Download Chapter 3: Interaction of Radiation with Matter in

11/19/2014
2014-2015 Residents' Core Physics Lectures
Mondays 7:00-8:00 am in VA Radiology and UCSDMC Lasser Conference Rooms
1
2
3
4
5
Topic
Introduction and Basic Physics
Interaction of Radiation and Matter
RSNA Week No Lecture
Computers
X-Ray Production
Christmas and New Year’s Holiday
Generators
Chapters
1, 2
3
4
5
5
Date
M 11/17
M 11/24
M 12/01
M 12/08
M 12/15
M 12/22,
12/29
M
01/05/2015
Faculty
Andre
Andre
Hall
Andre
Andre
Nuclide Families
Family
Example
Isotopes
Atomic number (Z)
I131, I125:
Isobars
Mass number (A)
Mo99, Tc99: A=99
Isotones
Neutron number (A-Z)
131:
53I
Isomers
A and Z same but different
energy state
Tc99m and Tc99:
Z=43, A=99, ΔE=142
keV
Textbook: The Essential Physics of Medical Imaging, Bushberg, et al., Philadelphia: Lippincott
Williams & Wilkins, 2002, 2nd Edition
Course Web Site??: http://3dviz.ucsd.edu/~radiology_residents/Home.html
• Stable isotopes found
along line N/Z = 1 at
low Z
• Stable isotopes found
along line N/Z = 1.5 at
high Z
• Odd N and odd Z tend
to be unstable
• Odd Z elements offer
potential for NMR
(unpaired p+)
Nuclides with Same:
A
ZX
Z=53
131-53=78
X = element symbol
Z = number of protons
A = number of protons + neutrons
2
Chapter 3: Interaction of Radiation with Matter
The Basis of X-Ray Imaging
Next time
we address
these
devices
“Huge relevance
to a Resident”
A
ZX
X = element symbol
Z = number of protons
A = number of protons + neutrons
or digital detector
AAPM/ABR Syllabus
Chapter 3: Interaction of Radiation with Matter
in Radiology and Nuclear Medicine
•
•
•
•
•
Particle Interactions
X- and Gamma-Ray Interactions
Attenuation of X- and Gamma-Rays
Absorption of Energy from X- and Gamma-Rays
Imparted Energy, Equivalent Dose and Effective Dose
Lots of new definitions here!
Important to us for radiographic and CT image contrast,
patient dose, x-ray production, Rad Tx, and more…
Recall: Contrast, Sharpness, Noise, Distortion, Dose
This topic affects Contrast, Noise and Dose
Module 4: Interactions of Ionizing Radiation with Matter
After completing this module, the resident should be able to apply the “Fundamental Knowledge” and “Clinical
Applications” learned from the module to example tasks, such as those found in “Clinical Problem-Solving.”
Fundamental Knowledge:
1. Describe how charged particles interact with matter and the resulting effects these interactions can have on the
material.
2. Describe the processes by which x-ray and γ-ray photons interact with individual atoms in a material and the
characteristics that determine which processes are likely to occur.
3. Indentify how photons are attenuated (i.e., absorbed and scattered) within a material and the terms used to
characterize the attenuation.
Clinical Application:
1. Identify which photon interactions are dominant for each of the following imaging modalities: mammography,
projection radiography, fluoroscopy, CT, and nuclear medicine imaging procedures.
2. Understand how image quality and patient dose are affected by these interactions.
3. What are the appropriate x-ray beam energies to be used when iodine and barium contrast agents are used?
4. How does the type of photon interaction change with increasing energy, and what is the associated clinical
significance?
Clinical Problem-Solving:
1. Select an appropriate thyroid imaging agent based on its particulate emissions for pediatric imaging and for adult
imaging. Would these agents use the same isotopes or different isotopes? How does dose differ between these
imaging isotopes?
2. What is the purpose of adding Cu filters in vascular imaging?
3. What makes a contrast agent radiolucent instead of radio-opaque?
6
1
11/19/2014
Recall: Chapter 2
• Energy: Definition?
– Ability to do Work
• Radiation: Definition?
