Interaction of Radiation with Matter in Radiology and Nuclear Medicine Which Is/Are True? the Energy of a Photon Is: • Particle Interactions

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2014-2015 Residents' Core Physics Lectures Mondays 7:00-8:00 am in VA Radiology and UCSDMC Lasser Conference Rooms Nuclide Families Topic Chapters Date Faculty 1 Introduction and Basic Physics 1, 2 M 11/17 Andre 2 Interaction of Radiation and Matter 3 M 11/24 Andre RSNA Week No Lecture M 12/01 Family Nuclides with Same: Example 3 Computers 4 M 12/08 Hall 4 X-Ray Production 5 M 12/15 Andre 131 125 Christmas and New Year’s Holiday M 12/22, Isotopes Atomic number (Z) I , I : Z=53 12/29 5 Generators 5 M Andre 99 99 01/05/2015 Isobars Mass number (A) Mo , Tc : A=99

131 Isotones Neutron number (A-Z) 53I : 131-53=78 Isomers A and Z same but different Tc99m and Tc99: energy state Z=43, A=99, ΔE=142 keV Textbook: The Essential Physics of Medical Imaging, Bushberg, et al., Philadelphia: Lippincott Williams & Wilkins, 2002, 2nd Edition X = element symbol A Course Web Site??: http://3dviz.ucsd.edu/~radiology_residents/Home.html ZX Z = number of protons A = number of protons + neutrons 2

• Stable isotopes found Chapter 3: Interaction of Radiation with Matter along line N/Z = 1 at The Basis of X-Ray Imaging low Z Next time • Stable isotopes found we address along line N/Z = 1.5 at these high Z devices • Odd N and odd Z tend to be unstable “Huge relevance • Odd Z elements offer to a Resident” potential for NMR (unpaired p+) 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 Module 4: Interactions of Ionizing Radiation with Matter After completing this module, the resident should be able to apply the “Fundamental Knowledge” and “Clinical in Radiology and Nuclear Medicine Applications” learned from the module to example tasks, such as those found in “Clinical Problem-Solving.” Fundamental Knowledge: • Particle Interactions 1. Describe how charged particles interact with matter and the resulting effects these interactions can have on the material. • X- and Gamma-Ray Interactions 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. • Attenuation of X- and Gamma-Rays 3. Indentify how photons are attenuated (i.e., absorbed and scattered) within a material and the terms used to characterize the attenuation. • Absorption of Energy from X- and Gamma-Rays Clinical Application: • Imparted Energy, Equivalent Dose and Effective Dose 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. Lots of new definitions here! 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: Important to us for radiographic and CT image contrast, 1. Select an appropriate thyroid imaging agent based on its particulate emissions for pediatric imaging and for adult patient dose, x-ray production, Rad Tx, and more… imaging. Would these agents use the same isotopes or different isotopes? How does dose differ between these imaging isotopes? Recall: Contrast, Sharpness, Noise, Distortion, Dose 2. What is the purpose of adding Cu filters in vascular imaging? This topic affects Contrast, Noise and Dose 3. What makes a contrast agent radiolucent instead of radio-opaque? 6

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Recall: Chapter 2 • Energy: Definition? – Ability to do Work Which is/are true? The energy of a photon is: • Radiation: Definition? – A. Proportional to its wavelength – Propagation of energy through space – B. Proportional to its frequency • Types in Medicine – C. Inversely proportional to the exposure time – Heat (infrared) [EM] – D. Inversely proportional to its wavelength – Visible light [EM] 1 eV e- 1V – E. Can be expressed in terms of potential – X-Rays [EM] difference (volts) – γ-Rays [EM] – Microwaves (MRI) [EM] – Particulate [Mass, charge, kinetic energy] – Sound [Mechanical]

Chapter 3: Interaction of Radiation with Matter in Radiology and Nuclear Medicine Which is/are true? The energy of a photon is: • Particle Interactions – A. Proportional to its wavelength • X- and Gamma-Ray Interactions – B. Proportional to its frequency • Attenuation of X- and Gamma-Rays – C. Inversely proportional to the exposure time • Absorption of Energy from X- and Gamma-Rays • Imparted Energy, Equivalent Dose and Effective Dose – D. Inversely proportional to its wavelength – E. Can be expressed in terms of potential Lots of new definitions here! difference (volts) E = h f = h c / λ Important to us for radiographic and CT image contrast, patient dose, x-ray production, Rad Tx, and more… E (keV) = 12.4 / λ (Å) Recall: Contrast, Sharpness, Noise, Distortion, Dose This topic affects Contrast, Noise and Dose

