5/19/2018 L5 Interaction of Radiation With Matter
1/54
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
5/19/2018 L5 Interaction of Radiation With Matter
2/54
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
5/19/2018 L5 Interaction of Radiation With Matter
3/54
IAEA
5: Interaction of radiation with matter
Overview
To become familiar with the basicknowledge in radiation physics and image
formation process.
5/19/2018 L5 Interaction of Radiation With Matter
4/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 1: Introduction to the atomic basic structure
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
5/19/2018 L5 Interaction of Radiation With Matter
5/54
IAEA
5: Interaction of radiation with matter
Electromagnetic spectrum
1041031021013 eV
0.0010.010.1110
0.12 keV
100
1.5
Angstrm
keV
Xand raysUVIR light
E
40008000
IR: infrared, UV= ultraviolet
5/19/2018 L5 Interaction of Radiation With Matter
6/54
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 electricallyneutral
5/19/2018 L5 Interaction of Radiation With Matter
7/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 2: Quantities and units
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
5/19/2018 L5 Interaction of Radiation With Matter
8/54IAEA 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
5/19/2018 L5 Interaction of Radiation With Matter
9/54IAEA 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: 1.67210-27 kg
5/19/2018 L5 Interaction of Radiation With Matter
10/54IAEA 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
5/19/2018 L5 Interaction of Radiation With Matter
11/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 3: Bremsstrahlung production
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
5/19/2018 L5 Interaction of Radiation With Matter
12/54IAEA 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.
5/19/2018 L5 Interaction of Radiation With Matter
13/54
IAEA 5: Interaction of radiation with matter
Electrons strike the nucleus
N N
n(E) E
E1
E2E3
n1
n3n2
E1
E2E3
n1E1
n2E2
n3E3
E
Emax
Bremsstrahlung
spectrum
5/19/2018 L5 Interaction of Radiation With Matter
14/54
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 increaseswith increasing electron energy
X Rays are dominantly produced byBremsstrahlung
5/19/2018 L5 Interaction of Radiation With Matter
15/54
IAEA 5: Interaction of radiation with matter
Bremsstrahlung continuous spectrum
Energy (E) of Bremsstrahlung photons may takeany value between zero and the maximum
kinetic energy of incident electrons
Number of photons as a function of E isproportional to 1/E
Thick target continuous linear spectrum
5/19/2018 L5 Interaction of Radiation With Matter
16/54
IAEA 5: Interaction of radiation with matter
Bremsstrahlung spectra
dN/dE (spectral density)dN/dE
From a thin target EE0
EE0
E0= energy of electrons, E = energy of emitted photons
From a thick target
5/19/2018 L5 Interaction of Radiation With Matter
17/54
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
I i ti d i t d
5/19/2018 L5 Interaction of Radiation With Matter
18/54
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
5/19/2018 L5 Interaction of Radiation With Matter
19/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 4: Characteristic X Rays
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Spectral distribution of characteristic
5/19/2018 L5 Interaction of Radiation With Matter
20/54
IAEA 5: Interaction of radiation with matter
Spectral distribution of characteristic
X Rays (I)
Starts with ejection of e- mainly from k shell (alsopossible 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 transitionsbetween energy levels
Energy of emitted photons is characteristic of theatom
Spectral distribution of characteristic
5/19/2018 L5 Interaction of Radiation With Matter
21/54
IAEA 5: Interaction of radiation with matter
L
K
M
NOP
Energy
(eV)
654
3
2
0
- 20- 70- 590
- 2800
- 11000
- 69510 0 10 20 30 40 50 60 70 80
100
80
60
40
20
L
L
L
K1
K
2
K
2
K1
(keV)
Spectral distribution of characteristic
X Rays (II)
5/19/2018 L5 Interaction of Radiation With Matter
22/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 5: Primary and secondary ionization
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
5/19/2018 L5 Interaction of Radiation With Matter
23/54
IAEA 5: Interaction of radiation with matter
Stopping power
Loss of energy along track through both collisions andBremsstrahlung
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, thehigher the amount transferred
most Bremsstrahlung photons are of low energy
collisions (hence ionization) are the main source ofenergy loss
except at high energies or in media of high Z
5/19/2018 L5 Interaction of Radiation With Matter
24/54
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 perunit of track length of the particle
Unit: e.g. [keV.m-1]
5/19/2018 L5 Interaction of Radiation With Matter
25/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 6: Photoelectric effect and Compton
scattering
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
5/19/2018 L5 Interaction of Radiation With Matter
26/54
IAEA 5: Interaction of radiation with matter
Photoelectric effect
Incident photon with energy h
all photon energy absorbed by a tightly boundorbital 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
Factors influencing photoelectric
5/19/2018 L5 Interaction of Radiation With Matter
27/54
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
5/19/2018 L5 Interaction of Radiation With Matter
28/54
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
5/19/2018 L5 Interaction of Radiation With Matter
29/54
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)
5/19/2018 L5 Interaction of Radiation With Matter
30/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 7: Beam attenuation and Half value
thickness
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Exponential attenuation law of
