radiation physics.ppt

Radiation physics
Prof Dr Naglaa Shawki El kilani
Prof and Head of Oral Medicine,
Periodontology, Radiology and Diagnosis
Head of Laser Center
Al Azhar University
Prof of Laser MUST University

DISCOVERY OF X-RADIATION
In a darkened room on the night of
November 8, 1895, a tiny bit of
fluorescence, which was to enlighten the
entire medical world, was noticed by
Wilhelm Conrad Roentgen. At that time
Roentgen was 50 years of age and, after a
hectic academic career, professor of
physics at the Physical Institute of the
Julius Maximilian University at Würzburg.

It was on a Friday evening that he had
darkened his laboratory to obscure the
fluorescence produced by a Hittorf-
crookes tube. To his surprise, he noticed a
faint glow on a table some distance from
the tube. The source of the glimmering
was another fluorescent screen covered
with barium platinocyanide.

As a result of his experience with cathode
rays, Roentgen realized that the distance
between the Hittorf-crookes tube and the
glowing screen was well beyond the
distance (6 to 8 cm) that cathode rays
from the tube could be detected. He
quickly realized that he was observing a
new form of energy.

In the succeeding weeks Roentgen devoted
himself to almost feverish study of the
properties of these strange emanations. He
soon determined that the new energy (x rays
he called it because of its unknown nature), in
addition to traveling much farther in air than
cathode rays, was able to penetrate dense
materials to varying degrees.

while these investigations were under way,
he unintentionally placed his hand
between the tube and a fluorescent
screen. The observation of the tint but
startling image of the bones within his
hand was the beginning of radiology. He
subsequently demonstrated that such
images of the body could also be recorded
on photographic plates.

Some of the significant contributions to
dental radiology in the United States,
however, were made by a New Orleans
dentist and inventor, Dr. C. Edmund Kells.
Kells had drawn worldwide attention, as
early as the 1880s, by fitting his office with
electric equipment-all of his own design. In
an article published in 1888 he described
some of these innovations: the first
compressed air system, an electrically
driven hand piece, and the first suction
apparatus.

When Kells heard of Roentgen's
discovery, its applications to dentistry were
immediately apparent to him. kells
acquired a Ruhmkorff coil and several
Hittorf-Crookes tubes and took the first
intraoral radiograph in the United States
on a live person (his assistant). In early
spring 1896, less than 4 months after
Roentgen's discovery. Kells, as did the
other early radiographers designed his
own dental x-ray apparatus

Kells became one of the early martyrs who
fell victim to the harmful effects of x rays
and the practice of "setting" the tubes. In
as much as he had gained considerable
prominence in the profession, his
misfortune did much to alert dentists to the
dangers that accompanied the use of x
rays.

Although it is true that the early workers in
the field were for the most part unaware of
the disastrous effects overexposure to
x rays could cause, there were a few
people who very early recognized the
dangers and sounded the warning. Both
Harrison and Walkhoff had reported
injuries caused by x rays.

Another warning came from William
Herbert Rollins, a Boston dentist. After
erythema developed on his hands, he
tested the effects of x rays on animals.
He announced as early as 1898 in
Electrical Review that x rays (x-light he
called it) would produce burns and even
death without burns

Had Rollins' warnings been heeded, many
lives would have been saved, including
Kells', in as much as he did not acquire his
burns until some 20 years later. However,
Kells seems to have considered himself
the authority, and often rejected and
criticized other workers' conclusions on the
basis of his own presumptions.

As late as 1912, 10 years before his
radiogenic neoplasia developed, he was
writing that, although some people may be
sensitive to x rays, he was "completely
immune." Kells endured 10 to 12 years of
increasing discomfort progressing to
excruciating pain, described as
"unequaled by any other disease," and 35
operations (some report 100) including
several amputations. Finally at the age of
72 years, he ended his life with a bullet to
the brain, on May 7,1928.

Radiation physics
Characteristics of radiation
X-ray machine
Mechanism of X-ray production
Interactions of X-ray and matter
Factors affecting the X-ray beam

Terminology
Radiology: science that deals with
diagnosis, therapeutic and research
application of high energy radiation
Dental Radiology: it is the branch of
science that deals with the use of radiation
in the diagnosis of dental diseases.
Radiograph: It is the image received on a
film due to the passage of the X ray from
the object to the sensitized film

DEFINITION OF TERMS USED
IN X-RAY INTERACTIONS
Scattering - change in direction of a
photon with or without a loss of energy.
Absorption- Deposition of energy, i.e.
removal of energy from the beam.
Attenuation- Reduction in the intensity of
the X ray beam caused by absorption and
scattering.
Attenuation = Absorption + Scattering

States of atom
Ground (stable) state: any atom that is
electrically neutral with equal numbers of
protons and electrons.
Excited state: It is the process by which
sufficient energy ejects an electron from its
normal level to a higher energy level.
Ionized state: it is the process by which
an atom loses its electric neutrality and
becomes an ion either by addition or
removal of an electron.

