11/19/2014. Chapter 3: Interaction of Radiation with Matter in Radiology and Nuclear Medicine. Nuclide Families. Family Nuclides with Same: Example

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1 Residents' Core Physics Lectures Mondays 7:00-8:00 am in VA Radiology and UCSDMC Lasser Conference Rooms 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 3 Computers 4 M 12/08 Hall 4 X-Ray Production 5 M 12/15 Andre Christmas and New Year s Holiday M 12/22, 12/29 5 Generators 5 M 01/05/2015 Andre Textbook: The Essential Physics of Medical Imaging, Bushberg, et al., Philadelphia: Lippincott Williams & Wilkins, 2002, 2 nd Edition Course Web Site??: Nuclide Families Family Nuclides with Same: Example Isotopes Atomic number (Z) I 131, I 125 : Z=53 Isobars Mass number (A) Mo 99, Tc 99 : A=99 Isotones Neutron number (A-Z) 53I 131 : =78 Isomers A and Z same but different Tc 99m and Tc 99 : energy state Z=43, A=99, ΔE=142 kev ZX A X = element symbol Z = number of protons A = number of protons + neutrons 2 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+) ZX A X = element symbol Z = number of protons A = number of protons + neutrons Next time we address these devices Chapter 3: Interaction of Radiation with Matter The Basis of X-Ray Imaging or digital detector Huge relevance to a Resident 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 AAPM/ABR Syllabus 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

2 Recall: Chapter 2 Energy: Definition? Ability to do Work Radiation: Definition? Propagation of energy through space Types in Medicine Heat (infrared) [EM] Visible light [EM] X-Rays [EM] γ-rays [EM] Microwaves (MRI) [EM] 1 ev e - 1V Particulate [Mass, charge, kinetic energy] Sound [Mechanical] 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) 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 = h f = h c / λ E (kev) = 12.4 / λ (Å) 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 Particle Particles in Medicine Symbol Relative Charge Mass (amu) Energy Equivalent (MeV) Alpha α, 4 He Proton p, 1 H Electron e -, β Positron e +, β Neutron n Particles interact with matter through Scattering: Elastic (no net Kinetic Energy loss) Inelastic (KE imparted) Excitation Ionization Radiation loss 1 ev e - 1V 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

3 Ionization Light vs. Heavy Charged Particles Imparted E > B.E. Ion pair results Secondary ionization Light Heavy Linear Energy Transfer LET = Energy/unit path length (ev/cm) LET proportional to Q 2 /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 Z 2 of absorber Results in spectrum of x-ray energies E Loss by Bremsstrahlung = K.E.(MeV) Z E Loss by Excitation + Ionization 820 Excitation Why is this important to you? Bremsstrahlung is the principal source of x-ray production in radiology (Chapter 5, next time) Summary of Particle Interactions Scattering Excitation Ionization (Direct and Indirect) Radiation (Bremsstrahlung) Electron-Positron annihilation (Chapter 22, PET) Two 180º opposed MeV photons Neutron interactions (Chapter 19) Interact with nuclei, mainly Hydrogen in tissue Split nucleus (fission) Or captured by nucleus 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 3

4 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 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 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 - φ Compton Scattering Occurs for loosely bound electrons with negligible B.E. Input: photon Output: photon + electron h inc = h scat + K.E. e- Scattered photon: Scattered electron: 0 φ 90 Involves only Low B.E. e - φ Compton Scattering h inc h scat h inc 1 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/E inc P (C) is not dependent on Z P (C) electron density ~ (g/cm 3 ) φ Compton Scattering 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 E k (electron) = 1.4 kev 2. Example: 2 MeV incident photon at 180 scatter h 2 = 226 kev E k = 1774 kev (Motivation for Megavoltage Rx) φ 4

