Unit 5 - Photon Interactions w/ Matter + Image Production
Call Kai
Learn
Practice Test
Spaced Repetition
Match
Flashcards
Knowt Play1/126
There's no tags or description
Looks like no tags are added yet.
127 Terms
What are the 3 possible interactions that can occur when an X-ray beam passes through matter?
Transmission, Absorption, Scattering
What is transmission and what is its interaction with X-ray beams when they pass through matter?
X-ray photons pass completely through the patient without interacting with matter
What is absorption and what is its interaction with X-ray beams when they pass through matter?
X-ray photons are absorbed within the patient's tissues, removing them from the beam
What is scattering and what is its interaction with X-ray beams when they pass through matter?
X-ray photons interact with matter and change direction, scattering out of the original beam
What is the original X-ray beam before it enters the patient?
Primary beam
What is the primary beam?
the original X-ray beam before it enters the patient
What happens to X-ray photons as the beam passes through the patient?
Some photons are absorbed or scattered, while others pass through without interacting
What is the remnant beam also called?
Exit beam
What is the remnant beam (exit beam)?
Radiation that exits the patient after attenuation, consisting of transmitted and scattered photons
What two types of photons make up the remnant beam?
Transmitted photons and scattered photons
When is the remnant beam formed?
After the x-ray beam passes through the patient and undergoes attenuation
What is attenuation?
The reduction in the number of X-ray photons remaining in the beam as it passes through matter
How does attenuation occur?
When X-ray photons interact with matter and are either absorbed or scattered
What four factors affect attenuation?
Part thickness, tissue density, atomic number, and X-ray beam energy
What happens to attenuation as part thickness or tissue density increases?
Attenuation increases because more photons are absorbed or scattered
How does increased attenuation affect the number of photons reaching the image receptor?
Fewer photons reach the image receptor
What type of pattern does attenuation follow?
Exponential attenuation
What does exponential attenuation mean?
Each additional thickness of material reduces the remaining photons by a fraction of the previous amount
What 4 factors affect beam attenuation?
part thickness, tissue density, atomic number, and beam energy
How does an increase in part thickness affect attenuation?
Attenuation increases because the X-ray beam travels through more matter, creating more opportunities for photon interactions
How does an increase in tissue density affect attenuation?
Attenuation increases because more matter is packed into the same volume, resulting in more photon interactions
How does an increase in atomic number affect attenuation?
Attenuation increases because higher-atomic-number materials are more likely to absorb X-ray photons, particularly through photoelectric interactions
How does an increase in beam energy (kVp) affect attenuation?
Attenuation decreases because higher-energy photons have greater penetrating ability and are more likely to pass through the patient without interacting
What is photoelectric absorption?
An interaction where an X-ray photon transfers all its energy to an inner-shell electron, ejecting it from the atom
Which electron shells are typically involved in photoelectric absorption?
The K or L shells
What happens to the incident X-ray photon during photoelectric absorption?
It transfers all its energy to the electron and is completely absorbed
What is a photoelectron?
An inner-shell electron ejected from an atom during photoelectric absorption
Why does photoelectric absorption cause ionization?
An inner-shell electron is ejected, leaving the atom with a positive charge
When are photoelectric interactions most likely to occur?
When the incident photon energy is slightly greater than the binding energy of an inner-shell electron
What photon energies favor photoelectric absorption?
Lower-energy X-ray photons, provided their energy exceeds the electron's binding energy
How does atomic number affect photoelectric absorption?
Higher atomic numbers increase the likelihood of photoelectric absorption
What happens to the atom after an inner-shell electron is ejected?
The atom becomes unstable because an inner-shell vacancy is created.
How is the inner-shell vacancy filled after photoelectric absorption?
An outer-shell electron drops into the vacancy
What is released when an outer-shell electron fills an inner-shell vacancy?
Energy in the form of a characteristic X-ray photon
What is a characteristic cascade?
A series of electron transitions that occur as vacancies are filled, releasing characteristic radiation
What formula is used to calculate the kinetic energy of a photoelectron?
Eke(kinetic) = Ei(incident) − Eb(binding)
What does Eke represent?
Kinetic energy of the photoelectron
What does Ei represent?
Energy of the incident X-ray photon
What does Eb represent?
Binding energy of the electron
How is the incident photon's energy used during photoelectric absorption?
Part of the energy overcomes the electron's binding energy, and the remainder becomes the photoelectron's kinetic energy
An incident photon has 80 keV of energy, and the electron has a binding energy of 30 keV. What is the photoelectron's kinetic energy?
50 keV (80 − 30 = 50 keV)
How is the kinetic energy of a photoelectron calculated?
By subtracting the electron's binding energy from the incident X-ray photon's energy
What happens to the incident photon's energy during photoelectric absorption?
All its energy is transferred to the electron
How is the transferred energy divided during photoelectric absorption?
Part overcomes the electron's binding energy, and the remainder becomes the photoelectron's kinetic energy
What happens to the energy remaining after the electron's binding energy is overcome?
It becomes the kinetic energy of the ejected photoelectron
How is secondary radiation produced?
When an outer-shell electron fills an inner-shell vacancy, releasing energy as a characteristic photon
What causes an inner-shell vacancy during photoelectric absorption?
An inner-shell electron is ejected from the atom
Why does an atom become unstable after photoelectric absorption?
An inner-shell electron is removed, leaving a vacancy
How does an unstable atom restore stability after photoelectric absorption?
An outer-shell electron moves down to fill the inner-shell vacancy
What determines the energy of the characteristic photon released?
The difference between the binding energies of the two electron shells
What is a characteristic cascade?
A series of electron transitions between shells that release characteristic radiation
When is characteristic radiation considered secondary radiation?
