Creation/Production
Define: Photon A packet of electromagnetic energy. X-rays consist of photons.
Define: Kinetic Energy Energy of motion, such as the energy carried by a moving electron.
Define: Binding Energy Energy required to remove an electron from its shell in an atom.
Define: Inner-shell electron An electron close to the nucleus that is relatively tightly bound to the atom.
Define: Outer-shell electron An electron farther from the nucleus that is relatively loosely bound to the atom.
Define: Shell Vacancy An unoccupied electron position in an atomic shell.
Define: Ionization The removal or addition of an electron, leaving an atom electrically charged.
Define: Excitation An increase in an atom’s energy without ejecting an electron. In the anode, an orbital electron may temporarily move to a higher energy level.
Define: keV Kiloelectron volt—a unit used to express photon or electron energy.
Define: Incident Electron An electron approaching a target atom. In the x-ray tube, it originates at the filament and accelerates toward the anode. Also called a projectile or filament electron in this context.
Define: Characteristic Radiation X-rays emitted when an outer-shell electron fills an inner-shell vacancy, releasing energy equal to the difference between the shell binding energies.
Define: Characteristic Photon Energy The inner-shell binding energy minus the binding energy of the shell from which the filling electron came.
Define: Characteristic Cascade A series of electron transitions that fill successive vacancies as electrons move into shells closer to the nucleus.
Define: Bremsstrahlung Radiation X-rays produced when an incident electron changes motion and loses energy while interacting with a nucleus’s electric field.
Define: Bremsstrahlung photon energy The difference between the incident electron’s kinetic energy before and after the interaction.
Define: Target Material The anode material in which incident electrons interact to produce x-rays, commonly tungsten.
Define: Incident Photon An x-ray photon approaching an atom before an interaction.
Define: Photoelectric Absorption An interaction in which an incident photon is completely absorbed and an inner-shell electron is ejected.
Define: Photoelectron The electron ejected during photoelectric absorption. Its kinetic energy equals the incident photon’s energy minus the electron’s binding energy.
Define: Compton Scattering An interaction in which an incident photon transfers part of its energy to an outer-shell electron, ejecting it. A scattered photon leaves with less energy and in a different direction. Also called modified or incoherent scattering.
Define: Recoil electron The electron ejected during Compton scattering. It carries kinetic energy transferred from the incident photon.
Define: Scattered Photon A photon traveling in a different direction after an interaction. It has less energy after Compton scattering and unchanged energy after coherent scattering.
Define: Scattering angle The angle between the scattered photon’s direction and the incident photon’s original forward direction.
Define: Backscatter Radiation scattered back toward the source.
Define: Coherent Scattering Scattering in which the photon changes direction without changing energy or ejecting an electron. Also called classical or unmodified scattering.
Define: Attenuation Reduction of the primary beam as photons are absorbed or scattered out of it.
Define: Pair Production An interaction in which a high-energy photon converts into an electron and a positron in the electric field near a nucleus. It requires energy beyond diagnostic radiography.
Define: Positron The electron’s antiparticle, with the same mass but a positive charge. A positively charged electron.
Define: Annihilation An interaction in which a positron and electron convert their mass and energy into photons, usually two.
Define: Photodisintegration An interaction in which a nucleus absorbs a high-energy photon and ejects a nuclear particle or fragment. It occurs beyond diagnostic radiography energies.
In the x-ray tube, what interacts with the anode target to produce x-rays? High-speed electrons.
How are electrons released before they accelerate toward the anode? By thermionic emission.
What causes electrons to accelerate toward the anode? The applied tube voltage (kVp).
Approximately what percentage of incident electron kinetic energy becomes heat at the anode? Approximately 99%; about 1% or less becomes x-rays.
What happens to photon-production efficiency as incident electron kinetic energy increases? The efficiency of photon production increases.
What are the two types of x-ray-producing anode target interactions and their spectra? Bremsstrahlung produces a continuous spectrum; characteristic interactions produce a discrete spectrum.
What does bremsstrahlung mean? Braking radiation, or slowing.
During a brems interaction, what attracts the negatively charged incident electron? The positively charged nucleus.
What happens to the incident electron during a brems interaction? It loses kinetic energy, slows down or brakes, and its course is diverted.
What happens to the energy lost during bremsstrahlung braking? It is emitted as an x-ray photon.
An electron enters a brems interaction with 60 keV and exits with 20 keV. What is the photon energy? 40 keV.
Why does bremsstrahlung produce a continuous spectrum? Individual brems photon energies vary over a range of energy levels.
What is the maximum possible brems photon energy? The incident electron’s full kinetic energy.
Can one incident electron undergo more than one brems interaction? Yes, a single incident electron can undergo multiple brems interactions.
What must an incident electron do to begin a characteristic interaction? Eject an inner-shell electron from the target atom by transferring enough energy to overcome its binding energy.
What happens to the incident electron after ejecting an inner-shell electron in a characteristic interaction? It usually continues but in a slightly different direction.
What fills the inner-shell vacancy during a characteristic interaction? An outer-shell electron.
What is released when an outer-shell electron fills an inner-shell vacancy? The difference in shell binding energies is released as an x-ray photon.
In tungsten, what shell transitions produce characteristic photons useful for general diagnostic radiography? Transitions into the K shell.
A K-shell binding energy is 69.5 keV and an L-shell binding energy is 12.1 keV. What is the K-characteristic photon energy? 57.4 keV.
For the same inner-shell vacancy, what happens when the filling electron comes from a more distant shell? A higher-energy characteristic photon is produced.
What does an x-ray emission spectrum show? The number of photons at each energy, including bremsstrahlung and characteristic photons.
On an x-ray emission spectrum, what does the horizontal axis represent? Photon energy in keV.
