered by Wilhelm Conrad Roentgen on 8th
November, 1895.
He was a professor of physics at the University of Wuzberg in
Germany.
He was working with Hittorf-Crookes tube, through which an electric
current from a battery was flowing in a darkened room with black
cardboard covering the tube. There were many fluorescent plates in the
room.
One evening, while he was working on the tubes, he noticed something
coming from the tube causing the fluorescent plates to glow.
He did not know what it was, he called it X-rays.
He won the Nobel prize for his discovery in 1901.
25. He first conducted the experiment on his wife’s
hand with a 15 minutes exposure.
He published 3 papers:
• A new kind of Rays: a preliminary
communication.
• A new kind of Rays: continued.
• Further observations on a new kind of Rays.
In June 1896, within 6 months after Roentgen
announced his discovery, X-Rays were being
used by battlefield physicians to locate bullets in
wounded soldiers.
26. Within 2weeks of its discovery, the first dental radiograph was taken by German
dentist Dr. Otto Walkoff and Prof.Wilhelm Koening in 1896 with the help of a
radiochemist Giesel.
He used a small glass photographic plate wrapped in black paper and
covered with rubber dam that he placed in his own mouth, because of the
plate’s positioning his mouth the image showed parts of the upper and lower teeth
and he was actually taking a bitewing radiograph.
27. Dr. C. Edmond Kells, a New Orleans dentist, is credited with taking
the first intra oral radiographs in the US in April 1896.
Dr. William Rollins developed the first dental X-ray unit in 1896.
He developed burns on his skin of his hands and recommended lead
shielding of both the tube and the patient.
Dr. Howard Riley was the first to introduce radiology into the
dental school curriculum at the University of Indiana.
In 1913, film was used instead of glass photographic plates.
28. PROPERTIES OF X-RAY:
1. They have a very short wavelength. It has wavelength of 0.10.001nm.
2. They have a selective penetration and absorption power.
3. It causes certain substances to fluorescence.
4. They cause ionization of atoms.
5. They have biological damaging effects.
6. They travel at the speed of light i.e. 3x108 m/s
7. Invisible, odorless, cannot be felt or heard.
8. Weightless, massless, charge less.
9. They cannot be focused or collected by lens.
11.They cannot be reflected or refracted.
12.They cannot be deviated by a magnetic field.
32. THE X-RAY TUBE:
All dental and medical x-ray tubes are called
Coolidge tubes because they follow the original design
of W. C. Coolidge introduced in 1913.
The basic apparatus for generating x rays, the x-ray
tube, is composed of a cathode and an anode.
33. THE TUBE:
The tube is an Evacuated glass tube with two extending arms or
electrodes extending in two opposite directions, which are the
cathode and the anode.
The tube is evacuated for the following reasons:
1) This will prevent the collision of the moving electrons with air
molecules.
2) This evacuation will prevent the oxidation and burnout of the
filaments.
37. THE FILAMENT:
•The filament is the source of electrons within the x-ray tube.
• It is a coil of tungsten wire about 2mm in diameter and 1cm or
less in length.
•It is mounted on two stiff wires that support it and carry the electric
current. These two mounting wires lead through the glass envelope
and connect to both the high- and low-voltage electrical sources.
•The filament is heated to incandescence by the flow of current from
the low-voltage source and emits electrons at a rate proportional to
the temperature of the filament.
38. FOCUSSING CUP:
•The filament lies in a focusing cup, a negatively charged concave
reflector made of Molybdenum.
•The focusing cup electro statically focuses the electrons emitted by
the incandescent filament into a narrow beam directed at a small
rectangular
area on the anode called the focal spot.
•The electrons move in this direction because they are repelled by
the negatively charged cathode and attracted to the positively
charged anode.
42. TARGET:
•The purpose of the target in an x-ray tube is to convert the kinetic energy
of the electrons generated from the filament into x-ray photons.
•This is an inefficient process with more than 99% of the electron kinetic
energy converted to heat.
•The target is made of tungsten, a material that has several characteristics
of an ideal target material.
It has a high atomic number (74)- Best for production of X-Rays
High melting point (3410o C)- Withstand heat generated at anode
High thermal conductivity- dissipate heat to copper stem
Low vapour pressure at the working temperatures of an x-ray tube.maintains vacuum.
43. COPPER BLOCK:
•The tungsten target is typically embedded in a large block of
copper to dissipate heat.
•Copper, a good Thermal conductor, dissipates heat from the
tungsten, thus reducing the risk of the target melting.
•In addition, insulating oil between the glass envelope and the
housing of the tube head carries heat away from the copper
stem. This type of anode is a stationary anode.
44. LINE FOCUS PRINCIPLE:
The focal spot is the area of the anode from which the x-rays are emitted. The
focal spot impacts the geometric resolution of the x-ray image.
By angling the anode target, one makes the effective focal spot much smaller
than the actual area of interaction. The angling of the target is know as the line
focus principle.
The Effective Focal Spot is the beam projected onto the patient. As the anode
angle decreases, the effective focal spot decreases. Diagnostic tube target
angles range from 5 to 15°.
