Week 3 - Interactions Flashcards
Why are the processes of interaction between radiation and matter important?
Because a basic understanding is necessary to set technical exposure factors when imaging
What are the important technical exposure factors worth knowing?
kVp (peak kilovoltage) and mAs (milliampere-seconds)
What is kVp?
Highest energy level photons in the x-ray beam equal to highest voltage established across the tube
What does kVp control?
Quality/penetrating power of the photons in the beam and to some degree the quantity of photons
What is mAs?
The product of milliampere and the amount of time in seconds the tube is activated
What is mA?
The current of the electron tube
What is the formula for mAs?
mA x s = mAs
What does mAs control?
The Quantity of radiation that is directed toward the patient during exposure
What is the radiographer responsible for when imaging?
- Setting the technical exposure factor that control quality/quality of the beam
- Selecting the dose the patient receives
What can selecting appropriate techniques achieve with imaging?
- Minimized dose to patients
- Images of optimal-quality
What do x-ray carry?
Manmade electromagnetic energy
What are the 3 ways x-rays enter a material such as human tissues?
- Interact with atoms of the tissue and are absorbed
- Interact with atoms of the tissue and are scattered, causing indirect transmission
- Pass through without interaction
What is absorption?
When electromagnetic energy is transferred from the x-rays to the atoms of a patient’s tissue
What is absorbed dose?
The amount of energy per unit mass
How can biologic damage be minimized?
By keeping the amount of electromagnetic energy transferred to patient as small as possible
Why is absorption important to diagnostically useful images?
Helps distinguish anatomic structures in images
What are the benefits to keeping patient dose minimal?
- Less radiation is scattered from the patient
- Reduces occupational hazards to the radiographer
How is a diagnostic x-ray beam produced?
When a stream of high-speed electrons bombard a positively charged target in a evacuated glass tube
What are anodes composed of?
Tungsten (metal) or Tungsten rhenium (metal alloy)
Why are tungsten and tungsten rhenium used as target materials?
- Have high melting points
- High atomic numbers
Do all photons in an x-ray beam have the same energy?
No, they have variable energy levels
What is the max energy a photon can have?
No more energy that the electrons that bombard the target
What is the energy of the electrons inside the tube specified in?
In terms of electrical voltage applied across the tube
In diagnostic radiology, what is the energy of electrons in the tube is expressed in?
kV, but because the voltage fluctuates its usually expressed in kVp
What is attenuation?
When an x-ray beam passes through a patient
What is direct transmission?
When primary photons traverse the patient without interacting and hit the image receptor
What is indirect transmission?
When primary photons traverse the patient with interacting and hit the image receptor
What is attenuation?
The reduction in intensity of the x-ray beam after its passed through a patient
What two mechanisms produce attenuation?
Absorption and Scattering
What is scattering?
A change in direction of a photon after interaction with an atom
What is small- angle scatter?
When the beam changes direction only slightly so that its still directed towards the IR
How does small-angle scatter interfere with image production?
It produces radiation fog
When is an optimal x-ray image formed?
When only direct transmission x-rays photons reach the IR
What methods can be used to limit the effects of indirect transmission photons?
Radiographic grids and air gap techniques
What types of transmission form radiographic images?
Direct and Indirect transmitted x-ray photons
What problems are caused by radiographic fog?
Interferes with the radiologists ability to distinguish different structures in the image
How can radiographic fog be reduced?
By reducing the amount of tissue irradiated, which is done via collimation
How can the probability of photon interactions with biologic matter be described?
Random
What can be predicted when photons enter tissue?
What will happen on average when a large number of photons enter the body
What does predicting photon interactions determine?
The characteristics of the image that results from these types of interactions
When primary radiation emerges from the x-ray tube what properties do the photons have?
X-ray photons of various energies
What are primary photons?
Photons that emerge from the x-ray source before they hit the body
What are exit, or image-formation, photons?
Photons that pass through the patient being radiographed and reach the image receptor
What are attenuated photons?
Photons that have interacted with the patient and have been scattered so that they don’t reach the IR
What does the probability of a photon interacting with tissue dependent on?
Dependent on photon energy and the characteristics of the tissue
What are the 5 possible interactions of photons with matter?
- Coherent scattering
- Photoelectric absorption
- Compton scattering
- Pair production
- Photodisintegration
Which of the two interactions are important for the energies used in diagnostic x-ray?
Compton scattering and Photoelectric absorption
What diagnostic kV range do Compton and Photoelectric interactions?
50-150 kV
When is Compton scattering more likely?
When using higher Kv
When is Photoelectric absorption more likely?
When using lower Kv
What is Coherent scatter also known as?
Classical, elastic, unmodified or Rayleigh scattering
What type of energy loss results from Coherent scattering?
No loss of energy as x-rays scatter
What type of energy does Coherent scattering occur in?
Low energy photons less than 10 kEv
What is the net result of Coherent scattering?
A change in direction of less than 20 degrees from the incident photon
What is the process of coherent scattering?
The incoming low-energy photon interacts with an atom and transfers its energies causing the electrons to vibrate. When the electrons vibrate they release energy in the form of electromagnetic waves which nondestructively combine to form a scattered wave.
