HDR202
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
CHAPTER 4
PHYSICS FOR RADIOGRAPHERS 2
RADIATION ATTENUATION
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Learning Objectives
At the end of the lesson, the student should be able to:-
Define the scattering and absorption.
Describe the probability of occurrence of interactions.
Explain the photon energy, atomic number, K-edge, density, and
thickness of attenuator.
Explain the x-ray interaction with matters.
Explain the particles and photons.
Explain the direction and energy of scattered radiation.
Explain the inverse square law.
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List of Contents
4.1 Scattering and Absorption
4.2 Probability of occurrence of interaction
4.3 Photon energy, atomic number, k-edge, density, and thickness
of attenuator
4.4 X-ray interaction with matter
4.5 Particles and Photons
4.6 Direction and Energy of Scattered Radiation
4.7 Inverse Square Law
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Definition
Scattering
The diversion of radiation (thermal, electromagnetic, or nuclear) from
its original path as a result of interaction or collisions with atoms,
molecules, or larger particles in the atmosphere or other media
between the source of the radiation (e.g., a nuclear explosion) and a
point at some distance away.
As a result of scattering, radiation (especially gamma rays and
neutrons) will be received at such a point from many directions
instead of only from the direction of the source.
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Definition
Attenuation
The process by which radiation losses the energy as it travels through
matter and interacts with it. Beam attenuation is the basis of the
contrast observed in all X-ray based imaging methods.
Absorption
The process by which radiation losses the intensity as it passes through
a material medium by conversion of the energy of the radiation to an
equivalent amount of energy appearing within the medium; the
radiant energy is converted into heat or some other form of molecular
energy.
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X-ray Interaction with Matter
There are 5 interactions with matter:-
1. Coherent Scattering
2. Photodisintegration
3. Pair Production
4. Compton Effect or scattering
5. Photoelectric Effect
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a) Coherent Scattering
A term sometimes used for Rayleigh scattering and Thomson scattering.
They are both examples of coherent scattering, in which the incident photon
undergoes a change in direction without a change in wavelength.
Notice in this diagram that the direction of the photon is changed but
the wavelength remains the same.
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b) Photodisintegration
Collision of a high energy photon with an atomic nucleus.
The photon is completely absorbed in the process, and a neutron, proton, or
alpha particle is ejected from the excited nucleus. Need at least 10 MeV for
photodisintegration. This is more energy than a normal x-ray.
This diagram shows an x-ray interacting with the nucleus of an atom and
expelling a piece of the nucleus.
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c) Pair Production
The process in which a high-energy photon is completely transformed into an
electron and a positron.
Thus, this is a process whereby energy is transformed into matter. It occurs
only in the vicinity of atoms which act as a sort of "catalyst".
Since according to Einstein's theory of relativity, the energy (E) and the mass
(m) are proportional to each other with the constant of proportionality being
the square of the velocity of light (c),
and the resting masses of electron and positron are 511 keV each, the
minimum photon energy required for pair production to occur is 1.022 MeV.
The inverse reaction to pair production is the annihilation reaction. This is
also more energy than normal x-ray.
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d) Compton Scattering
During Compton scattering, a photon impinges on an electron in matter, and
in this process transfers part of its energy to it.
The excited electron is termed a Compton electron and is ejected or moved
into an excited atomic state, while due to the law of conservation of energy
the photon energy is reduced.
This diagram shows a photon
interacting with an electron,
ejecting it and giving some of its
energy to the electron. The photon
is scattered by an angle, luckily
we do not have to calculate the
angle, and the wavelength is
changed.
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e) Photoelectric Effect
The effect discovered by Einstein (for which he received the Nobel prize in
1921) in which a photon transfers its entire energy to an electron in the
material on which it impinges.
The electron thereby acquires enough energy either to free itself from the
material to which it is bound or to be elevated into the conduction band of a
semiconductor or insulator (solid).
This diagram shows a
photon interacting
with an electron and
giving all of its energy
to the electron. The
electron is then
ejected from the atom.
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Series of Photoelectric Effect
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The probability of photoelectric effect:-
The probability of the photoelectric effect is inversely proportional to the
cube of the x-ray energy .
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The probability of photoelectric effect is directly proportional to the cube of
the atomic number of the absorbing material .