Particles in Medicine Excitation Energy Relative Mass Particle Symbol Equivalent Charge (amu) (MeV) 2+ Alpha α, 4He +2 4.0028 3727 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: Excitation De-excitation with radiation • Imparted E < Binding Energy • Photon (low energy) •Elastic (no net Kinetic Energy loss) - •Inelastic (KE imparted) • Results in e at higher energy • Auger electron state • Excitation - 1 eV e 1V • Ionization • 70% of all particulate • Radiation loss interactions are non-ionizing

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Light vs. Heavy Charged Particles Ionization

• Imparted E > B.E. • Ion pair results • Secondary ionization Light Heavy

• 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 Why is this important to you? energies Excitation Bremsstrahlung is the principal source of x-ray production in E Loss by Bremsstrahlung = K.E.(MeV) • Z radiology (Chapter 5, next time) E Loss by Excitation + Ionization 820

X- and Gamma-Ray Interactions Summary of Particle Interactions • Attenuation Absorption + Scattering • Scattering * • Methods of Interaction: • Excitation • Ionization (Direct and Indirect) 1. “Coherent or Rayleigh or Classical” Scattering • Radiation (Bremsstrahlung) 2. Compton Scattering • Electron-Positron annihilation (Chapter 22, PET) 3. Photoelectric Absorption Two 180º opposed 0.511 MeV photons 4. Pair Production • Neutron interactions (Chapter 19) 5. Photo-disintegration – Interact with nuclei, mainly Hydrogen in tissue * – Split nucleus (fission) – Or captured by nucleus

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Rayleigh Scattering X- and Gamma-Ray Interactions • Attenuation Absorption + Scattering * • No net loss of energy by • Methods of Interaction: incident photon, no ionization 1. “Coherent,” “Rayleigh” or “Classical” Scattering • Excites entire atom 2. Compton Scattering (incoherent) • Results in change of direction 3. Photoelectric Absorption of photon 4. Pair Production • Occurs in tissue only at low x-ray energies, E = h  5. Photodisintegration therefore low frequencies, long wavelengths • #2 and #3 are * • Less significant for diagnostic radiology dominant • <5% of interactions above 70 keV in radiology • Maximum occurrence of 12% at 30 keV

Compton Scattering Involves only Compton Scattering - (Incoherent) Involves only Low B.E. e Low B.E. e- • 30 keV to 30 MeV: • Occurs for loosely bound Photon interactions electrons with negligible B.E. in soft tissue are • Input: photon predominantly φ Output: photon + electron Compton  • Main source of • hinc = hscat + K.E. e- undesirable φ • Scattered photon: 0°    180° scattered radiation • Scattered electron: 0°  φ  90° which reduces  image contrast

Compton Scattering Compton Scattering h When low energy photon h  inc scat h undergoes Compton 1 inc 1 cos  interaction, majority of energy 511 keV is retained by scattered photon • hscat = Energy of scattered photon and only slight amount is • h = Energy of incident photon transferred to electron. inc φ φ •  = scatter angle of photon 1. Example: 20 keV photon • As E of incident photon increases,  scattered at 180° 

 (and φ) decrease, so they hit receptor h 2 = 18.6 keV E (electron) = 1.4 keV • 2(scattered) = 1(incident) +  [conserve E] k •  (E loss) is maximum when  = 180° (backscatter) 2. Example: 2 MeV incident • Probability of Compton interaction photon at 180° scatter h 2 = 226 keV P (C)  1/hinc = 1/Einc P (C) is not dependent on Z Ek = 1774 keV P (C)  electron density ~   (g/cm3) (Motivation for Megavoltage Rx)

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X- and Gamma-Ray Interactions Photoelectric Effect • Products of interaction: • Attenuation Absorption + Scattering * – 1. Photoelectron (ejected electron) • Methods of Interaction: – 2. Positive ion (remaining atom) 1. “Coherent,” “Rayleigh” or “Classical” Scattering – 3. Characteristic radiation (discrete x-rays emitted when 2. Compton Scattering (incoherent) electron cascades to fill vacant shells) or Auger electrons 3. Photoelectric Absorption – 4. Original photon disappears • X-ray energy is unique to the element (characteristic) 4. Pair Production 5. Photodisintegration • #2 and #3 are dominant * in radiology

