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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 . Topics. Introduction to the atomic basic structure Quantities and units

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radiation protection in diagnostic and interventional radiology

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY

L 5: Interaction of radiation with matter

topics
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

5: Interaction of radiation with matter

overview
Overview
  • To become familiar with the basic knowledge in radiation physics and image formation process.

5: Interaction of radiation with matter

part 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

electromagnetic spectrum
Electromagnetic spectrum

E

keV

1.5

0.12 keV

1

10

102

103

3 eV

104

X and  rays

IR

UV

light

100

10

1

0.1

0.01

4000

0.001

8000

Angström

IR: infrared, UV = ultraviolet

5: Interaction of radiation with matter

the atomic structure
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

5: Interaction of radiation with matter

basic units in physics si system
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

5: Interaction of radiation with matter

quantities and units
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

5: Interaction of radiation with matter

atom characteristics
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

5: Interaction of radiation with matter

electron nucleus interaction i
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.

5: Interaction of radiation with matter

electrons strike the nucleus

N

N

Bremsstrahlung

spectrum

E

E

n(E)

n1E1

n2E2

n1

n3E3

n2

n3

Emax

E1

E1

E2

E2

E3

E3

Electrons strike the nucleus

5: Interaction of radiation with matter

electron nucleus interaction ii
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

5: Interaction of radiation with matter

bremsstrahlung continuous spectrum
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

5: Interaction of radiation with matter

bremsstrahlung spectra
Bremsstrahlung spectra

dN/dE

dN/dE (spectral density)

E0

E

E0

E

From a “thin” target

From a “thick” target

E0= energy of electrons, E = energy of emitted photons

5: Interaction of radiation with matter

x ray spectrum energy
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

Bremsstrahlung

after filtration

keV

keV

50 100 150 200

5: Interaction of radiation with matter

ionization and associated energy transfers
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

5: Interaction of radiation with matter

spectral distribution of characteristic x rays i
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

5: Interaction of radiation with matter

spectral distribution of characteristic x rays ii
Spectral distribution of characteristic X Rays (II)

Energy

(eV)

K1

100

80

60

40

20

- 20

- 70

- 590

- 2800

- 11000

- 69510

6

5

4

3

2

0

P

O

K2

N

K1

M

L

L

L

K2

L

K

0 10 20 30 40 50 60 70 80

(keV)

5: Interaction of radiation with matter

part 5 interaction of radiation with matter22

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

Part 5: Interaction of radiation with matter

Topic 5: Primary and secondary ionization

stopping power
Stopping power
  • Loss of energy along track through both collisions and Bremsstrahlung
  • The linear stopping power of the medium

S = 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

5: Interaction of radiation with matter

linear energy transfer
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·µm-1

5: Interaction of radiation with matter

part 5 interaction of radiation with matter25

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

photoelectric effect
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

5: Interaction of radiation with matter

factors influencing photoelectric effect
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

5: Interaction of radiation with matter

compton scattering
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

5: Interaction of radiation with matter

compton scattering and tissue density
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)

5: Interaction of radiation with matter

part 5 interaction of radiation with matter30

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

exponential attenuation law of photons i
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

5: Interaction of radiation with matter

attenuation coefficients
Attenuation coefficients

Linear attenuation depends on:

  • characteristics of the medium (density )
  • photon beam energy

Mass attenuation coefficient:  / [m2kg-1]

    •  / same for water and water vapor (different )
    •  / similar for air and water (different µ)

5: Interaction of radiation with matter

attenuation of an heterogeneous beam
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

5: Interaction of radiation with matter

half value layer hvl
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)

5: Interaction of radiation with matter

photon interactions 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

(simplified representation)

5: Interaction of radiation with matter

dependence on z and photon energy
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

e.g., lead (Z = 82) for photons (E > 0.5 MeV)

5: Interaction of radiation with matter

part 5 interaction of radiation with matter37

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

x ray penetration and attenuation in human tissues
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 than water
  • lungs: weak due to density
      • bones can allow to visualize lung structures can be visualized with higher kVp (reducing photoelectric effect)
      • body cavities are made visible by means of contrast products (iodine, barium).

5: Interaction of radiation with matter

x ray penetration in human tissues

60 kV - 50 mAs

70 kV - 50 mAs

80 kV - 50 mAs

X Ray penetration in human tissues

5: Interaction of radiation with matter

x ray penetration in human tissues40
X Ray penetration in human tissues

Improvement of image contrast (lung)

5: Interaction of radiation with matter

x ray penetration in human tissues41
X Ray penetration in human tissues

Improvement of image contrast (bone)

5: Interaction of radiation with matter

x ray penetration in human tissues42
X Ray penetration in human tissues

70 kV - 25 mAs

70 kV - 50 mAs

70 kV - 80 mAs

5: Interaction of radiation with matter

x ray penetration in human tissues43
X Ray penetration in human tissues

5: Interaction of radiation with matter

x ray penetration in human tissues44
X Ray penetration in human tissues

5: Interaction of radiation with matter

purpose of using contrast media
Purpose of using contrast media
  • To enhance the contrast within a specific organ
  • To improve the image quality
  • Main used substances
    • Barium: abdominal parts
    • Iodine: urography, angiography, etc.

5: Interaction of radiation with matter

x ray absorption characteristics of iodine barium and body soft tissue
X Ray absorption characteristics of iodine, barium and body soft tissue

100

Iodine

10

Barium

X Ray ATTENUATION COEFFICIENT (cm2 g-1)

Soft Tissue

1

(keV)

0.1

20 30 40 50 60 70 80 90 100

5: Interaction of radiation with matter

photoelectric absorption and radiological image
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

5: Interaction of radiation with matter

contribution of photoelectric and compton interactions to attenuation of x rays in water muscle

10

1.0

Total

X Ray ATTENUATION COEFFICIENT (cm2 g-1)

0.1

Compton + Coherent

Photoelectric

(keV)

0.01

20 40 60 80 100 120 140

Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle)

5: Interaction of radiation with matter

contribution of photoelectric and compton interactions to attenuation of x rays in bone

10

1.0

Total

X Ray ATTENUATION COEFFICIENT (cm2 g-1)

0.1

Compton + Coherent

Photoelectric

(keV)

0.01

20 40 60 80 100 120 140

Contribution of photoelectric and Compton interactions to attenuation of X Rays in bone

5: Interaction of radiation with matter

x ray penetration in human tissues50
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: bodycavities can be made visible by means of contrast media: iodine, barium

5: Interaction of radiation with matter

effect of compton scattering
Effect of Compton scattering

Effects of scattered radiation on:

  • image quality
  • patient absorbed energy
  • scattered radiation in the room

5: Interaction of radiation with matter

summary
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.

5: Interaction of radiation with matter

where to get more information 1
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.
  • The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011

5: Interaction of radiation with matter

where to get more information 2
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.

5: Interaction of radiation with matter