IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 5: Interaction of radiation with matter
IAEA
International Atomic Energy Agency
Topics
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Introduction to the atomic basic structure
Quantities and units
Bremsstrahlung production
Characteristic X Rays
Primary and secondary ionization
Photo-electric effect and Compton scattering
Beam attenuation and half value thickness
Principle of radiological image formation
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5: Interaction of radiation with matter
Overview
• To become familiar with the basic knowledge
in radiation physics and image formation
process.
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 1: Introduction to the atomic basic structure
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International Atomic Energy Agency
Electromagnetic spectrum
E
1.5
0.12 keV 1
3 eV
IR light
8000 4000
10
102
103
104
X and  rays
UV
100
10
1

0.1
IR: infrared, UV = ultraviolet
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keV
5: Interaction of radiation with matter
0.01 0.001
Angström
The atomic structure
• The nuclear structure
• protons and neutrons = nucleons
• Z protons with a positive electric charge
• (1.6 10-19 C)
• neutrons with no charge (neutral)
• number of nucleons = mass number A
• The extranuclear structure
• Z electrons (light particles with electric
charge)
• equal to proton charge but negative
• The atom is normally electrically
neutral
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 2: Quantities and units
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International Atomic Energy Agency
Basic units in physics (SI system)
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Time: 1 second [s]
Length: 1 meter [m]
Mass: 1 kilogram [kg]
Energy: 1 joule [J]
Electric charge: 1 coulomb
[C]
• Other quantities and units
• Power: 1 watt [W] (1 J/s)
• 1 mAs = 0.001 C
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5: Interaction of radiation with matter
Quantities and units
• electron-volt [eV]:
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1.603 10-19 J
1 keV = 103 eV
1 MeV = 106 eV
1 electric charge: 1.6
10-19 C
mass of proton: 1.672
10-27 kg
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5: Interaction of radiation with matter
Atom characteristics
A, Z and associated quantities
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Hydrogen
A=1
Z=1
EK= 13.6 eV
Carbon
A = 12
Z=6
EK= 283 eV
Phosphor
A = 31
Z = 15
EK= 2.1 keV
Tungsten
A = 183
Z = 74
EK= 69.5 keV
Uranium
A = 238
Z = 92
EK= 115.6 keV
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 3: Bremsstrahlung production
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International Atomic Energy Agency
Electron-nucleus interaction (I)
• Bremsstrahlung:
• radiative energy loss (E) by electrons
slowing down on passage through a
material
•  is the deceleration of the incident
electron by the nuclear Coulomb
field
•  radiation energy (E) (photon) is
emitted.
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5: Interaction of radiation with matter
Electrons strike the nucleus
N
N
Bremsstrahlung
spectrum
E
E
n(E)
n1E1
n2E2
n3E3
n1
n2
n3
E1
E2
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Emax
E1
E3
E2
E3
5: Interaction of radiation with matter
Electron-nucleus interaction (II)
• With materials of high atomic number
• the energy loss is higher
• The energy loss by Bremsstrahlung
• > 99% of kinetic E loss as heat production, it increases
with increasing electron energy
• X Rays are dominantly produced by
Bremsstrahlung
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5: Interaction of radiation with matter
Bremsstrahlung continuous spectrum
• Energy (E) of Bremsstrahlung photons may take
any value between “zero” and the maximum
kinetic energy of incident electrons
• Number of photons as a function of E is
proportional to 1/E
• Thick target  continuous linear spectrum
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5: Interaction of radiation with matter
Bremsstrahlung spectra
dN/dE (spectral density)
E0 E
From a “thin” target
dN/dE
E0
E
From a “thick” target
E0= energy of electrons, E = energy of emitted photons
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5: Interaction of radiation with matter
X Ray spectrum energy
• Maximum energy of Bremsstrahlung photons
• kinetic energy of incident electrons
• In X Ray spectrum of radiology installations:
• Max (energy) = Energy at X Ray tube peak voltage
E
Bremsstrahlung
50 100 150 200
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Bremsstrahlung
after filtration
keV
keV
5: Interaction of radiation with matter
Ionization and associated energy
transfers
• Example: electrons in water
• ionization energy: 16 eV (for a water molecule
• other energy transfers associated to ionization
• excitations (each requires only a few eV)
• thermal transfers (at even lower energy)
• W = 32 eV is the average loss per ionization
• it is characteristic of the medium
• independent of incident particle and of its energy
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 4: Characteristic X Rays
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International Atomic Energy Agency
Spectral distribution of characteristic
X Rays (I)
• Starts with ejection of e- mainly from k shell (also
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possible for L, M,…) by ionization
e- from L or M shell fall into the vacancy created in
the k shell
Energy difference is emitted as photons
A sequence of successive electron transitions
between energy levels
Energy of emitted photons is characteristic of the
atom
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5: Interaction of radiation with matter
Spectral distribution of characteristic
X Rays (II)
Energy
(eV)
K1
100
- 20
- 70
- 590
- 2800
- 11000
- 69510
80
P
O
N
M
L
6
5
4
3
2
40
L L
20
K
0
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K2
60
0
K2
L
10 20
K1
30 40
5: Interaction of radiation with matter
50 60
70 80
(keV)
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 5: Primary and secondary ionization
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International Atomic Energy Agency
Stopping power
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Loss of energy along track through both collisions and
Bremsstrahlung
The linear stopping power of the medium
