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Y2 Radiography > 4. Radiation Physics > Flashcards

4. Radiation Physics Flashcards

1
Q

Definition of radiation:

A

Transmission of energy through space & matter

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2
Q

What is Ionization? (definition)

A

neutral atom loses an electron to form an ion pair (positive ion + free electron)

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3
Q

what are the components of an xray tube?

A
  1. tube housing
  2. glass envelope
  3. cathode assembly
  4. anode
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4
Q

Function of the tube housing?

A
  • lead lining minimizes leakage radiation
  • contains oil to dissipate heat
  • physical protection and electrical insulation
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5
Q

Function of the glass envelope?

A
  • creates vaccuum for xray production
  • provides electrical insulation
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6
Q

Function of cathode assembly?

A
  • tungsten wire filament: source of electrons
  • thermionic emission: filament is heated and electrons are ‘boiled off’
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7
Q

Function of the nickel electron focusing cup?

A
  • concentrates the electrons into a sharp/narrow beam
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8
Q

Function of the anode?

A
  • 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
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9
Q

What is Bremsstrahlung radiation?

A
  • 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
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10
Q

Maximum ________ energy corresponds to the maximum ________ applied across the tube

A

Maximum photon energy == maximum voltage applied across the tube

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11
Q

What is Characteristic radiation?

A
  1. Incident e interacts w k shell e
  2. K shell e is removed, leaving a vacancy.
  3. Vacancy is filled by an e from the M or L shell
  4. Excess energy (difference in electron binding energies of the 2 shells) is given off as a characteristic x ray photon
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12
Q

The quantity of xrays produced is determined by..

A

Tube current: mAs

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13
Q

What are the energy requirements at the cathode filament?

A

Filament current: high amperage (3-5A)
Filament voltage: low voltage (10V)

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14
Q

What are the energy requirements at the X-ray tube?

A

Tube current: low amperage (10mA)
Tube voltage: high voltage (60-100kV)

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15
Q

Energy requirement relationship between the cathode filament and xray tube? Why?

A

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
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16
Q

List the factors affecting the xray beam

A
  1. Exposure time (s)
  2. Tube current (mA)
  3. Tube voltage (kVp)
  4. Filtration
  5. Collimation
  6. Distance (inverse square law)
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17
Q

What are the factors affecting the xray beam?

A
  1. Time (s)
  2. Tube current (mA)

=> xray quantity (mAs)

18
Q

How does increased mAs affect the xray?

A

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)

19
Q

How does decreased mAs affect xray?

A
  • decreased xray quantity, no change in xray quality
  • decrease in density
    => image becomes noisy/grainy (cuz it isnt receiving enough information)
20
Q

What does kVp tell us?

A
  • measure of quality/penetrability of the xray beam
  • high kVp => passes thru everything very easily, no contrast
  • lower kVp => higher contrast
21
Q

How does an increase in kVp affect the xray?

A

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
22
Q

How does a decrease in kVp affect the xray?

A

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)
23
Q

What is filtration of xrays for?

A

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
24
Q

What are the 2 types of filtration involved?

A
  1. Inherent filtration
    - provided by components of the xray assembly
    - primary purpose not for filtration
    - eg. Tube window, port cover, oil
  2. 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
25
Q

How does filtration affect the xray image?

A

Filtration = 1. Decrease in xray qty 2. Increase in average & min energy of xray

=> 1. Decreased density 2. Decreased contrast

26
Q

Purpose of collimation?

A
  • reduce the dose to patient
  • improve contrast by reducing scatter
27
Q

Mechanism of collimation?

A
  • restricting the width of the xray beam using a metallic barrier
  • reducing the volume of irradiated tissue
  • minimising non-useful compton scatter radiation
28
Q

How does circular vs rectangular collimation affect xray image contrast?

A

Circular => decrease in contrast
Rectangular => increase in contrast

  • contrast increases w decreased kVp (energy)
  • rectangular further restricts xray beam
29
Q

What is the inverse square law?

A
  • intensity of an xray beam is inversely proportional to the square of the distance from the source

Tldr, further away = less intense

30
Q

Which is the main contributor to scattering?

A

Compton scattering

31
Q

What are the different types of scatter?

A
  1. Coherent scattering
  2. Photoelectric absorption
  3. Compton scattering
32
Q

What is Compton scattering?

A
  1. Incident photon interacts w outer shell electron
  2. Electron gets ejected
  3. Excess energy (incident KE-electron binding energy) exits atom as a scattered photon
33
Q

How does compton scattering affect image quality?

A
  • worsens image quality due to decreased contrast
  • majority of scatter is projected onto the film/receptor, resulting in radiation fog
34
Q

What does compton scattering do to px’s body?

A

Biological damage: ionizing in the px’s body

  • scatter radiation that is emitted is hazardous to radiographer too
35
Q

How does photoelectric absorption affect image quality?

A

Improved contrast
- PE absorption more likely in matter w high atomic number (bone>soft tissue)
- contributes to differential absorption

36
Q

What is differential absorption?

A

Variation in the amount of transmission and absorption, resulting in differential exposure of the film/receptor

37
Q

How does keV affect scatter?

A

Higher amt of keV (energy) => higher amt of scatter

38
Q

What is attenuation?

A

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

39
Q

What is the minimum keV for characteristic radiation?

A

69.5keV
- binding energy of K-shell electrons in tungsten

40
Q

[T/F] X-rays are polyenergetic

A

True
- monoenergetic : fixed energy
- polyenergetic : energy changes due to attenuation

41
Q

Compton scatter caused by _________kVp results in __________ contrast.

A

Increased kVp results in decreased contrast

Y2 Radiography (10 decks)

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