What type of interaction does Coherent scattering produce?
Excitation
What photon energy range does Coherent scattering occur in?
1-50 kVp
What is the site of interaction for Coherent scattering?
The atom
What is the by-product of Coherent scattering?
Nothing
What does the probability of Coherent scattering depend on?
Photon energy and mass density
How is probability of Coherent scattering related to photon energy?
As photon energy increase probability of Coherent scattering decreases
How is probability of Coherent scattering related to mass density?
As mass density increases the probability of Coherent scattering increases
What is the most important mode of interaction between x-ray photons and atoms?
Photoelectric Absorption
What does absorption depend strongly on?
The type of tissue, characterized by the atomic number
What is used with the atomic differences between tissues are not great enough?
Contrast media
What is the process of photoelectric absorption?
- X-ray photon encounters an inner-shell electron and surrenders all its energy to the electron, and the photon ceases to exist
- The atom ejects the electron, which is now called a photoelectron, and creates a vacancy
- The opening is then filled with an electron dropping down from an outer shell releasing energy in the form of a characteristic photon. (occurs until atom electron equilibrium is reached)
What is another probability that occurs in photoelectric absorption?
An auger electron is ejected
What is the Auger effect?
A radiationless effect that occurs as a result of photoelectric absorption
What is the process of the Auger Effect?
- Characteristic energy, that is released when an inner shell vacancy is filled, is transferred to another electron in an outer shell and ejects it
What is the liberated electron called?
Auger electron
What type of interaction does photoelectric absorption produce?
Absorption
What photon energy range does Photoelectric absorption occur in?
Low to moderate 1-50 kVp
What is the site of interaction of Photoelectric Absorption?
Inner orbital electron, usually K or L shell
What is the by-product of interaction of photoelectric absorption?
- Photoelectron
- 1 of more characteristic photons
What does the probability of Photoelectric absorption depend on?
Photon Energy, Atomic number and Mass density
How is probability of Photoelectric Absorption related to photon energy?
Inversely proportional to keV^3
The higher energy photons are less likely to be absorbed
How is probability of Photoelectric Scattering related to atomic number of tissue?
Proportional to Z^3
How is probability of Photoelectric Absorption related to mass density?
Proportional to mass density
What color does positive contrast medium reflect in x-ray images?
White
What color does negative contrast medium reflect in x-ray images?
Black
What is the impact of photoelectric absorption on contrast?
The greater the different in the amount of absorption, the greater the contrast in the image
What is Compton scattering also known as?
Incoherent, inelastic and modified scattering
Why is compton scattering important to radiography?
When photons change direction, they can exit the patient at a different direction that initiated and produce scatter fog which reduces image quality
What is the process of compton scattering?
- A photon interacts with an outer shell electron and is deflected with loss of energy
- Some of the photons energy is transferred to the electron (amount of energy transfers increases with the angle of scatter)
When does compton scatter occur?
When the x-ray photon energy is very high compared to electron binding energy
What type of interaction does compton scattering produce?
Ionization
What photon energy range does Compton scattering occur in?
Moderate to high 50-150 kVp
What is the site of interaction of compton scattering?
Outer orbital electron
What is the by-product of interaction of compton scattering?
- Compton scattered electron, secondary/recoil electron
- Compton scattered proton with low energy
What does the probability of compton scattering depend on?
- Photon energy
- Mass energy
How is probability of compton scattering related to photon energy?
Probability increases with increasing energy
How is probability of Compton scattering related to mass density?
Probability increase with mass density increase
What is the process of pair production?
- A photon with energy > 1.02 MeV interacts with the nuclear field of an atom
- It transfers all its energy to the atom to produce an electron-positron pair
- As a result, the unstable positron combines with an electron to produce annihilation radiation
What type of interaction does pair production produce?
Energy to matter transformation
What photon energy range does pair production occur in?
Greater that 1.022 Mev
What is the site of interaction of pair production?
A nuclear electric field
What is the by- product of interaction of pair production?
A negatron and positron, which splits into two 0.511 meV photons
What does the probability of pair production depend on?
- Photon energy
- Atomic # of absorber
- Mass density
How is probability of Pair production related to photon energy?
Probability increase with increase in energy
How is probability of Pair production related to Mass density?
Proportional to mass density
How is probability of pair production related to atom number?
Probability increases with atomic number
What are the uses of annihilation radiation in PET?
- sources of positrons
- process of positron decay
- formation of annihilation photons
What is photodisintegration interaction?
An interaction that occurs at more the 10 MeV high energy radiation in radiation therapy machines
What effect does photodisintegration have on the nucleus of an atom?
Causes the nucleus of an atom to change in some way or break apart
Why type of interaction does photodisintegration produce?
Nuclear disintegration
What photon energy range does Photodisintegration occur in?
Very high photon range >10 MeV
Where is the site of interaction of photodisintegration?
Nucleus
What is the by-product of photodisintegration?
Nuclear fragments
What does the probability of photodisintegration depend on?
- Photon energy
- Atomic number of absorber
- Mass density
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