S = E / x [MeV.cm-1]
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E: energy loss
x: element of track
for distant collisions: the lower the electron energy, the
higher the amount transferred
most Bremsstrahlung photons are of low energy
collisions (hence ionization) are the main source of
energy loss
except at high energies or in media of high Z
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5: Interaction of radiation with matter
Linear Energy Transfer
• Biological effectiveness of ionizing
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radiation
Linear Energy Transfer (LET): amount of
energy transferred to the medium per
unit of track length of the particle
Unit: e.g. [keV.m-1]
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5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 6: Photoelectric effect and Compton
scattering
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International Atomic Energy Agency
Photoelectric effect
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Incident photon with energy h
 all photon energy absorbed by a tightly bound
orbital electron
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ejection of electron from the atom
Kinetic energy of ejected electron: E = h - EB
Condition: h > EB (electron binding energy)
Recoil of the residual atom
Attenuation (or interaction) coefficient
 photoelectric absorption coefficient
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5: Interaction of radiation with matter
Factors influencing photoelectric
effect
• Photon energy (h) > electron binding energy EB
• The probability of interaction decreases as h
increases
• It is the main effect at low photon energies
• The probability of interaction increases with Z3 (Z:
atomic number)
• High-Z materials are strong X Ray absorber
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5: Interaction of radiation with matter
Compton scattering
• Interaction between photon and electron
• h = Ea + Es (energy is conserved)
• Ea: energy transferred to the atom
• Es: energy of the scattered photon
• momentum is conserved in angular distributions
• At low energy, most of initial energy is scattered
• ex: Es > 80% (h) if h <1 keV
• Increasing Z  increasing probability of interaction.
Compton is practically independent of Z in diagnostic
range
• The probability of interaction decreases as h increases
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5: Interaction of radiation with matter
Compton scattering and tissue
density
• Variation of Compton effect according to:
• energy (related to X Ray tube kV) and material
• lower E  Compton scattering process  1/E
• Increasing E  decreasing photon deviation angle
• Mass attenuation coefficient  constant with Z
• effect proportional to the electron density in the medium
• small variation with atomic number (Z)
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 7: Beam attenuation and Half value
thickness
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International Atomic Energy Agency
Exponential attenuation law of
photons (I)
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Any interaction  change in photon energy and or
direction
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Accounts for all effects: Compton, photoelectric,…
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dI/I = -  dx
Ix = I0 exp (- x)
• I: number of photons per unit area per second [s-1]
• : the linear attenuation coefficient [m-1]
•  / [m2.kg-1]: mass attenuation coefficient
•  [kg.m-3]: material density
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5: Interaction of radiation with matter
Attenuation coefficients
Linear attenuation depends on:
• characteristics of the medium (density )
• photon beam energy
Mass attenuation coefficient:  / [m2kg-1]
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 / same for water and water vapor (different )
 / similar for air and water (different µ)
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5: Interaction of radiation with matter
Attenuation of an heterogeneous
beam
• Various energies  No more exponential
attenuation
• Progressive elimination of photons through the
matter
• Lower energies preferentially
• This effect is used in the design of filters
 Beam hardening effect
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5: Interaction of radiation with matter
Half Value Layer (HVL)
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HVL: thickness reducing beam intensity by 50%
Definition holds strictly for monoenergetic beams
Heterogeneous beam  hardening effect
I/I0 = 1/2 = exp (-µ HVL)
HVL = 0.693 / µ
HVL depends on material and photon energy
HVL characterizes beam quality
 modification of beam quality through filtration
 HVL (filtered beam)  HVL (beam before filter)
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5: Interaction of radiation with matter
Photon interactions with matter
Scattered photon
Compton effect
Secondary
photons
Fluorescence photon
(Characteristic radiation)
Annihilation photon
Incident
photons
Non interacting photons
Recoil electron
Secondary
electrons
Photoelectron
(Photoelectric effect)
Electron pair
E > 1.02 MeV
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(simplified
representation)
5: Interaction of radiation with matter
Dependence on Z and photon energy
• Z < 10  predominating Compton effect
• higher Z increase photoelectric effect
• at low E: photoelectric effect predominates in bone compared
to soft tissue
• (total photon absorption)
• contrast products  photoelectric absorption
high Z (Barium 56, Iodine 53)
• use of photoelectric absorption in radiation protection
ex: lead (Z = 82) for photons (E > 0.5 MeV)
IAEA
5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 5: Interaction of radiation with
matter
Topic 8: Principle of radiological image formation
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International Atomic Energy Agency
X Ray penetration and attenuation in
human tissues
Attenuation of an X Ray beam:
• air:
negligible
• bone:
significant due to relatively high
density (atom mass number of Ca)
• soft tissue (e.g. muscle,.. ): similar to water
• fat tissue: less important than water
• lungs:
weak due to density
• bones can allow to visualize lung structures with higher kVp
(reducing photoelectric effect)
• body cavities are made visible by means of contrast products
(iodine, barium).