53I

Photoelectric Effect in Iodine Photoelectric Effect • 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. 53I If h inc< EB.E. interaction does not occur

Photoelectric Effect: K-Edge K-shell electron binding energies Semi-log plot • Prob. of Absorption or “absorption edges” (Photoelectric mass Atomic Number, Z Material K-Edge, keV attenuation K-edge = 37.4 keV coefficients) for 7.4 Avg Tissue 0.5 – Tissue (Z=7), – Iodine (Z=53), 20 Calcium 4.04 – Barium (Z=56) K-edge = 33.2 keV 53 Iodine 33.2 • Huge increase in Prob. Absorption K-edge < 1 keV 56 Barium 37.4 above the K-shell AbsorptionProbability of B.E. 74 Tungsten 69.5 82 Lead 88.0

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Radiological Significance of Photoelectric Effect Effect of Scatter on Radiographic Contrast

Scatter masks image contrast (noise) • 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 Not collimated Collimated • Iodine and barium image contrast are highest Scatter included Scatter reduced when kVp is set match the k-edge (grid)

Pair Production • h  > 1.02 MeV Photodisintegration • Excess is K.E. of β’s • High energy photon ejects a nuclear particle. • Probability of pair • Except for beryllium, this occurs for h  > 7 MeV. production – P (PP)  Z • Not significant for diagnostic radiology but – P (PP)  h > 1.02 important for Rx. MeV – P (PP)   (g/cm3)

Which of the following is false? A photon can undergo a Which of the following is false? A photon can undergo a _____ interaction followed by a _____ interaction. _____ interaction followed by a _____ interaction.

a. Compton, pair production a. Compton, pair production b. Compton, another Compton b. Compton, another Compton c. Compton, photoelectric c. Compton, photoelectric d. Photoelectric, Compton d. Photoelectric, Compton

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Attenuation of X- and •  = Rayleigh + Compton + Photoelectric + Pair Prod + Photodisint Gamma-Rays •  is function of: E (h), Z,  -2 • Removal of photons from • / = mass attenuation coefficient (cm /g) beam, or sum of scatter and absorption (from all interactions) • For monochromatic (single energy) radiation of intensity I0 -x -x – I = Io e or N = No e –  = linear attenuation coefficient (cm-1) –  = ln 2/HVL – HVL (cm) = 0.693/ = thickness of absorber that

attenuates beam by 1/2 ProbabilityAbsorption of –  is function of: E (h), Z, 

Which is/are False? The linear attenuation coefficient: Which is/are False? The linear attenuation coefficient: a. Is equal to the mass attenuation coefficient multiplied by a. Is equal to the mass attenuation coefficient multiplied the density of the absorbing material. by the density of the absorbing material. b. Varies mainly due to changes in electron density. b. Varies mainly due to changes in electron density. c. Is equal to the fractional reduction in the intensity per c. Is equal to the fractional reduction in the intensity unit absorber thickness. per unit absorber thickness. d. Becomes less dependent on Compton interactions than d. Becomes less dependent on Compton interactions than on photo-electric interactions at higher energies. on photo-electric interactions as energy increases. e. Is a constant for monoenergetic photon beam in a given e. Is a constant for monoenergetic photon beam in a absorbing material. given absorbing material.

Measuring Attenuation of X- and Gamma-Rays

• For monochromatic (single energy) radiation of intensity I0 -x -x Ice cubes – I = Io e or N = No e –  = linear attenuation coefficient (cm-1) Air bubbles –  = ln 2/HVL – HVL = 0.693/ = thickness of absorber that attenuates beam by 1/2 –  is function of: h , Z, 

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I Avg Energy (quality) and HVL increases  ex Beam Hardening Photon intensity (quantity) decreases I0