Smaller target angles will produce smaller effective focal spots and sharper
images. To cover a 17” the angle must be 12°. To cover 36” the angle must be
14°.
45. HEEL EFFECT:
Because of the use of line-focus principle the consequence is that
the radiation intensity on the cathode side of the x-ray field is
higher than that on the anode side.
Because the e- on the anode side must travel further than the ethat are close to the cathode side of the target, the anode side xrays have slightly lower energy than the cathode side x-rays.
The smaller the anode angle, the larger the heel affect.
46.
47.
48. A SIMPLIFIED DIAGRAM OF X-RAY TUBE
Step Down
Transformer
Cathode
Filament
Anode
Target
Copper
Block
Insulating Oil
Focussing Cup
X-Ray
Metal Housing
Evacuated Glass Tube
Useful Beam
Step Up
Transformer
49. POWER SUPPLY:
The primary functions of the power supply of an x-ray machine are to
(1) provide a low-voltage current to heat the x-ray tube filament by
use of a step-down transformer.
(2) generate a high potential difference between the anode and
cathode by use of a high voltage transformer.
These transformers and the x-ray tube lie within an electrically
grounded metal housing called the head of the x-ray machine. An
electrical insulating material, usually oil, surrounds the transformers.
50. TRANSFORMER: It is an electric device, which increases or reduces the
voltage of an alternating current by mutual induction between primary and
secondary coils.
Step down Transformer: A transformer in which the secondary voltage is
less than primary voltage.
Step up Transformer: A transformer in which the secondary voltage is
greater than the primary voltage.
51. The Principles of X-Ray
Production
When an electric current, which is composed of a stream of
negatively charged electrons having kinetic energy passes
through a filament or wire, it will be heated so the orbiting
electrons within its atom will acquire sufficient energy to
escape from their shells. Finally, this electron cloud will be
given from the heated wire of filament.
If these electrons are suddenly stopped, they will lose the
accompanying kinetic energy and convert it into heat and XRay radiation.
52. Application of this Principle on dental X-Ray machine:
The step down transformer will decrease the electric voltage to 812 volts
This voltage is sufficient enough to heat the tungsten filament of the
cathode and produce electrons according to the degree of heating by
thermo ionic emission.
These electrons will form a cloud around the cathode, which will be
collected by the concave focussing cup but they have no velocity to
move.
53. The step-up transformer will raise the potential difference between the
cathode and the anode by raising the voltage to 60-70 kVP.
This increase in potential difference will accelerate the electron cloud to
move towards the anode, as there is a force of attraction between the
cathode and anode.
By the action of the focussing cup, the electrons will hit only the
tungsten target of the anode, losing their kinetic energy in the form of
99.8% heat and only 0.2% X-Rays.
The produced X-rays are conducted out from the tube housing through
the filters and collimators to be used as a useful beam.
54. FACTORS CONTROLLING X-RAY BEAM:
EXPOSURE TIME: When it increases, there is increase in the number of
photons generated keeping current and voltage constant. When the
exposure time is doubled, the number of photons generated at all energies
in the x-ray emission spectrum is doubled, but the range of photon
energies is unchanged. Therefore changing the time
simply controls the quantity of the exposure, the
number of photons generated.
TUBE CURRENT: The number of photons is directly proportional to the
tube current (mA)
55. TUBE VOLTAGE: Increasing the kVp increases the potential difference
between the cathode and anode, thus increasing the energy of each
electron when it strikes
the target. This results in an increased efficiency of conversion of electron
energy into x-ray photons, and thus
an increase in
(1) the number of photons generated,
(2) their mean energy, and
(3) their maximal energy.
The increased number of photons produced per unit
time by use of higher kVp results from the greater efficiency in the
production of bremsstrahlung photons
that occurs when increased numbers of higher-energy
electrons interact with the target.
58. PRODUCTION OF X-RAYS:
Electrons traveling from the filament to the target convert
some of their kinetic energy into x-ray photons by the
formation of
Bremsstrahlung and
Characteristic radiation.
59. BREMSSTRAHLUNG:
Bremsstrahlung interactions, the primary source of x-ray photons
from an x-ray tube, are produced by the sudden stopping or
slowing of high-speed electrons at the target. (Bremsstrahlung
means "braking radiation“ in German.)
Most high-speed electrons, however, have near or wide misses with
atomic nuclei.
In these interactions, a negatively charged high-speed electron
attracted toward the positively charged nuclei.
The closer the high speed electron approaches the nuclei, the greater
is the electrostatic attraction on the electron, the braking effect, and
the energy of the resulting bremsstrahlung photons.
63. CHARACTERISTIC RADIATION:
•Characteristic radiation occurs when an electron from the filament
displaces an electron from a shell of a tungsten target atom, thereby
ionizing the atom.
•When this happens, a higher energy electron in an outer shell of the
tungsten atom is quickly attracted to the void in the deficient inner shell.
•When the outer-shell electron replaces the displaced electron, a photon
is emitted with an energy equivalent to the difference in the two orbital
binding energies.