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5: Interaction of radiation with matter
X Ray penetration in human tissues
60 kV - 50 mAs
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70 kV - 50 mAs
80 kV - 50 mAs
5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (lung)
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5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (bone)
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5: Interaction of radiation with matter
X Ray penetration in human tissues
70 kV - 25 mAs
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70 kV - 50 mAs
70 kV - 80 mAs
5: Interaction of radiation with matter
X Ray penetration in human tissues
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5: Interaction of radiation with matter
X Ray penetration in human tissues
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5: Interaction of radiation with matter
Purpose of using contrast media
• To make visible soft tissues normally transparent
to X Rays
• To enhance the contrast within a specific organ
• To improve the image quality
• Main used substances
• Barium: abdominal parts
• Iodine: urography, angiography, etc.
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5: Interaction of radiation with matter
X Ray absorption characteristics of
iodine, barium and body soft tissue
X Ray ATTENUATION COEFFICIENT (cm2 g-1)
100
10
1
0.1
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(keV)
20
30
40
50 60 70 80 90 100
5: Interaction of radiation with matter
Photoelectric absorption and
radiological image
• In soft or fat tissues (close to water), at low
energies (E< 25 - 30 keV)
• The photoelectric effect predominates
•  main contributor to image formation on
the radiographic film
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5: Interaction of radiation with matter
X Ray ATTENUATION COEFFICIENT (cm2 g-1)
Contribution of photoelectric and Compton interactions
to attenuation of X Rays in water (muscle)
10
1.0
Total
0.1
Compton + Coherent
Photoelectric
0.01
20
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40
60
80
100
120
5: Interaction of radiation with matter
(keV)
140
Contribution of photoelectric and Compton interactions
to attenuation of X Rays in bone
X Ray ATTENUATION COEFFICIENT (cm2 g-1)
10
1.0
Total
0.1
0.01
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Compton + Coherent
Photoelectric
20
40
60
80
100
120
5: Interaction of radiation with matter
(keV)
140
X Ray penetration in human tissues
• Higher kVp reduces
photoelectric effect
• The image contrast is lowered
• Bones and lungs structures can
simultaneously be visualized
Note: body cavities can be
made visible by means of
contrast media: iodine, barium
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5: Interaction of radiation with matter
Effect of Compton scattering
Effects of scattered radiation on:
• image quality
• patient absorbed energy
• scattered radiation in the room
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5: Interaction of radiation with matter
Summary
• The elemental parts of the atom constituting
both the nucleus and the extranucleus
structure can be schematically represented.
• Electrons and photons have different types of
interactions with matter
• Two different forms of X Rays production
Bremsstrahlung and characteristic radiation
contribute to the image formation process.
• Photoelectric and Compton effects have a
significant influence on the image quality.
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5: Interaction of radiation with matter
Where to Get More Information (1)
• Part 2: Lecture on “Radiation quantities and Units”
• Attix FH. Introduction to radiological physics and
radiation dosimetry. New York, NY: John Wiley &
Sons, 1986. 607 pp. ISBN 0-47101-146-0.
• Johns HE, Cunningham JR. Solution to selected
problems form the physics of radiology 4th edition.
Springfield, IL: Charles C. Thomas, 1991.
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5: Interaction of radiation with matter
Where to Get More Information (2)
• Wahlstrom B. Understanding Radiation.
Madison, WI: Medical Physics Publishing,
1995. ISBN 0-944838-62-6.
• Evans RD. The atomic nucleus. Malabar, FL:
R.E. Kriege, 1982 (originally 1955) ISBN 089874-414-8.
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5: Interaction of radiation with matter