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: An attenuation curve for a 120 kVp x-ray beam yields the following data: Added filtration (mm Al) Relative Intensity Added filtration (mm Al) Relative Intensity 100 0 100% 100 0 100% 75 0.5 50 75 0.5 50 1 40 1 40 50 50 2 27 2 27 25 3 20 25 3 20 0 4 15 0 4 15 0 1 2 3 4 5 5 12 0 1 2 3 4 5 5 12 The second half value layer Add 1 mm to the beam. What The second half value layer Add 1 mm to the beam. What is approximately: is the HVL now? is approximately: is the HVL now? a. 1.0 mm a. 1.0 mm a. 1.0 mm a. 1.0 mm b. 1.7 mm b. 1.5 mm b. 1.7 mm b. 1.5 mm c. 2.0 mm c. 2.0 mm c. 2.0 mm c. 2.0 mm d. 2.2 mm d. 2.5 mm d. 2.2 mm d. 2.5 mm e. 3.0 mm e. 3.0 mm e. 3.0 mm e. 3.0 mm

An attenuation curve for a 120 kVp x-ray beam yields the following data: Next Session Added filtration (mm Al) Relative Intensity 100 0 100% 75 0.5 50 1 40 50 • Monday December 8, 7:00 a.m. @ VA 2 27 25 3 20 – Chapter 4: Computers, Dr. Hall 0 4 15 0 1 2 3 4 5 5 12 • Monday December 15, 7:00 a.m. @ VA The second half value layer Add 1 mm to the beam. What – Chapter 5: X-Ray Production is approximately: is the HVL now? a. 1.0 mm a. 1.0 mm • No Lectures Monday December 22 or 29 b. 1.7 mm b. 1.5 mm c. 2.0 mm c. 2.0 mm d. 2.2 mm d. 2.5 mm e. 3.0 mm e. 3.0 mm

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Attenuation of X- and Gamma-Rays

N Photon Fluence  A A narrow monoenergetic photon beam interacts with an absorber. Which is/are True?

a. The photon fluence decreases exponentially with N h Photon Energy Fluence    increasing depth in the absorber. A b. The photon fluence becomes zero beyond a maximum range determined by the photon energy. c. The LET depends on the depth in the absorber. N d. The photon fluence is reduced by the same fraction, as Photon Flux  A  Fluence Rate the beam passes through equal thickness of the absorber t at any depth.

AAPM/ABR Syllabus

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 A narrow monoenergetic photon beam interacts with an “Clinical Problem-Solving.” absorber. Which is/are True? Fundamental Knowledge: 1. Recognize that there are 2 different systems for units of measurement (i.e. SI and Classical) used to describe physical quantities. a. The photon fluence decreases exponentially with 2. Describe the SI and Classical units for measuring the ionization resulting from radiation increasing depth in the absorber. interactions in air (e.g., exposure-related quantities). b. The photon fluence becomes zero beyond a maximum 3. Describe the concepts of dose‐related quantities and their SI and Classical units. range determined by the photon energy. Clinical Application: c. The LET depends on the depth in the absorber. 1. Discuss the appropriate use or applicability of radiation quantities in the health care applications of imaging, therapy, and safety. d. The photon fluence is reduced by the same fraction, as the beam passes through equal thickness of the Clinical Problem-Solving: absorber at any depth. 1. Explain radiation exposure and dose quantities in lay language to a patient.

Units of Radiation

• Exposure (R) 1 R = 2.58 x 10-4 C/kg • Absorbed Dose (Gy) 1 Gy = 100 rad = 1 J/kg = 1 erg/gm • Kerma (Gy) K.E. transferred to charged particles

K = Ψ (tr/)E

• Equivalent Dose (Sv) H = wR D = 100 rem

• Effective Dose (Sv) E = ΣT wT HT • Activity (Bq) 3.7x1010 Bq = 1 Ci

(Also known as Quality Factor, largely based on LET)

• Effective Dose (Sv) E = ΣT wT HT

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Which of the following is not equal to one Gray? Which of the following is not equal to one Gray? a. 1.0 Joule/kg a. 1.0 Joule/kg b. 100 rads b. 100 rads c. 1.0 Sv/Quality Factor c. 1.0 Sv/Quality Factor d. (100 R)•(f-factor) d. (100 R)•(f-factor) e. 100 ergs/gm 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)

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 7.69 MeV αlpha in air B.

a. 16 •Specific Ionization increases with charge of particle b. 8 •Decreases with velocity of incident particle c. 4 •E.g., alpha may be as high as 7,000 IP/mm in air d. 2 - compared to e of 50-100 IP/cm e. 0.5 •As α slows, Bragg peak occurs •Bragg peak may be useful for Rad Tx

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