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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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CHAPTER 4: RADIATION ATTENUATION

CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

Series of Photoelectric Effect

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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

Differential Absorption

Differential Absorption occurs because of Compton scattering,

photoelectric effect, and x-rays transmitted through the patient.

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CONTENTS

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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CONTENTS

Dependence on Atomic Number

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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

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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CONTENTS

End of Lecture

Thank You

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