Effective Atomic Numbers
Types of Substance Effective Atomic Number
Human Tissue
Fat 6.3
Soft Tissue 7.4
Lung 7.4
Bone 13.8
Contrast Material
Air 7.6
Iodine 53
Barium 56
Other
Concrete 17
Molybdenum 42
Tungsten 74
Lead 82
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Features of Photoelectric Effect
Most likely to occur
a) With inner-shell electronsb) With tightly bound electronsc) When x-ray energy is just higher than electron-binding energy
As x-ray energy increases
a) Increased penetration through tissue without interactionb) Less photoelectric effect relative to Compton effectc) Reduced absolute photoelectric effect
As atomic number of absorber increasesIncreases proportionately with the cube of the atomic number
As mass density of absorber increasesProportional increase in photoelectric absorption
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Differential Absorption
Differential Absorption occurs because of Compton scattering,
photoelectric effect, and x-rays transmitted through the patient.
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Radiopaque - or opaque, the relative capacity of matter to obstruct the
transmission of radiant energy. When x-rays are obstructed the film is light,
for example from bone.
Radiolucent - or nonopaque, being permeable to radiation or penetrable by X-
rays. The opposite term is radiopaque. When x-rays are not obstructed the
film is dark.
The difference between the radiopaque and the radiolucent areas of the body
give the contrast or differential absorption.
Differential absorption increases as the kVp is reduced.
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Dependence on Atomic Number
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Dependence on Mass Density
Mass density is the mass per unit volume of a substance.
The interaction between x-rays and tissue is proportional to the mass density
of the tissue.Mass Density of Materials in Radiology
Substance Mass Density
Human Tissue
Lung 320
Fat 910
Soft tissue, muscle 1000
Bone 1850
Contrast Material
Air 1.3
Barium 3500
Iodine 4930
Other
Calcium 1550
Concrete 2350
Molybdenum 10,200
Lead 11,350
Rhenium 12,500
Tungsten 19,300
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Characteristics of Differential Absorption
As x-ray energy increases
a) Fewer Compton interactionsb) Many fewer photoelectric interactionsc) More transmission through tissue
As tissue atomic number increases
a) No change in Compton interactionsb) Many more photoelectric interactionsc) Less x-ray transmission
As tissue mass density increases
a) Proportional increase in Compton interactionsb) Proportional increase in photoelectric interactionc) Proportional reduction in x-ray transmission
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Exponential Attenuation
Attenuation - process by which radiation loses power as it travels through
matter and interacts with it. Beam attenuation is the basis of the contrast
observed in all X-ray based imaging methods.
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K-Edge
K-edge describes a sudden increase in the attenuation coefficient of photons
occurring at a photon energy just above the binding energy of the K shell
electron of the atoms interacting with the photons.
The sudden increase in attenuation is due to photoelectric absorption of the
photons. For this interaction to occur, the photons must have more energy
than the binding energy of the K shell electrons.
A photon having an energy just above the binding energy of the electron is
therefore more likely to be absorbed than a photon having an energy just
below this binding energy.
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Particles
Particles are the one that makes the atom and usually it is called subatomic
particles.
The atom is sometimes also called particles.
The three main subatomic particles that form an atom are protons, neutrons,
and electrons.
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Photons
A photon is a discrete bundle (or quantum) of electromagnetic (or light)
energy.
Photons are always in motion and, in a vacuum, have a constant speed of light
to all observers, at the vacuum speed of light (more commonly just called the
speed of light) of c = 2.998 x 108 m/s.
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Basic Properties of Photons
move at a constant velocity, c = 2.9979 x 108 m/s (i.e. "the speed of light"), in
free space.
have zero mass and rest energy.
carry energy and momentum.
can be destroyed/created when radiation is absorbed/emitted.
can have particle-like interactions (i.e. collisions) with electrons and other
particles, such as in the Compton effect.
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Inverse Square Law
an inverse-square law is any physical law stating that a specified physical
quantity or strength is inversely proportional to the square of the distance
from the source of that physical quantity.
generally applies when some force, energy, or other conserved quantity is
radiated outward radially in three-dimensional space from a point source.
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Inverse Square Law
It can be applied in (for examples):-
I. Gravitational
II. Electrostatics
III. Electromagnetic Radiation (e.g. x-ray, gamma ray, etc)
Where, I is Intensity
P is Point of Source
A is Area
The equation is:-
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End of Lecture
Thank You