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

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Interaction of Radiation with Matter in Radiology and Nuclear Medicine Which Is/Are True? the Energy of a Photon Is: • Particle Interactions – A 11/19/2014 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 1 11/19/2014 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 2 11/19/2014 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 3 11/19/2014 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 • hinc = hscat + 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 • hscat = 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/hinc = 1/Einc P (C) is not dependent on Z Ek = 1774 keV P (C) electron density ~ (g/cm3) (Motivation for Megavoltage Rx) 4 11/19/2014 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.
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