2Y03 Lecture 10 - 2023 (Student) PDF

Radiographic Physics and Instrumentation I Unit 1.5: Photon Interactions with Matter Lecture 10: Photon Interactions with Matter 1 Objectives • Attenuation • Interaction with matter • Scattering • Photoelectric interaction • Pair production • Photodisintegration Attenuation • Reduction in the...

Radiographic Physics and Instrumentation I Unit 1.5: Photon Interactions with Matter Lecture 10: Photon Interactions with Matter 1 Objectives • Attenuation • Interaction with matter • Scattering • Photoelectric interaction • Pair production • Photodisintegration Attenuation • Reduction in the number of x-ray photons • Reduction in the intensity of the beam • Losing energy through interactions • Due to both absorption and scatter X-Ray Photon Interaction • Interaction • Interact = whole atom, an orbital electron or with the nucleus • Interaction depends on the energy of the incoming photon Five Basic Interactions • Coherent Scattering • Compton Scattering • Photoelectric Effect or Absorption • Pair Production • Photodisintegration Coherent Scattering • Interaction is between very low energy x-rays and matter • Also called Classical or Unmodified scatter • 2 types • Thompson and Rayleigh Classical or Coherent Scattering Thompson Scattering • Photon (10 keV) interacts with a single electron • Vibrates or oscillates the electron • Scattered photon = same energy and wavelength as initial photon but travels in a slightly different direction Rayleigh Scattering • Incident photon interacts with the whole atom • Causes excitation of all electrons • Very low energy range 15 to 30 keV • Scattered photon Rayleigh Scattering • Mammography • ↓ image quality • Low probability in diagnostic energy range Bushberg 3-6 Compton Scattering • Predominant interaction of x-ray in the diagnostic energy range with soft tissue • 26 keV to 30 MeV • Interaction of incident x-ray photon with loosely bound outer shell electron Compton Scattering Compton Scattering • The electron is ejected from the atom • Compton or recoil electron • The incident photon leaves the atom in a slightly different direction and with some reduction in energy • Scattered photon Energy of the Scattered Photon • Depends on: • Incident photon energy • Angle of scatter relative to incident direction θ Bushberg 3-7 Transfer of Energy • In diagnostic energy range (50 to 150 keV), the majority of the energy is transferred to the scattered photon • Scattered photon tends to scatter in a more forward direction and has a higher chance of reaching image receptor • Degrades image quality Scattered Photon • Scattered photon can deflect at any angle • Deflection of 00, no energy is transferred • As the angle of deflection ↑, more energy is transferred to recoil electron and less is imparted to scattered photon Probability of Compton Interaction • Incident photon energy ↑, probability of Compton interaction ↑ relative all other interactions taking place at higher energies • Overall, the number of compton interaction ↓, as photon energy increases Probability of Compton Interactions Photoelectric Effect • Incident photon interacts with an inner shell electron • Energy of the incident photon is SLIGHTLY GREATER than the B.E of the inner shells (K or L) Photoelectric Effect • Photoelectron • EP.E = EO – EB.E (Bushberg) • Ei = Eb + E KE (Bushong) • Atom is ionized with an inner shell electron vacancy Photoelectric Effect • The electron is ejected from the atom • Similar to characteristic radiation sequence except the Z is very small, therefore, the photon emitted does not have enough energy to escape the interaction site Photoelectric Effect Photoelectric Effect • The incident x-ray photon energy must be greater than the B.E of the inner shell electron • Vacancy = filled by an electron from outer shell and this will generate a chain reaction of electron cascade Probability of P.E. Interaction • Probability is approximately proportional to Z3/E3 • Eg: BARIUM (Z=56) and CALCIUM (Z=20) • (56/20)3 = 22 Probability of P.E. Interaction • Therefore, probability of P.E is 22 times greater in Ba than in Ca for a photon of a particular energy • P.E directly proportional to Z3 • P.E inversely proportional E3 Relative Probability of P.E. Probabilities of Compton and P.E Probability of Compton and P.E. Example • Consider a monoenergetic beam where E = x • Total # of interactions = 2000 • # of Photoelectric interactions = 1000 • # of Compton Interactions = 1000 • Proportion of Compton to P.E. interactions = 1 : 1 • What happens if the energy is doubled? Probability of Compton and P.E. Example • Energy Doubled • Compton proportional to 1/E = ½ • 1000/2 = 500 Compton Interactions • Photoelectric proportional to 1/E3 = 1/2 3 = 1/8 • 1000/8 = 125 Photoelectric interactions • Total # interactions = 500 + 125 = 625 • Proportion of Compton to P.E. interactions = 500 :125 = 4:1 Probability of Interaction and Density • ↑ density of attenuator → ↑ interactions • ↑ density means more electrons per unit of volume • More electrons = more possible interaction sites • Able to visualize different tissues due to: • Photoelectric effect → proportional to Z3 • All interactions ↑ with ↑ density Pair Production - Threshold is 1.02 MeV - Can only occur at energies exceeding 1.02 MeV - Interaction is with the electric field of the nucleus - Photon energy is split into electron – positron pair Pair Production • Any energy in excess of the threshold is imparted to the electrons as K.E. • Electron is absorbed by other nearby atoms • Positron = more volatile Pair Production • The electron and positron lose their K.E by excitation and ionization • Positron → rest, it interacts with an electron → gives off 2 annihilation photons. • Two photons move in opposite direction are given off • Each photon energy = 0.511 MeV • No significance in diagnostic imaging Pair Production Bushong Photodisintegration • Above 10 MeV • Nucleus • Photon totally absorbed and excites the nucleus • Nucleus instantly emits a nucleon or other nuclear fragments Photodisintegration ▪ Interaction between high energy photon and the nucleus • Interaction = above 10 MeV • Excites the nucleus • Responds by emitting a nuclear fragment • No relevance to diagnostic imaging Photodisintegration Two Interactions Significant to General Radiography Differential Absorption • Responsible for difference in optical densities on the image • Essentially the difference between photons absorbed photoelectrically, and photons passing through to the IR without interaction • Increases as kVp decreases • Dose increases as kVp decreases • Must compromise between contrast and dose Summary • Attenuation • Coherent scattering • Compton scattering • Photoelectric effect • Pair production • Photodisintegration Readings • Unit 1.5 course manual & study guide • Bushberg chapter 3, section 3.2 • Bushong chapter 10