When it is produced in irradiated matter outside the X-ray target
What is the difference between primary and secondary characteristic radiation?
Primary characteristic radiation is produced at the X-ray target, while secondary characteristic radiation is produced in irradiated matter outside the target
How do photoelectric interactions affect patient dose?
They increase patient dose because the incident X-ray photon is completely absorbed within the patient's tissues.
Why do photoelectric interactions increase patient dose?
The absorbed photon transfers its energy to the patient, and the ejected photoelectron can cause additional ionizations and tissue damage
How do photoelectric interactions affect occupational dose?
They do not directly contribute to occupational dose because the incident photon is completely absorbed rather than scattered toward personnel
Why don't photoelectric interactions directly contribute to occupational or bystander dose?
The incident photon is completely absorbed within the patient instead of being scattered into the examination room
How do photoelectric interactions affect image quality?
They improve image quality by increasing radiographic contrast
Why does photoelectric absorption increase radiographic contrast?
Different tissues absorb different amounts of radiation, creating differences in image brightness.
Why does bone appear lighter on a radiograph?
Bone absorbs more X-ray photons, resulting in fewer photons reaching the image receptor
Why do air-containing structures, such as the lungs, appear darker on a radiograph?
They absorb fewer X-ray photons, allowing more photons to reach the image receptor
What are the three main effects of photoelectric interactions?
Increased patient dose, no direct occupational dose from the absorbed incident photon, and increased radiographic contrast
How does increasing photon energy affect the probability of photoelectric absorption?
The probability decreases as photon energy increases
How does decreasing photon energy affect the probability of photoelectric absorption?
The probability increases, provided the photon has enough energy to overcome the electron's binding energy
When are photoelectric interactions most likely to occur?
When the incident photon energy is slightly greater than the binding energy of an inner-shell electron
What is the relationship between photon energy and the probability of photoelectric absorption?
The probability is approximately inversely proportional to the third power of photon energy (1/E³).
What happens to the probability of photoelectric absorption when photon energy doubles?
It decreases to approximately 1/8 of the original probability, assuming no absorption edge is crossed
How does increasing kVp affect photoelectric absorption?
It generally decreases the probability of photoelectric absorption by increasing photon energy and penetrating ability
Why are higher-energy photons less likely to undergo photoelectric absorption?
They have greater penetrating ability and are more likely to pass through the patient without interacting
How does increasing atomic number affect the probability of photoelectric absorption?
The probability increases as atomic number increases
How does decreasing atomic number affect the probability of photoelectric absorption?
The probability decreases as atomic number decreases.
What is the relationship between atomic number and the probability of photoelectric absorption?
The probability is approximately directly proportional to the third power of atomic number (Z³)
What happens to the probability of photoelectric absorption when atomic number doubles?
It increases by approximately 8 times, assuming photon energy and other factors remain constant
Why are photoelectric interactions more likely in high-atomic-number materials?
Higher-atomic-number materials have more tightly bound electrons and a greater probability of photoelectric absorption
What are examples of materials with greater photoelectric absorption?
Bone, iodine, and barium
Why are iodine and barium used as contrast agents?
Their high atomic numbers increase photoelectric absorption compared with surrounding tissues
How do high-atomic-number contrast agents improve image quality?
They increase radiographic contrast by absorbing more X-ray photons than surrounding tissues
What is the approximate relationship between photon energy, atomic number, and photoelectric absorption?
Photoelectric absorption probability is proportional to Z³/E³, where Z is atomic number and E is photon energy
What is coherent scattering?
An interaction where a very-low-energy X-ray photon interacts with an entire atom and is scattered in a different direction without ionization
What does the incident photon interact with during coherent scattering?
The entire atom rather than an individual electron
What happens to the incident photon during coherent scattering?
Its energy is temporarily absorbed by the atom and then released as a scattered photon
What happens to the electrons during coherent scattering?
They vibrate at the same frequency as the incident photon
What happens after the atom absorbs energy during coherent scattering?
It releases the energy as a scattered photon traveling in a different direction
Does coherent scattering cause ionization?
No, because no electrons are ejected from the atom
What photon energies are most associated with coherent scattering?
Very-low-energy X-ray photons
How does the energy of a coherent scattered photon compare to the incident photon?
It remains the same
How does the wavelength of a coherent scattered photon compare to the incident photon?
It remains the same
How does the frequency of a coherent scattered photon compare to the incident photon?
It remains the same
What is the only change to a photon during coherent scattering?
Its direction of travel
What are the three main characteristics of coherent scattering?
No ionization, no change in photon energy, and a change in photon direction
How do coherent interactions affect patient dose?
They have no significant effect on patient dose because no ionization occurs and no energy is permanently absorbed
Why do coherent interactions have no significant effect on patient dose?
No electrons are ejected, and no energy is permanently absorbed within the patient's tissues.
How do coherent interactions affect occupational dose?
They have a negligible effect because coherent scattering occurs at very low photon energies and contributes little scattered radiation
Why is occupational dose from coherent scattering negligible?
Coherent scattering occurs at very low photon energies and produces very little scattered radiation
How do coherent interactions affect image quality?
They slightly decrease image quality by reducing radiographic contrast
Why does coherent scattering decrease radiographic contrast?
Scattered photons change direction and may reach the image receptor, reducing contrast
What are the three main effects of coherent scattering?
No significant patient dose, negligible occupational dose, and a slight decrease in image quality
When is coherent scattering most likely to occur?
When very-low-energy X-ray photons, typically below 10 keV, interact with matter
What happens to the incident photon's energy during coherent scattering?
The atom temporarily absorbs the energy and releases it as a scattered photon
Does coherent scattering cause ionization?
No, because no electrons are ejected.