The phenomena associated with
radiology employs the quantum
mechanical model proposed by Niels
Bohr in 1913.
Bohr conceived the atom as a
miniature solar system , at the center
of which is the nucleus , analogous
to the sun .

The electrostatic attraction between the
positively charged nucleus & its negatively
charged electrons balances the centrifugal
force of the rapidly revolving electrons &
maintains them in their orbits .
Consequently ,the amount of energy
required to remove an electron from a
given shell must exceed the electrostatic
force of attraction between it & the
nucleus .(Electron binding energy ,
Ionization energy).

For an electron to move from a specific
orbit to another or bit farther from the
nucleus, energy must be supplied in an
amount equal to the difference in binding
energies between the two orbits,& vice
versa.
Electrons can move from shell to shell but
cannot exist between shells- an area
known as the forbidden zone

Nature of Radiation
Radiation is the transmission of energy through
space and matter.
It may occur in two forms
particulate electromagnetic.
Particulate radiation: It is the type of radiation
given off from radium, radioisotopes and during
splitting of the atom. It consists of atomic nuclei
or subatomic particles moving at high velocity.

Properties of Corpuscular radiation
1. They are minute particles.
2. They have mass
3. They have very high velocity .
4. They travel in straight lines.
5. They have charge except for neutrons.
6. They are used in therapeutic means.

Examples of Particulate Radiation.
Alpha particles helium nuclei
consisting of two protons and two
neutrons.
- Densely ionize matter through which
they pass because of their double charge
and heavy mass.
Beta particles electrons emitted
by radioactive nuclei.
- greater depth than alpha particles, to a
maximum of 1.5 cm in tissue. This deeper
penetration occurs because beta particles
are smaller and lighter and carry a single
negative charge.

Cathode rays also high-speed
electrons but are produced by
manufactured devices (e.g., x-ray tubes).

ELECTROMAGNETIC
RADIATION
It is that type of radiation formed of units of
pure energy (photons) which is
propagated in the form of waves. It is
generated when the velocity of an
electrically charged particle is altered.

Some of the properties of electromagnetic
radiation are best expressed by wave theory,
whereas others are most successfully described
by quantum theory.
The wave theory of electromagnetic radiation
maintains that radiation is propagated in the
form of waves, not unlike the waves resulting
from a disturbance in water.
Quantum theory considers electromagnetic
radiation as small bundles of energy called
Photons. Each photon travels at the speed of
light and contains a specific amount of energy.
The unit of photon energy is the electron volt
(e V).

Properties of electromagnetic
radiation
1. Made of pure energy units ” photons”
2. They have no mass.
3. They have no charge.
4. Propagate in the form of waves.
5. They are in a combination of electric and
magnetic fields.
6. They travel in the same speed of light
which is (3× 10 8 m/ sec) in straight lines.

7. The magnetic and electric fields are at
right angles to each other and to the
direction of motion or propagation of
photons.
8. They have wavelength( ‫ג‬ ),frequency (V)
and velocity (C).C = ‫ג‬ × ν.
9. They differ in properties( ex. Their power
of penetration). This difference depends
on their ‫ג‬.

10. The relationship between wavelength
and photon energy is as follows:
E = h x C / ‫ג‬
where E is energy (k e V), h is Planck's
constant ( 4.3 x 1O-18keV), C is the
velocity of light, and ‫ג‬ is wavelength in
nanometers. This expression may be
simplified as follows:
E = 1.24/‫ג‬

Examples of electromagnetic
radiation
1. Cosmic rays
2. Gamma rays
3. X rays
4. Ultraviolet rays
5. Visible light
6. Infra-red rays
7. Micro-waves
8. Radio, Radar, T.V waves

Roentgen rays or X-rays
Are a form of pure energy units belonging
to electromagnetic spectrum characterized
by having a very short wave length and
having the ability of casting or producing
shadows, images of the body tissues.

A- Special or specific
properties
1. They have a very short wavelength.
2. They have selective penetration,
absorption.
3. It affects photographic films emulsion.
4. It can cause certain substance to
fluorescence.
5. They cause ionization of atoms.
6. They have biological damaging effects.