On an x-ray emission spectrum, what does the vertical axis represent? The number of photons.
Which interaction produces most photons within the diagnostic x-ray range? Bremsstrahlung target interactions.
About what fraction of kVp is the average brems photon energy in the PowerPoint spectrum example? About one-third of the kVp.
What happens to photon quantity when mAs increases? Photon quantity increases proportionally.
What happens to the spectrum when mAs increases? It becomes taller and its total area increases.
What happens to average and maximum photon energies when mAs changes alone? They remain unchanged.
What happens to lower-energy photons when tube filtration is added? They are absorbed.
How does added filtration affect photon quantity and average photon energy? It decreases photon quantity but increases average photon energy.
What happens to maximum photon energy when added filtration is increased? It remains unchanged.
What happens to average and maximum photon energies when kVp increases? Both increase.
What happens to photon quantity when kVp increases? Photon quantity increases.
How does the emission spectrum change when kVp increases? It extends farther right and its total area increases.
At the same kVp and mAs, what does lower voltage ripple produce? More photons and a higher average photon energy.
What happens to maximum photon energy when voltage ripple is lowered at the same kVp and mAs? It remains unchanged.
What are the two reasons radiographers should understand x-ray interactions with matter? To make informed technique choices that minimize unnecessary patient dose and understand effects on image contrast and anatomical visibility.
What is radiation emitted by the x-ray tube that has not yet passed through the patient or object? The primary beam.
What is radiation that exits the patient or object and reaches the image receptor? The remnant beam.
What six categories are listed as radiographic variables? Technical variables; geometric variables; patient characteristics and condition; image receptor systems; image processing; and viewing conditions.
What is the reduction in x-ray beam intensity as photons are absorbed or scattered out of the beam? Attenuation.
What are four possible fates of x-ray photons listed in the PowerPoint? They may pass through matter without interacting, interact with the whole atom, interact with orbital electrons, or interact with the nucleus at sufficiently high photon energies.
How does higher atomic number generally affect K-shell binding energy? Higher atomic number generally means greater K-shell binding energy.
How do electron energy states change farther from the nucleus? Electrons in shells farther from the nucleus occupy higher energy states.
As you move outward through electron shells, what happens to electron energy and binding energy? Electron energy increases while binding energy decreases.
What are the two primary interactions affecting diagnostic images? Photoelectric absorption and Compton scattering.
Which interactions are not encountered in diagnostic x-ray energies? Pair production and photodisintegration.
During photoelectric absorption, which electron does the incident photon interact with? An inner-shell electron.
What happens to the incident photon during photoelectric absorption? It is completely absorbed.
What happens after the photoelectron is ejected in photoelectric absorption? An outer-shell electron fills the inner-shell vacancy and a characteristic photon (secondary radiation) is released.
What energy must the incident photon have to eject an electron with kinetic energy? More energy than the electron’s binding energy.
When is photoelectric absorption more likely in relation to shell binding energy? When photon energy is just above the shell’s binding energy.
What material property makes photoelectric absorption more likely, with other factors comparable? Higher atomic number.
What is the kinetic energy of a photoelectron equal to? Incident photon energy minus the electron’s binding energy.
What materials are specifically identified as higher-atomic-number materials favoring photoelectric absorption? Iodine and barium.
How do differences in photoelectric absorption affect the radiographic image? Differences in absorption contribute to subject contrast.
What are the alternate names for the Compton effect? Modified scattering or incoherent scattering.
Which electron does an incident photon interact with during Compton scattering? A loosely bound orbital electron, usually in an outer shell.
What happens to the electron during Compton scattering? It is removed from its shell and becomes a recoil electron.
What happens to the scattered photon during Compton scattering? It exits the atom in a different direction and with less energy.
What is the incident photon’s energy used for during Compton scattering? To free the electron, give it kinetic energy, and produce the scattered photon.
What type of radiation is a Compton-scattered photon? Secondary radiation.
What two factors determine scattered photon energy in the Compton effect? The incident photon’s energy and the scattering angle.
For the same incident energy, what happens to retained photon energy as scattering angle increases? Less energy is retained by the photon.
What is backscatter radiation? Radiation scattered back toward the source.
What is the major source of occupational exposure? Compton scatter from the patient.
What happens when scatter reaches the image receptor? It adds unwanted exposure (fog) and reduces contrast.
What are the alternate names for coherent scattering? Classical scatter or unmodified scatter.
What two types of coherent scattering are listed? Thomson and Rayleigh scattering.
What photon energies are most associated with coherent scattering? Low-energy photons.
Does coherent scattering eject an electron? No; the photon interacts with a bound electron or the electron cloud without ejecting an electron.
What happens to photon energy and direction during coherent scattering? The photon keeps the same energy but travels in a different direction.
What minimum photon energy is required for pair production? At least 1.022 MeV.
Where does the photon interact during pair production? In the field around the nucleus.
What two particles are produced during pair production? A positron and an electron.
What happens when the positron later encounters an electron? They undergo annihilation, producing two photons.
At what energies does photodisintegration occur? At photon energies above those used in diagnostic radiography, generally even higher than pair production interactions.
What happens during photodisintegration? The nucleus absorbs a high-energy photon and emits a particle or nuclear fragment.
As kVp increases, what happens to the proportion of photons passing through the patient without interacting? A greater proportion passes through without interacting.
How does increasing photon energy above the binding-energy threshold affect photoelectric absorption? Photoelectric absorption becomes less likely.
How does increasing kVp affect the relative predominance of Compton scattering? Compton scattering becomes more predominant relative to photoelectric absorption.
Which interaction is more likely at lower photon energies above the binding-energy threshold? Photoelectric absorption.
Which interaction is predominant in soft tissue through much of the diagnostic energy range? Compton scattering.