4. Radiation Physics Flashcards
Definition of radiation:
Transmission of energy through space & matter
What is Ionization? (definition)
neutral atom loses an electron to form an ion pair (positive ion + free electron)
what are the components of an xray tube?
- tube housing
- glass envelope
- cathode assembly
- anode
Function of the tube housing?
- lead lining minimizes leakage radiation
- contains oil to dissipate heat
- physical protection and electrical insulation
Function of the glass envelope?
- creates vaccuum for xray production
- provides electrical insulation
Function of cathode assembly?
- tungsten wire filament: source of electrons
- thermionic emission: filament is heated and electrons are ‘boiled off’
Function of the nickel electron focusing cup?
- concentrates the electrons into a sharp/narrow beam
Function of the anode?
- tungsten target
- xrays are produced here via: bramsstrahlung & characteristic interactions
- inefficient conversion of kinetic energy from electrons to xray photons results in 99% of the energy converted to heat
What is Bremsstrahlung radiation?
- also know as ‘braking’ radiation
- electrons from cathode electrostatically attracted towards positively charged anode
- when electrons path is deflected & decelerated, electrons lose KE
- energy is given off in the form of xray photons
Maximum ________ energy corresponds to the maximum ________ applied across the tube
Maximum photon energy == maximum voltage applied across the tube
What is Characteristic radiation?
- Incident e interacts w k shell e
- K shell e is removed, leaving a vacancy.
- Vacancy is filled by an e from the M or L shell
- Excess energy (difference in electron binding energies of the 2 shells) is given off as a characteristic x ray photon
The quantity of xrays produced is determined by..
Tube current: mAs
What are the energy requirements at the cathode filament?
Filament current: high amperage (3-5A)
Filament voltage: low voltage (10V)
What are the energy requirements at the X-ray tube?
Tube current: low amperage (10mA)
Tube voltage: high voltage (60-100kV)
Energy requirement relationship between the cathode filament and xray tube? Why?
Electron goes frm low to high voltage.
Cathode filament gotta be low V(10V) and xray tube gotta be high V(60-100kV)
- to create a potential difference for electrons to be accelerated from the cathode to the anode
List the factors affecting the xray beam
- Exposure time (s)
- Tube current (mA)
- Tube voltage (kVp)
- Filtration
- Collimation
- Distance (inverse square law)
What are the factors affecting the xray beam?
- Time (s)
- Tube current (mA)
=> xray quantity (mAs)
How does increased mAs affect the xray?
Increase mAs = increase in xray quantity => ‘increase in exposure time’
- no change in xray quality
=> increase in density, image becomes darker
(more black metallic silver grains develop)
How does decreased mAs affect xray?
- decreased xray quantity, no change in xray quality
- decrease in density
=> image becomes noisy/grainy (cuz it isnt receiving enough information)
What does kVp tell us?
- measure of quality/penetrability of the xray beam
- high kVp => passes thru everything very easily, no contrast
- lower kVp => higher contrast
How does an increase in kVp affect the xray?
Increase kVp = 1. increase xray qty! + 2. increase in average & max energy of xray
=> 1. Increase in density & 2. Decrease in contrast
- xray graph shifts up and right
How does a decrease in kVp affect the xray?
Decrease kVp = 1. Decrease xray qty + 2. Decrease in average & max energy of xrays
=> 1. Decrease in density & 2. Increased contrast
- xray photon energy graph shifts down and left (smaller)
What is filtration of xrays for?
To: reduce dose to patient
By: removing low energy photons which do not contribute to image formation
- cuz soft xrays/low energy xrays dont get any images but still damaging to px
- results in ‘beam hardening’ => increase in average xray quality
What are the 2 types of filtration involved?
- Inherent filtration
- provided by components of the xray assembly
- primary purpose not for filtration
- eg. Tube window, port cover, oil - Added filtration
- filters added outside the xray tube to meet regulatory requirements
- made of aluminum or other composite metals
- units: mm Al or mm Al eq
- can only be added by manufacturer
How does filtration affect the xray image?
Filtration = 1. Decrease in xray qty 2. Increase in average & min energy of xray
=> 1. Decreased density 2. Decreased contrast
Purpose of collimation?
- reduce the dose to patient
- improve contrast by reducing scatter
Mechanism of collimation?
- restricting the width of the xray beam using a metallic barrier
- reducing the volume of irradiated tissue
- minimising non-useful compton scatter radiation
How does circular vs rectangular collimation affect xray image contrast?
Circular => decrease in contrast
Rectangular => increase in contrast
- contrast increases w decreased kVp (energy)
- rectangular further restricts xray beam
What is the inverse square law?
- intensity of an xray beam is inversely proportional to the square of the distance from the source
Tldr, further away = less intense
Which is the main contributor to scattering?
Compton scattering
What are the different types of scatter?
- Coherent scattering
- Photoelectric absorption
- Compton scattering
What is Compton scattering?
- Incident photon interacts w outer shell electron
- Electron gets ejected
- Excess energy (incident KE-electron binding energy) exits atom as a scattered photon
How does compton scattering affect image quality?
- worsens image quality due to decreased contrast
- majority of scatter is projected onto the film/receptor, resulting in radiation fog
What does compton scattering do to px’s body?
Biological damage: ionizing in the px’s body
- scatter radiation that is emitted is hazardous to radiographer too
How does photoelectric absorption affect image quality?
Improved contrast
- PE absorption more likely in matter w high atomic number (bone>soft tissue)
- contributes to differential absorption
What is differential absorption?
Variation in the amount of transmission and absorption, resulting in differential exposure of the film/receptor
How does keV affect scatter?
Higher amt of keV (energy) => higher amt of scatter
What is attenuation?
Decrease in amt of photoelectrons reaching the receptor
Aka the absorption of photoelectrons
Think of hard and soft tissue, hard tissue will cause more attenuation
What is the minimum keV for characteristic radiation?
69.5keV
- binding energy of K-shell electrons in tungsten
[T/F] X-rays are polyenergetic
True
- monoenergetic : fixed energy
- polyenergetic : energy changes due to attenuation
Compton scatter caused by _________kVp results in __________ contrast.
Increased kVp results in decreased contrast