B-Other Properties
1. Travel in straight lines in wave motion.
2. Travel with the same speed of light.
3. Their band width is from 0.1-1A.
4. Invisible, can t be felt ,smelled or heard.
5. They have no weight or mass or charge.
6. They can not be focused or collected by
lens, reflected by mirror ,refracted by
fluids, deviated by a magnet, but can be
deflected (deviated into another linear
path in heavy metals) .

X-Ray Machine
The basic apparatus for generating x rays, the
x-ray tube, is composed of a cathode and an
anode .
The cathode serves as the source of electrons
that flow to the anode. The cathode and anode
lie within an evacuated glass envelope or tube.
When electrons from the cathode strike the
target in the anode, they produce x rays.
For the x-ray tube to function, a power supply is
necessary

Cathode
The cathode in an x-ray tube consists of :
a filament and a focusing cup. The filament is
the source of electrons within the x-ray tube. It is a
coil of tungsten wire about 2 mm in diameter and 1
cm or less in length.
It is connected 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.
The filament lies in a focusing cup a negatively
charged concave reflector made of molybdenum.

The x-ray tube is evacuated to prevent
collision of the moving electrons with gas
molecules, which would significantly
reduce their speed. This also prevents
oxidation and "burnout" of the filament.

Anode
The anode consists of a tungsten target
embedded in a copper stem. 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.
The target is made of tungsten, a material that
has several characteristics of an ideal target
material.
It has:
1. a high atomic number
2. high melting point,
3. high thermal conductivity,
4. low vapor pressure at the working temperatures
of an x-ray tube.
The tungsten target is typically embedded in a
large block of copper to dissipate heat.

The focal spot
is the area on the target to which the focusing
cup directs the electrons from the filament.
The sharpness of the radiographic image
increases as the size of the focal spot (the
radiation source decreases) but the heat
generated per unit target area, however,
becomes greater.
To take advantage of a small focal spot while
distributing the electrons over a larger area of
the target, the target is placed at an angle to the
electron beam( 20 degrees to the central ray of
the x-ray beam), thus the projection of the focal
spot perpendicular to the electron beam (the
effective focal spot) is smaller than the actual
size of the focal spot. This is in the stationary
type

Power Supply, housing
The primary functions of the power supply of an
x-ray machine are:
(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. Oil facilitates the
removal of heat.
A surrounding lead casing absorbs unwanted X rays as
a radiation protection measure since X-rays are emitted
in all directions.

Sequence of events in the
production of X-rays :
I. The filament is electrically heated and a cloud of
electrons is produced around the filament.
2. The high-voltage (potential difference) across the tube
accelerates the electrons at very high speed towards the
anode.
3. The focusing device aims the electron stream at the
focal spot on the target.
4. The electrons bombard the target and are brought
suddenly to rest.
5. The energy lost by the electrons is transferred into either
heat (about 99%) or X-rays (about 1%).
6. The heat produced is removed and dissipated by the
copper block and the surrounding oil.
7. The X-rays are emitted in all directions from the target.
Those emitted through the small window in the lead
casing, constitute the beam used for diagnostic
purposes.

INTERACTIONS AT THE
ATOMIC LEVEL
The high-speed electrons bombarding the target are
involved in two main types of collision with the tungsten
atoms:
1. Heat-producing collisions .
2. X-ray-producing collisions.
Heat-producing collisions
-The incoming electron is deflected by the cloud of outer-
shell tungsten electrons, with a small loss of energy, in
the form of heat
-The incoming electron collides with an outer shell
tungsten electron displacing it to an even more
peripheral shell (excitation) or displacing it from the atom
(ionization), again with a small loss of energy in the form
of heat.

Important points to note
Heat-producing interactions are the most
common because there are millions of
incoming electrons and many outer-shell
tungsten electrons with which to interact.
Each individual bombarding electron can
undergo many heat-producing collisions
resulting in a considerable amount of heat
at the target.
Heat needs to be removed quickly and
efficiently to prevent damage to the target.

X-ray-producing collisions
I-Bremsstrahlung or braking radiation:
II-CHARACTERISTIC RADIATION

X-ray-producing collisions
I-Bremsstrahlung or braking radiation:
The incoming electron penetrates the
outer electron shells and passes close to
the nucleus of the tungsten atom. The
incoming electron is dramatically slowed
down and deflected by the nucleus with a
large loss of energy which is emitted in the
form of X-rays. Bremsstrahlung
interactions generate x-ray photons with a
continuous spectrum of energy.