– Propagation of energy through space
• Types in Medicine
–
–
–
–
–
–
–
Heat (infrared) [EM]
1 eV
Visible light [EM]
X-Rays [EM]
γ-Rays [EM]
Microwaves (MRI) [EM]
Particulate [Mass, charge, kinetic energy]
Sound [Mechanical]
e- 1V
Which is/are true? The energy of a photon is:
– A. Proportional to its wavelength
– B. Proportional to its frequency
– C. Inversely proportional to the exposure time
– D. Inversely proportional to its wavelength
– E. Can be expressed in terms of potential
difference (volts)
Chapter 3: Interaction of Radiation with Matter
in Radiology and Nuclear Medicine
Which is/are true? The energy of a photon is:
– A. Proportional to its wavelength
– B. Proportional to its frequency
– C. Inversely proportional to the exposure time
– D. Inversely proportional to its wavelength
– E. Can be expressed in terms of potential
difference (volts)
E = hf = hc/λ
E (keV) = 12.4 / λ (Å)
Particles in Medicine
Particle
Symbol
Relative
Charge
Mass
(amu)
Energy
Equivalent
(MeV)
3727
Alpha
α, 4He2+
+2
4.0028
Proton
p, 1H+
+1
1.007593
938
Electron
e-, β-
-1
0.000548
0.511
Positron
e + , β+
+1
0.000548
0.511
Neutron
n0
0
1.008982
940
Particles interact with matter through Scattering:
•Elastic (no net Kinetic Energy loss)
•Inelastic (KE imparted)
• Excitation
1 eV e 1V
• Ionization
• Radiation loss
•
•
•
•
•
Particle Interactions
X- and Gamma-Ray Interactions
Attenuation of X- and Gamma-Rays
Absorption of Energy from X- and Gamma-Rays
Imparted Energy, Equivalent Dose and Effective Dose
Lots of new definitions here!
Important to us for radiographic and CT image contrast,
patient dose, x-ray production, Rad Tx, and more…
Recall: Contrast, Sharpness, Noise, Distortion, Dose
This topic affects Contrast, Noise and Dose
Excitation
Excitation
De-excitation with radiation
• Imparted E < Binding Energy • Photon (low energy)
• Results in e- at higher energy • Auger electron
state
• 70% of all particulate
interactions are non-ionizing
2
11/19/2014
Light vs. Heavy Charged Particles
Ionization
• Imparted E > B.E.
• Ion pair results
• Secondary ionization
Heavy
Light
• Linear Energy Transfer
• LET = Energy/unit path length (eV/cm)
• LET proportional to Q2/K.E.
• LET (eV/cm) = Spec. Ion.(IP/cm) • Avg. E per IP (eV/IP)
• LET largely determines “biological effectiveness”
• High LET: α , p+
• Low LET: β+, β-, electromagnetic
Bremsstrahlung
[“Braking”]
Radiation
• Decelerate e- ( velocity)
• Bremsstrahlung x-ray
E = h  = K.E. loss of e• Probability of interaction is
proportional to Z2 of absorber
• Results in spectrum of x-ray
energies
Why is this important to you?
Excitation
E Loss by Bremsstrahlung
= K.E.(MeV) • Z
E Loss by Excitation + Ionization
820
Summary of Particle Interactions
•
•
•
•
•
Scattering
Excitation
Ionization (Direct and Indirect)
Radiation (Bremsstrahlung)
Electron-Positron annihilation (Chapter 22, PET)
Two 180º opposed 0.511 MeV photons
• Neutron interactions (Chapter 19)
– Interact with nuclei, mainly Hydrogen in tissue
– Split nucleus (fission)
– Or captured by nucleus
Bremsstrahlung is the principal
source of x-ray production in
radiology (Chapter 5, next time)
X- and Gamma-Ray Interactions
•
•
Attenuation
Absorption + Scattering *
Methods of Interaction:
1. “Coherent or Rayleigh or Classical” Scattering
2. Compton Scattering
3. Photoelectric Absorption
4. Pair Production
5. Photo-disintegration
*
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11/19/2014
X- and Gamma-Ray Interactions
Rayleigh Scattering
• No net loss of energy by
incident photon, no ionization
• Excites entire atom
• Results in change of direction
of photon
• Occurs in tissue only at low x-ray energies, E = h 
therefore low frequencies, long wavelengths
• Less significant for diagnostic radiology
• <5% of interactions above 70 keV
• Maximum occurrence of 12% at 30 keV
Compton Scattering
(Incoherent)
• 30 keV to 30 MeV:
Photon interactions
in soft tissue are
predominantly
Compton
• Main source of
undesirable
scattered radiation
which reduces
image contrast
Involves only
Low B.E. e-
Attenuation
Absorption + Scattering *
Methods of Interaction:
1. “Coherent,” “Rayleigh” or “Classical” Scattering
2. Compton Scattering (incoherent)
3. Photoelectric Absorption
4. Pair Production
5. Photodisintegration
• #2 and #3 are
*
dominant
in radiology
• hinc = hscat + K.E. e• Scattered photon: 0°    180°
• Scattered electron: 0°  φ  90°
φ
Involves only
Low B.E. e-
Compton Scattering
• Occurs for loosely bound
electrons with negligible B.E.