5 Probability of Absorption 11/19/2014 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 Photoelectric Effect 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 in Iodine Photoelectric Effect Probability of photoelectric interaction per unit mass P (P.E.) Z 3 P (P.E.) 1/(h ) 3 = 1/E 3 P (P.E.) (g/cm 3 ) Higher probability when (h ) is close to E B.E. Higher probability with higher E B.E. such as K shell 53I E e- = h inc E B.E. If h inc < E B.E. interaction does not occur 53I 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. Photoelectric Effect: K-Edge Semi-log plot K-edge = 37.4 kev K-edge = 33.2 kev K-edge < 1 kev K-shell electron binding energies or absorption edges Atomic Number, Z Material K-Edge, kev 7.4 Avg Tissue Calcium Iodine Barium Tungsten Lead

6 Radiological Significance of Photoelectric Effect Effect of Scatter on Radiographic Contrast No scatter radiation (characteristic x-rays in tissue have very low E, < 1 kev), pure x-ray contrast P(P.E.) Z 3 means that P.E. enhances subject contrast (differences in attenuation between tissues), inversely proportional to E 3 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 Scatter included Scatter masks image contrast (noise) Scatter reduced (grid) Not collimated Collimated 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/cm 3 ) Pair Production 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. Compton, pair production b. Compton, another Compton c. Compton, photoelectric d. Photoelectric, Compton Which of the following is false? A photon can undergo a interaction followed by a interaction. a. Compton, pair production b. Compton, another Compton c. Compton, photoelectric d. Photoelectric, Compton 6

7 Probability of Absorption 11/19/2014 Attenuation of X- and Gamma-Rays Removal of photons from beam, or sum of scatter and absorption (from all interactions) For monochromatic (single energy) radiation of intensity I 0 I = I o e -x or N = N o 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, = Rayleigh + Compton + Photoelectric + Pair Prod + Photodisint is function of: E (h), Z, / = mass attenuation coefficient (cm -2 /g) 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. 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 I 0 I = I o e -x or N = N o 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, 7

8 I I 0 e x Avg Energy (quality) and HVL increases Beam Hardening Photon intensity (quantity) decreases Monochromatic X-Rays 1 st HVL = 2 nd HVL Polyenergetic X-Rays e.g., Diagnostic x-ray beam 2 nd HVL > 1 st HVL An attenuation curve for a 120 kvp x-ray beam yields the following data: Added filtration (mm Al) Relative Intensity % The second half value layer Add 1 mm to the beam. What is approximately: is the HVL now? a. 1.0 mm a. 1.0 mm 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 An attenuation curve for a 120 kvp x-ray beam yields the following data: Added filtration (mm Al) Relative Intensity % The second half value layer Add 1 mm to the beam. What is approximately: is the HVL now? a. 1.0 mm a. 1.0 mm 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 An attenuation curve for a 120 kvp x-ray beam yields the following data: Added filtration (mm Al) Relative Intensity % The second half value layer Add 1 mm to the beam. What is approximately: is the HVL now? a. 1.0 mm a. 1.0 mm 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 Next Session Monday December 8, 7:00 VA Chapter 4: Computers, Dr. Hall Monday December 15, 7:00 VA Chapter 5: X-Ray Production No Lectures Monday December 22 or 29 8

9 Attenuation of X- and Gamma-Rays Photon Fluence N A Photon Energy Fluence Photon Flux N A t N h A Fluence Rate A narrow monoenergetic photon beam interacts with an absorber. Which is/are True? a. The photon fluence decreases exponentially with increasing depth in the absorber. 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. d. 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. The photon fluence decreases exponentially with increasing depth in the absorber. 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. d. 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) 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 = w R D = 100 rem Effective Dose (Sv) E = Σ T w T H T Activity (Bq) 3.7x10 10 Bq = 1 Ci (Also known as Quality Factor, largely based on LET) Effective Dose (Sv) E = Σ T w T H T 9

10 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 VA Chapter 4: Computers, Dr. Hall Monday December 15, 7:00 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 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. 16 b. 8 c. 4 d. 2 e

11 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. 16 b. 8 c. 4 d. 2 e. 0.5 P (P.E.) Z 3 Mean Free Path 1 MFP HVL 1.44 HVL 11