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IAEAInternational Atomic Energy Agency
RADIATION PROTECTION INDIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 5: Interaction of radiation with matter
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Topics
• 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
IAEA 5: Interaction of radiation with matter
Overview
• To become familiar with the basic knowledge in radiation physics and image formation process.
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 1: Introduction to the atomic basic structure
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Electromagnetic spectrum
1041031021013 eV
0.0010.010.1110
0.12 keV
100
1.5
Angström
keV
X and raysUVIR light
E
40008000
IR: infrared, UV = ultraviolet
IAEA 5: Interaction of radiation with matter
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
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 2: Quantities and units
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Basic units in physics (SI system)
• 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
IAEA 5: Interaction of radiation with matter
Quantities and units
• electron-volt [eV]: 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
IAEA 5: Interaction of radiation with matter
Atom characteristics
A, Z and associated quantities• 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
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 3: Bremsstrahlung production
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
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.
IAEA 5: Interaction of radiation with matter
Electrons strike the nucleus
N N
n(E) E
E1
E2E3
n1
n3
n2
E1
E2E3
n1E1
n2E2
n3E3
E
Emax
Bremsstrahlungspectrum
IAEA 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
IAEA 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
IAEA 5: Interaction of radiation with matter
Bremsstrahlung spectra
dN/dE (spectral density) dN/dE
From a “thin” target EE0EE0
E0= energy of electrons, E = energy of emitted photons
From a “thick” target
IAEA 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
BremsstrahlungE
keV50 100 150 200
Bremsstrahlung after filtration
keV
IAEA 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
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 4: Characteristic X Rays
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Spectral distribution of characteristic X Rays (I)
• Starts with ejection of e- mainly from k shell (also 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
IAEA 5: Interaction of radiation with matter
LL
KK
MMNNOOPP
Energy(eV)
65432
0
- 20- 70- 590- 2800- 11000
- 695100 10 20 30 40 50 60 70 80
100
80
60
40
20
L L L
K1
K2
K2
K1
(keV)
Spectral distribution of characteristic X Rays (II)
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 5: Primary and secondary ionization
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Stopping power
• Loss of energy along track through both collisions and Bremsstrahlung
• The linear stopping power of the mediumS = E / x [MeV.cm-1]
• 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
IAEA 5: Interaction of radiation with matter
Linear Energy Transfer
• Biological effectiveness of ionizing 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]
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 6: Photoelectric effect and Compton scattering
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Photoelectric effect
• Incident photon with energy h • all photon energy absorbed by a tightly bound
orbital electron• 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
IAEA 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
IAEA 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
IAEA 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)
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 7: Beam attenuation and Half value thickness
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
Exponential attenuation law of photons (I)• Any interaction change in photon energy and or
direction
• Accounts for all effects: Compton, photoelectric,…
• 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
IAEA 5: Interaction of radiation with matter
Attenuation coefficients
Linear attenuation depends on:• characteristics of the medium (density )• photon beam energyMass attenuation coefficient: / [m2kg-1]• / same for water and water vapor (different )• / similar for air and water (different µ)
IAEA 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
IAEA 5: Interaction of radiation with matter
Half Value Layer (HVL)
• 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)
IAEA 5: Interaction of radiation with matter
Photon interactions with matter
Annihilation photon
Incidentphotons
Secondaryphotons
Secondaryelectrons
Scattered photonCompton effect
Fluorescence photon(Characteristic radiation)
Recoil electron
Electron pairE > 1.02 MeV
Photoelectron(Photoelectric effect)
Non interacting photons
(simplified representation)
IAEA 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 absorptionhigh Z (Barium 56, Iodine 53)
• use of photoelectric absorption in radiation protectionex: lead (Z = 82) for photons (E > 0.5 MeV)
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation with matter
Topic 8: Principle of radiological image formation
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
IAEA 5: Interaction of radiation with matter
X Ray penetration and attenuation in human tissuesAttenuation 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).
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (lung)
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (bone)
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
70 kV - 25 mAs 70 kV - 50 mAs 70 kV - 80 mAs
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
IAEA 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.
IAEA 5: Interaction of radiation with matter
X Ray absorption characteristics of iodine, barium and body soft tissue
100
20 30 40 50 60 70 80 90 100
10
1
0.1
IodineIodine
(keV)
X R
ay A
TTEN
UA
TIO
N C
OEF
FIC
IEN
T (c
m2 g
-1)
BariumBariumSoft Tissue
Soft Tissue
IAEA 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
IAEA 5: Interaction of radiation with matter
Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle)
20 40 60 80 100 120 140
10
1.0
0.1
0.01
Total
Compton + CoherentPhotoelectric
(keV)
X R
ay A
TTEN
UA
TIO
N C
OEF
FIC
IEN
T (c
m2 g
-1)
IAEA 5: Interaction of radiation with matter
Contribution of photoelectric and Compton interactions to attenuation of X Rays in bone
20 40 60 80 100 120 140
10
1.0
0.1
0.01
Total
Compton + CoherentPhotoelectric
(keV)
X R
ay A
TTEN
UA
TIO
N C
OEF
FIC
IEN
T (c
m2 g
-1)
IAEA 5: Interaction of radiation with matter
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
IAEA 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
IAEA 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.
IAEA 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.
IAEA 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 0-89874-414-8.