• Input:
photon
Output: photon + electron
φ


Compton Scattering
Compton Scattering
h scat 
•
•
h inc
h inc
1  cos  
511 keV
• hscat = Energy of scattered photon
• hinc = Energy of incident photon
•  = scatter angle of photon

• As E of incident photon increases,
 (and φ) decrease, so they hit receptor
• 2(scattered) = 1(incident) +  [conserve E]
•  (E loss) is maximum when  = 180° (backscatter)
• Probability of Compton interaction
P (C)  1/hinc = 1/Einc
P (C) is not dependent on Z
P (C)  electron density ~   (g/cm3)
1
φ
When low energy photon
undergoes Compton
interaction, majority of energy
is retained by scattered photon
and only slight amount is
transferred to electron.
1. Example: 20 keV photon
scattered at 180°
h 2 = 18.6 keV
Ek (electron) = 1.4 keV
2. Example: 2 MeV incident
photon at 180° scatter
h 2 = 226 keV
Ek = 1774 keV
(Motivation for Megavoltage Rx)
φ

4
11/19/2014
Photoelectric Effect
X- and Gamma-Ray Interactions
•
•
Attenuation
Absorption + Scattering *
Methods of Interaction:
1. “Coherent,” “Rayleigh” or “Classical” Scattering
2. Compton Scattering (incoherent)
3. Photoelectric Absorption
4. Pair Production
5. Photodisintegration
• #2 and #3 are
dominant
*
in radiology
• Products of interaction:
– 1. Photoelectron (ejected electron)
– 2. Positive ion (remaining atom)
– 3. Characteristic radiation (discrete x-rays emitted when
electron cascades to fill vacant shells) or Auger electrons
– 4. Original photon disappears
• X-ray energy is unique to the element (characteristic)
53I
Photoelectric Effect
Photoelectric Effect in Iodine
• Probability of photoelectric interaction per unit mass
– P (P.E.)  Z3
– P (P.E.)  1/(h )3 = 1/E3
– P (P.E.)   (g/cm3)
– Higher probability when (h ) is close to EB.E.
– Higher probability with higher EB.E. such as K shell
53I
Ee- = h inc – EB.E.
If h inc< EB.E. interaction does not occur
Photoelectric Effect: K-Edge
53I
K-shell electron binding energies
• Prob. of Absorption
(Photoelectric mass
attenuation
coefficients) for
– Tissue (Z=7),
– Iodine (Z=53),
– Barium (Z=56)
• Huge increase in
Prob. Absorption
above the K-shell
B.E.
Probability of Absorption
Semi-log plot
or “absorption edges”
K-edge = 37.4 keV
K-edge = 33.2 keV
K-edge < 1 keV
Atomic Number, Z
Material
K-Edge, keV
7.4 Avg
Tissue
0.5
20
Calcium
4.04
53
Iodine
33.2
56
Barium
37.4
74
Tungsten
69.5
82
Lead
88.0
5
11/19/2014
Radiological Significance of Photoelectric Effect
• No scatter radiation (characteristic x-rays in tissue
have very low E, < 1 keV), “pure” x-ray contrast
• P(P.E.)  Z3 means that P.E. enhances subject
contrast (differences in attenuation between
tissues), inversely proportional to E3
• Higher doses to patient when it occurs in tissue:
total absorption of photon, no energy escapes
• Iodine and barium image contrast are highest
when kVp is set match the k-edge
Effect of Scatter on Radiographic Contrast
Scatter masks image contrast (noise)
Not collimated
Scatter included
Collimated
Scatter reduced
(grid)
Pair Production
• h  > 1.02 MeV
• Excess is K.E. of
β’s
• Probability of pair
production
– P (PP)  Z
– P (PP)  h > 1.02
MeV
– P (PP)   (g/cm3)
Which of the following is false? A photon can undergo a
_____ interaction followed by a _____ interaction.
a.
b.
c.
d.