66. FILTRATION:
A thin sheet of pure aluminum (1.5-2.5mm thickness) is placed in the
way of the X-ray beam at the end of the X-ray tube in order to improve the
quality of the beam.
Inherent filtration consists of the materials that x-ray photons encounter
as they travel from the focal spot on the target to form the usable beam
outside the tube enclosure. These materials include the glass wall,
insulating oil, barrier material
Total filtration is the sum of the inherent filtration plus any added external
filtration supplied in the form of aluminum disks placed over the port in the
head of the x-ray machine.
69. COLLIMATION:
A collimator is a metallic barrier with an aperture in the middle used to
reduce the size of the x-ray beam and therefore the volume of irradiated
tissue
within the patient.
71. INVERSE SQUARE LAW:
The intensity of an x-ray beam at a given point (number of photons per crosssectional area per unit exposure time) depends on the distance of the measuring
device from the local spot. For a given beam the intensity is inversely proportional
to the square of the distance from the source.
The reason for this decrease in intensity is that the x-ray beam spreads out as it
moves from the source.
The relationship is as follows:
I1 / I2 = D22 / D2 1
where l is intensity and D is distance. Therefore changing the distance between
the x-ray tube and patient has a marked effect on beam intensity. Such a change
requires a corresponding modification of the kVp or mA if the exposure of the film
is to be kept constant.
72. INTERACTIONS OF XRAYS
WITH MATTER:
• The intensity of an x-ray beam is reduced by interaction with the matter it
encounters. This attenuation results from interactions of individual
photons in the beam with atoms in the absorber.
• The x-ray photons are either absorbed or scattered out of the beam.
• In a dental x-ray beam there are three means of beam attenuation:
(1) COHERENT SCATTERING
(2) PHOTOELECTRIC ABSORPTION
(3) COMPTON SCATTERING
73. COHERENT SCATTERING:
• Coherent scattering (also known as classical, elastic, or
Thompson scattering) may occur when a low-energy incident
photon passes near an outer electron of an atom.
• The incident photon interacts with the electron causing it to
become momentarily excited at the same frequency as the in
coming photon.
74. THE INCIDENT PHOTON
CEASES TO EXIST
THE EXCITED ELECTRON
THEN RUTURNS TO GROUND
STATE
SECONDARY PHOTON
EMITTED AT AN ANGLE
ALTERED FROM THE
INCIDENT BEAM
76. • This interaction accounts for only about 8% of the total number of
interactions (per exposure) in a dental examination.
•
Coherent scattering contributes very little to film fog because the
total quantity of scattered photons is small and its energy level is
too low for much of it to reach the film.
77. PHOTOELECTRIC ABSORPTION:
• It is critical in Diagnostic Imaging.
• This process occurs when an incident photon
collides with a bound electron in an atom of the absorbing medium.
• The electron is ejected from its shell and becomes a recoil electron
(photoelectron).
• The kinetic energy imparted to the recoil electron is equal to the
energy of the incident photon minus that used to overcome the
binding energy of the electron.
78. • The absorbing atom is now ionized because it has lost an electron.
• The recoil electron acquires most of the energy of the incident
photon.
• Most photoelectric interactions occur in the K shell because the
density of the electron cloud is greater in this region and a higher
probability of interaction exists.
• About 30% of photons absorbed from a dental x-ray beam are
absorbed by-the photoelectric process.
79. • Although this is beneficial in producing high-quality radiographs,
because no scattered radiation fogs the film, it is potentially
deleterious for patients because of increased radiation absorption.
• The difference in absorption in various tissues appears as a
difference in optical density in the radiographic image.
81. COMPTON SCATTERING
• In this interaction the incident photon collides with an outer
electron, which receives kinetic energy and recoils from the point
of impact.
• The path of the incident photon is deflected by its interaction and is
scattered from the site of the collision.
The energy of the scattered photon equals the energy of the
incident photon minus the sum of the kinetic energy gained by the
recoil electron and its binding energy.
82. • Compton scattering results in the loss of an electron and
ionization of the absorbing atom.
• The probability of a Compton interaction is directly proportional
to the electron density of the absorber. Therefore the probability
of Compton scattering is correspondingly greater in bone than in
tissue.
•
In a dental x-ray beam, approximately 62% of the photons
undergo Compton scattering.
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Elsevier Publication, 5th edition, 2004.
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Western Perspective (Revised ed.). World Scientific. ISBN 978-981-4324-57-1.
3, Bushberg J.T. : The essential physics of medical imaging, ed.2 Baltimore, 2001Lippincott
Williams an Wilkin.
4. Bushbong SC: Radiologic science for technologists: physics, biology and protection, ed 7. St
Louis, 2001, Mosby.
5. Curry TS, Dowdey JE, Murry RC: Christensen’s introduction to the physics of diagnostic
radiology, ed 4. Philadelphia, 1990, Lea & Febiger
6. International Commission on Radiological Protection & Safety In Medicine; ICRP Publication
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85. 7. WJ Merdith, JB Massey, Fundamental Physics of Radiology, Second Edition, Bristol: John
Wright and Sons 1972.
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