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L05 Interaction

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IAEA International Atomic Energy Agency RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 5: Interaction of radiation with matter IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Transcript
Page 1: L05 Interaction

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

Page 2: L05 Interaction

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

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IAEA 5: Interaction of radiation with matter

Overview

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

Page 4: L05 Interaction

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

Page 5: L05 Interaction

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

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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

Page 7: L05 Interaction

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

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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

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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

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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

Page 11: L05 Interaction

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

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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.

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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

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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

Page 15: L05 Interaction

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

Page 16: L05 Interaction

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

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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

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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

Page 19: L05 Interaction

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

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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

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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)

Page 22: L05 Interaction

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

Page 23: L05 Interaction

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

Page 24: L05 Interaction

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]

Page 25: L05 Interaction

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

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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

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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

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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

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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)

Page 30: L05 Interaction

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

Page 31: L05 Interaction

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

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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)

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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

Page 34: L05 Interaction

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)

Page 35: L05 Interaction

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)

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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)

Page 37: L05 Interaction

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

Page 38: L05 Interaction

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).

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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

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IAEA 5: Interaction of radiation with matter

X Ray penetration in human tissues

Improvement of image contrast (lung)

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IAEA 5: Interaction of radiation with matter

X Ray penetration in human tissues

Improvement of image contrast (bone)

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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

Page 43: L05 Interaction

IAEA 5: Interaction of radiation with matter

X Ray penetration in human tissues

Page 44: L05 Interaction

IAEA 5: Interaction of radiation with matter

X Ray penetration in human tissues

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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.

Page 46: L05 Interaction

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

Page 47: L05 Interaction

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

Page 48: L05 Interaction

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)

Page 49: L05 Interaction

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)

Page 50: L05 Interaction

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

Page 51: L05 Interaction

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

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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.

Page 53: L05 Interaction

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.

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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.


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