Compton, pair production
Compton, another Compton
Compton, photoelectric
Photoelectric, Compton
Photodisintegration
• High energy photon ejects a nuclear particle.
• Except for beryllium, this occurs for h  > 7 MeV.
• Not significant for diagnostic radiology but
important for Rx.
Which of the following is false? A photon can undergo a
_____ interaction followed by a _____ interaction.
a.
b.
c.
d.
Compton, pair production
Compton, another Compton
Compton, photoelectric
Photoelectric, Compton
6
11/19/2014
•  = Rayleigh + Compton + Photoelectric + Pair Prod + Photodisint
•  is function of: E (h), Z, 
• / = mass attenuation coefficient (cm-2/g)
• Removal of photons from
beam, or sum of scatter and
absorption (from all interactions)
• For monochromatic (single
energy) radiation of intensity I0
– I = Io e-x or N = No e-x
–  = linear attenuation coefficient (cm -1)
–  = ln 2/HVL
– HVL (cm) = 0.693/ = thickness of absorber that
attenuates beam by 1/2
–  is function of: E (h), Z, 
Which is/are False? The linear attenuation coefficient:
a. Is equal to the mass attenuation coefficient multiplied by
the density of the absorbing material.
b. Varies mainly due to changes in electron density.
c. Is equal to the fractional reduction in the intensity per
unit absorber thickness.
d. Becomes less dependent on Compton interactions than
on photo-electric interactions at higher energies.
e. Is a constant for monoenergetic photon beam in a given
absorbing material.
Probability of Absorption
Attenuation of X- and
Gamma-Rays
Which is/are False? The linear attenuation coefficient:
a. Is equal to the mass attenuation coefficient multiplied
by the density of the absorbing material.
b. Varies mainly due to changes in electron density.
c. Is equal to the fractional reduction in the intensity
per unit absorber thickness.
d. Becomes less dependent on Compton interactions than
on photo-electric interactions as energy increases.
e. Is a constant for monoenergetic photon beam in a
given absorbing material.
Measuring Attenuation of X- and Gamma-Rays
Ice cubes
Air bubbles
• For monochromatic (single energy) radiation of intensity I0
– I = Io e-x or N = No e-x
–  = linear attenuation coefficient (cm-1)
–  = ln 2/HVL
– HVL = 0.693/ = thickness of absorber that attenuates
beam by 1/2
–  is function of: h , Z, 
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11/19/2014
Avg Energy (quality) and HVL increases
I
 e  x
I0
Beam Hardening
Photon intensity (quantity) decreases
Monochromatic X-Rays
1st HVL = 2nd HVL
Polyenergetic X-Rays
e.g., Diagnostic x-ray beam
2nd HVL > 1st HVL
An attenuation curve for a 120 kVp x-ray beam yields the following data:
100
75
50
25
0
0
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
Relative Intensity
100%
50
40
27
20
15
12
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
An attenuation curve for a 120 kVp x-ray beam yields the following data:
100
75
50
25
0
0
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
Relative Intensity
100%
50
40
27
20
15
12
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
An attenuation curve for a 120 kVp x-ray beam yields the following data:
100
75
50
25
0
0
1
2
3
4
5
Added filtration (mm Al)
0
0.5
1
2
3
4
5
The second half value layer
is approximately:
a. 1.0 mm
b. 1.7 mm
c. 2.0 mm
d. 2.2 mm
e. 3.0 mm
Relative Intensity
100%
50
40
27
20
15
12
Add 1 mm to the beam. What
is the HVL now?
a. 1.0 mm
b. 1.5 mm
c. 2.0 mm
d. 2.5 mm
e. 3.0 mm
Next Session
• Monday December 8, 7:00 a.m. @ VA
– Chapter 4: Computers, Dr. Hall
• Monday December 15, 7:00 a.m. @ VA
– Chapter 5: X-Ray Production
• No Lectures Monday December 22 or 29
8
11/19/2014
Attenuation of X- and Gamma-Rays
Photon Fluence 
N
A
A narrow monoenergetic photon beam interacts with an
absorber. Which is/are True?
