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