5/19/2018 L5 Interaction of Radiation With Matter
31/54
IAEA 5: Interaction of radiation with matter
Exponential attenuation law ofphotons (I)
Any interaction change in photon energy and ordirection
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
5/19/2018 L5 Interaction of Radiation With Matter
32/54
IAEA 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] /same for water and water vapor (different )
/similar for air and water (different )
Attenuation of an heterogeneous
5/19/2018 L5 Interaction of Radiation With Matter
33/54
IAEA 5: Interaction of radiation with matter
Attenuation of an heterogeneousbeam
Various energies No more exponentialattenuation
Progressive elimination of photons through the
matter Lower energies preferentially
This effect is used in the design of filters
Beam hardening effect
5/19/2018 L5 Interaction of Radiation With Matter
34/54
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 (- HVL) HVL = 0.693 / HVL depends on material and photon energy
HVL characterizesbeam quality
modification of beam quality through filtration HVL (filtered beam) HVL (beam before filter)
5/19/2018 L5 Interaction of Radiation With Matter
35/54
IAEA 5: Interaction of radiation with matter
Photon interactions with matter
Annihilation photon
Incident
photons
Secondary
photons
Secondaryelectrons
Scattered photon
Compton effect
Fluorescence photon
(Characteristic radiation)
Recoil electron
Electron pair
E > 1.02 MeV
Photoelectron
(Photoelectric effect)
Non interacting photons
(simplified
representation)
5/19/2018 L5 Interaction of Radiation With Matter
36/54
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 comparedto 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)
5/19/2018 L5 Interaction of Radiation With Matter
37/54
IAEAInternational Atomic Energy Agency
Part 5: Interaction of radiation withmatter
Topic 8: Principle of radiological image formation
IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
X Ray penetration and attenuation in
5/19/2018 L5 Interaction of Radiation With Matter
38/54
IAEA 5: Interaction of radiation with matter
X Ray penetration and attenuation inhuman tissues
Attenuation of an X Ray beam: air: negligible
bone: significantdue to relatively highdensity (atom mass number of Ca)
soft tissue (e.g. muscle,.. ): similarto water fat tissue: less important than water
lungs: weakdue 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).
5/19/2018 L5 Interaction of Radiation With Matter
39/54
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
5/19/2018 L5 Interaction of Radiation With Matter
40/54
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (lung)
X R i i h i
5/19/2018 L5 Interaction of Radiation With Matter
41/54
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
Improvement of image contrast (bone)
X R t ti i h ti
5/19/2018 L5 Interaction of Radiation With Matter
42/54
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
X R t ti i h ti
5/19/2018 L5 Interaction of Radiation With Matter
43/54
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
X R t ti i h ti
5/19/2018 L5 Interaction of Radiation With Matter
44/54
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
P f i t t di
5/19/2018 L5 Interaction of Radiation With Matter
45/54
IAEA 5: Interaction of radiation with matter
Purpose of using contrast media
To make visible soft tissues normally transparentto 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.
X Ray absorption characteristics of
5/19/2018 L5 Interaction of Radiation With Matter
46/54
IAEA 5: Interaction of radiation with matter
X Ray absorption characteristics ofiodine, barium and body soft tissue
100
20 30 40 50 60 70 80 90 100
10
1
0.1(keV)
XRayATT
ENUATIONCOEFF
ICIENT(cm2g
-1)
Photoelectric absorption and
5/19/2018 L5 Interaction of Radiation With Matter
47/54
IAEA 5: Interaction of radiation with matter
Photoelectric absorption andradiological image
In soft or fat tissues (close to water), at lowenergies (E< 25 - 30 keV)
The photoelectriceffect predominates
main contributor to image formation onthe radiographic film
Contribution of photoelectric and Compton interactions
5/19/2018 L5 Interaction of Radiation With Matter
48/54
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)XRayA
TTENUATIONCOE
FFICIENT(cm2g
-1)
Contribution of photoelectric and Compton interactions
5/19/2018 L5 Interaction of Radiation With Matter
49/54
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 + Coherent
Photoelectric(keV)
XRayATTENUATIONCOE
FFICIENT(cm2g
-1)
X R t ti i h ti
5/19/2018 L5 Interaction of Radiation With Matter
50/54
IAEA 5: Interaction of radiation with matter
X Ray penetration in human tissues
Higher kVp reducesphotoelectric effect
The image contrast is lowered
Bones and lungs structures can
simultaneously be visualized
Note: bodycavities can bemade visible by means ofcontrast media: iodine, barium
Effect of Compton scattering
5/19/2018 L5 Interaction of Radiation With Matter
51/54
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
Summary
5/19/2018 L5 Interaction of Radiation With Matter
52/54
IAEA 5: Interaction of radiation with matter
Summary
The elemental parts of the atom constitutingboth the nucleus and the extranucleusstructure can be schematically represented.
Electrons and photons have different types ofinteractions with matter
Two different forms of X Rays productionBremsstrahlungand characteristicradiation
contribute to the image formation process. Photoelectric and Compton effects have asignificant influence on the image quality.
Where to Get More Information (1)
5/19/2018 L5 Interaction of Radiation With Matter
53/54
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.
Where to Get More Information (2)
5/19/2018 L5 Interaction of Radiation With Matter
54/54
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.