Photon Energy Fluence 
N
Photon Flux  A
t
N h

A
 Fluence Rate
a.
b.
c.
d.
The photon fluence decreases exponentially with
increasing depth in the absorber.
The photon fluence becomes zero beyond a maximum
range determined by the photon energy.
The LET depends on the depth in the absorber.
The photon fluence is reduced by the same fraction, as
the beam passes through equal thickness of the absorber
at any depth.
AAPM/ABR Syllabus
A narrow monoenergetic photon beam interacts with an
absorber. Which is/are True?
a.
b.
c.
d.
The photon fluence decreases exponentially with
increasing depth in the absorber.
The photon fluence becomes zero beyond a maximum
range determined by the photon energy.
The LET depends on the depth in the absorber.
The photon fluence is reduced by the same fraction,
as the beam passes through equal thickness of the
absorber at any depth.
Module 5: Radiation Units
After completing this module, the resident should be able to apply the “Fundamental Knowledge”
and “Clinical Applications” learned from the module to example tasks, such as those found in
“Clinical Problem-Solving.”
Fundamental Knowledge:
1. Recognize that there are 2 different systems for units of measurement (i.e. SI and Classical)
used to describe physical quantities.
2. Describe the SI and Classical units for measuring the ionization resulting from radiation
interactions in air (e.g., exposure-related quantities).
3. Describe the concepts of dose‐related quantities and their SI and Classical units.
Clinical Application:
1. Discuss the appropriate use or applicability of radiation quantities in the health care
applications of imaging, therapy, and safety.
Clinical Problem-Solving:
1. Explain radiation exposure and dose quantities in lay language to a patient.
52
Units of Radiation
• Exposure (R)
• Absorbed Dose (Gy)
• Kerma (Gy)
• Equivalent Dose (Sv)
• Effective Dose (Sv)
• Activity (Bq)
1 R = 2.58 x 10-4 C/kg
1 Gy = 100 rad = 1 J/kg = 1 erg/gm
K.E. transferred to charged particles
K = Ψ (tr/)E
H = wR D = 100 rem
E = ΣT wT HT
3.7x1010 Bq = 1 Ci
(Also known as Quality Factor, largely based on LET)
• Effective Dose (Sv)
E = ΣT wT HT
9
11/19/2014
Which of the following is not equal to one Gray?
a. 1.0 Joule/kg
b. 100 rads
c. 1.0 Sv/Quality Factor
d. (100 R)•(f-factor)
e. 100 ergs/gm
Which of the following is not equal to one Gray?
a. 1.0 Joule/kg
b. 100 rads
c. 1.0 Sv/Quality Factor
d. (100 R)•(f-factor)
e. 100 ergs/gm
Next Session
• Monday December 8, 7:00 a.m. @ VA
– Chapter 4: Computers, Dr. Hall
• Monday December 15, 7:00 a.m. @ VA
– Chapter 5: X-Ray Production
• No Lectures Monday December 23 or 30
Specific Ionization (Ion Pairs/mm)
7.69 MeV αlpha in air
•Specific Ionization increases with charge of particle
•Decreases with velocity of incident particle
•E.g., alpha may be as high as 7,000 IP/mm in air
compared to e- of 50-100 IP/cm
•As α slows, Bragg peak occurs
•Bragg peak may be useful for Rad Tx
Two materials are irradiated by monoenergetic photons.
Material A has an atomic number of 14 and B has an
atomic number of 7. The photoelectric component of the
mass attenuation coefficient of A is ______ times that of
B.
a.
b.
c.
d.
e.
16
8
4
2
0.5
10
11/19/2014
Two materials are irradiated by monoenergetic photons.
Material A has an atomic number of 14 and B has an
atomic number of 7. The photoelectric component of the
mass attenuation coefficient of A is ______ times that of
B.
a.
b.
c.
d.
e.
16
8
4
2
0.5
P (P.E.)  Z3
Mean Free Path
MFP 
1

1
0.693
HVL
 1.44 HVL

11