Resident Physics Lectures Christensen, Chapter 4 Basic Interactions Between X-Rays and Matter George David Associate Professor Medical College of Georgia Department of Radiology

Basic Interactions Coherent Scattering Pair Production Photodisintegration Photoelectric Effect Photoelectric Effect Compton Scattering Compton Scattering

Photon Phate absorbed completely removed from beam ceases to exist scattered change in direction no useful information carried source of noise Nothing Photon passes unmolested X *

Noise –covers valid information with distracting or obscuring garbage

Image Noise Caution! Image Noise –covers valid information with distracting or obscuring garbage

Another Example

Image Noise Example Caution! Image Noise

Coherent Scattering Also called unmodified scattering classical scattering Types Thomson photon interacts with single electron Rayleigh photon interacts with all electrons of an atom

Coherent Scattering Change in direction No change in energy frequency wavelength No ionization Contributes to scatter as film fog Less than 5% of interactions insignificant effect on image quality compared to other interactions

Pair Production Process high energy photon interacts with nucleus photon disappears electronpositron electron & positron (positive electron) created energy in excess of 1.02 MeV given to electron/positron pair as kinetic energy ~ ~ + ~ - + ***

Positron Phate Positron undergoes ANNIHILATION REACTION Two MeV photons created Photons emerge in exactly opposite directions

Pair Production Threshold energy for occurrence: 1.02 MeV energy equivalent of rest mass of 2 electrons Threshold is above diagnostic energies does not occur in diagnostic radiology ~ ~ + ~ - +

Photodisintegration photon causes ejection of part of atomic nucleus ejected particle may be neutron proton alpha particle cluster ~ ~ + ~ ? *

Photodisintegration Threshold photon energy for occurrence nuclear binding energy typically 7-15 MeV Threshold is above diagnostic energies does not occur in diagnostic radiology

Photoelectric Effect photon interacts with bound (inner-shell) electron electron liberated from atom (ionization) photon disappears Electron out Photon in - **

PHOTOELECTRIC EFFECT

Photoelectric Effect Exiting electron kinetic energy incident energy - electron’s binding energy electrons in higher energy shells cascade down to fill energy void of inner shell characteristic radiation Electron out Photon in M to L L to K - ****

Photoelectric Interaction Probability inversely proportional to cube of photon energy low energy event proportional to cube of atomic number more likely with inner (higher) shells tightly bound electrons 1 P.E. ~ energy 3 P.E. ~ Z 3

Photoelectric Effect Interaction much more likely for low energy photons high atomic number elements 1 P.E. ~ energy 3 P.E. ~ Z 3

Photoelectric Effect Photon Energy Threshold > binding energy of orbital electron binding energy depends on atomic number higher for increasing atomic number shell lower for higher (outer) shells most likely to occur when photon energy & electron binding energy are nearly the same

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 15 Which shells are candidates for photoelectric interactions?

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 15 Which shells are candidates for photoelectric interactions? NO

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 25 Which shells are candidates for photoelectric interactions?

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 25 Which shells are candidates for photoelectric interactions? NO YES

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 22 Which photon has a greater probability for photoelectric interactions with the m shell? Photon energy: 25 A B 1 P.E. ~ energy 3

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 55 Which shells are candidates for photoelectric interactions?

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 55 Which shells are candidates for photoelectric interactions? NO YES

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon in Photon energy: 105 Which shells are candidates for photoelectric interactions?

Photoelectric Threshold Binding Energies K: 100 L: 50 M: 20 Photon energy: 105 Which shells are candidates for photoelectric interactions? YES

Photoelectric Threshold Photoelectric interactions decrease with increasing photon energy BUT … 1 P.E. ~ energy 3

Photoelectric Threshold When photon energies just reaches binding energy of next (inner) shell, photoelectric interaction now possible with that shell  shell offers new candidate target electrons Photon Energy Interaction Probability K-shell interactions possible L-shell interactions possible L-shell binding energy ** K-shell binding energy

Photoelectric Threshold causes step increases in interaction probability as photon energy exceeds shell binding energies Photon Energy Interaction Probability L-edge K-edge

Characteristic Radiation Occurs any time inner shell electron removed energy states orbital electrons seek lowest possible energy state innermost shells M to L L to K **

Characteristic Radiation electrons from higher states fall (cascade) until lowest shells are full characteristic x-rays released whenever electron falls to lower energy state M to L L to K characteristic x-rays **

Characteristic Radiation only iodine & barium in diagnostic radiology have characteristic radiation which can reach image receptor

Photoelectric Effect Why is this important? photoelectric interactions provide subject contrast variation in x-ray absorption for various substances scatter photoelectric effect does not contribute to scatter photoelectric interactions deposit most beam energy that ends up in tissue always always use highest kVp technique consistent with imaging contrast requirements

Compton Scattering Source of virtually all scattered radiation Process incident photon (relatively high energy) interacts with free (loosely bound) electron some energy transferred to recoil electron electron liberated from atom (ionization) emerging photon has less energy than incident new direction Electron out (recoil electron) Photon in Photon out - ***

Compton Scattering What is a “free” electron? low binding energy outer shells for high Z materials all shells for low Z materials Electron out (recoil electron) Photon in Photon out -

Compton Scattering Incident photon energy split between electron & emerging photon Fraction of energy carried by emerging photon depends on incident photon energy angle of deflection similar principle to billiard ball collision Electron out (recoil electron) Photon in Photon out -

Compton Scattering & Angle of Deflection higher incident energy = less photon deflection high energy (1MeV) photons primarily scatter forward diagnostic energy photons scatter fairly uniformly forward & backward at diagnostic energy photons lose very little energy during Compton Scattering higher deflection = less energy retained photons having small deflections retain most incident incident energy Electron out (recoil electron) Photon in Photon out deflection angle -

Compton Scattering & Angle of Deflection Photons having small deflections retain most incident incident energy Photons will scatter many times, losing a little energy each time

Compton Scattering Formula  = (1-cos  ) where  = change in wavelength (A) for photon  = angle of photon deflection (0-180 degrees) recoil electron Photon in Photon out Angle  - 0 o results in no change in wavelength 180 o results in maximum change in wavelength

Compton Scattering Probability of Occurrence independent of atomic number (except for hydrogen) Proportional to electron density (electrons/gram) fairly equal for all elements except hydrogen (~ double)

Compton Scattering Probability of Occurrence decreases with increasing photon energy decrease much less pronounced than for photoelectric effect Photon Energy Interaction Probability Compton Photoelectric

Photon Interaction Probabilities Photoelectric Pair Production COMPTON Z protons E energy (MeV)