II-CHARACTERISTIC RADIATION
The incoming electron collides with an inner-
shell tungsten electron displacing it to an outer
shell (excitation) or displacing it from the atom,
then 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.

Characteristic radiation from the K shell
occurs as discrete increments compared
with bremsstrahlung radiation.
The energies of characteristic photons are
a function of the energy levels of various
electron orbital levels hence are
characteristic of the target atoms.
Characteristic radiation is only a minor
source of radiation from an x-ray tube.

The diagnostic X-ray beam can vary in its
intensity and in its quality:
Intensity = the number or quantity of X-ray
photons in the beam.
Quality = the energy carried by the X-ray
photons which is a measure of their
penetrating power.

Factors that can affect the
intensity and/or the quality of
the beam include:
Size of the tube voltage = beam energy (kV)
Size of the tube current = exposure rate (m A)
Distance from the target = target-patient dist .(d)
Beam shape (collimation )
Time = length of exposure (t)
Filtration
Target material

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 ability of x-ray photons to penetrate matter
depends on their energy. High-energy x-ray
photons have a greater probability of penetrating
matter, whereas relatively low-energy photons
have a greater probability of being absorbed.

shows the influence of changing tube voltage ( k V p) on the spectrum of
photon energies in an x-ray beam

Size of the tube current
mA setting, more power is applied to the
filament, which heats up and releases more
electrons that collide with the target to produce
radiation.
Therefore the quantity of radiation produced
by an x-ray tube (i.e., the number of photons that
reach the patient and film) is directly proportional
to the tube current (m A) and the time the tube is
operated.
The quantity of radiation produced is expressed
as the product of time and tube current.

illustrates the changes in the spectrum of photons that result from increasing
tube current (m A) while maintaining constant tube voltage (k V p) and
exposure time.

Filtration
An x-ray beam consists of a spectrum of x-
ray photons of different energies, only
photons with sufficient energy to penetrate
through anatomic structures and reach the
image receptor (usually film) are useful for
diagnostic radiology. Those that are of low
energy (long wavelength) contribute to patient
exposure (and risk) but do not have enough
energy to reach the film.
Consequently, to reduce patient dose, the less-
penetrating photons should be removed.

illustrates how the addition of an aluminum filter alters the energy distribution
of the unfiltered beam. The aluminum preferentially removes many of the
lower-energy photons with lesser effect on the higher-energy photons that
are able to penetrate to the film.

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.
Round and rectangular collimators are most
frequently used in dentistry.
Dental x-ray beams are usually collimated to a
circle 23/4inches (7 cm) in diameter.
Collimating the beam to reduce the exposure
area and thus the number of scattered photons
reaching the film can minimize the detrimental
effect of scattered radiation on the images.

Target-patient distance
The intensity of an x-ray beam at a given
point (number of photons per cross-
sectional area per unit exposure time)
depends on the distance of the measuring
device from the focal spot.
INVERSE SQUARE LAW : For a given beam
the intensity is inversely proportional to the
square of the distance from the source

Changing the distance between the x-ray tube and patient has a
marked effect on beam intensity.

portrays the changes in the x-ray spectrum that result when the
exposure time is increased while the tube current (m A) and
voltage (kV) remain 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.

INTERACTION OF X-RAYS
WITH MATTER

INTERACTION OF X-RAYS
WITH MATTER
When X-rays strike matter, such as a patient's tissues,
the photons have four possible fates.
The photons may be:
1. Completely scattered with no loss of energy .
2. Absorbed with total loss of energy .
3. Scattered with some absorption and loss of energy.
4. Transmitted unchanged.
INTERACTION OF X-RAYS AT THE ATOMIC LEVEL
Only 2 interactions are important in the X-ray energy range
used in dentistry:
- Photoelectric effect - pure absorption .
- Compton effect - scatter and absorption .

Terminology
Primary Radiation: It is that radiation coming
directly out of the target in all direction.
Useful Beam: It is that part of the primary
radiation passing through the aperture which is
not absorbed by the housing.
Central Ray: It is that part occupying the central
portion of the useful beam.
Secondary Radiation: It is that type of radiation
generated from the patient and surrounding
objects after the passage of primary beam.

Scattered Radiation: Secondary radiation that
have been deviated after the passage of X rays
through organs .
Stray Radiation: Type of radiation when primary
beam hits a metal heavier than aluminum.
Remnant Radiation: Radiation exposing the film
and producing the image after the passage of
primary beam through the patient.
Soft Radiation: Radiation of longer wavelength
and lower energy.
Hard Radiation: Radiation of shorter wavelength
and higher energy.

radiation physics.ppt

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