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MED-107 Medical Physics II: Medical Imaging and Radiotherapy > 1.1/1.2 ionizing radiation > Flashcards

1.1/1.2 ionizing radiation Flashcards

1
Q

what are the classifications of radiation?

A
  • electromagnetic radiation
  • particulate radiation
  • ionizing and non-ionizing radiation
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2
Q

electromagnetic waves, like all waves, can be characterized by their

A
  • amplitude
  • wavelength (λ)
  • frequency (f)
  • speed (v)
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3
Q

how is speed of a wave calculated?

A

c = λ x f
c = wavelength x frequency

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4
Q

what are photons commonly referred to as?

A

γ-rays

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5
Q

what unit is commonly used to denote photon energy?

A

the electron-volt (eV)

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6
Q

1 Joule is equal to

A

6.24 x 10^18 eV

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7
Q

in diagnostic radiology, the only particulate radiation that needs to be considered is the

A

electron

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8
Q

an electron has a rest mass of _____ and a rest energy of _____

A

9.1 x 10^-31
511keV

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9
Q

two ways radiation is classified

A
  • ionizing - can ionize matter
  • non-ionizing - cannot ionize matter
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10
Q

what frequency of electromagnetic radiation makes it ionizing?

A

frequency higher than the near-ultraviolet region of the electromagnetic spectrum

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11
Q

what types of electromagnetic radiation is non-ionizing?

A

visible light, infrared, radiofrequency

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12
Q

what are 3 ways photons can be absorbed?

A
  • photoelectric absorption
  • compton scattering
  • pair production
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13
Q

describe photoelectric absorption

A
  • a photon can be absorbed by transferring all of its energy to an inner orbital electron in an atom of the absorber
  • the electron is ejected from the atom and the photon disappears as it has lost all of its energy and never had mass
  • the atom is left with a vacant inner electron orbit which it will fill with one of the outer electrons
  • when it does this it releases a small amount of energy in the form of a characteristic X-ray photon
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14
Q

why is the X-ray called a characteristic photon?

A

because its energy is characteristic of the absorbing material

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15
Q

why does an X-ray photon have fixed energy?

A

because orbital electron have fixed energies which correspond to the orbit which they occupy

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16
Q

in what case is photoelectric absorption the most likely form of absorption?

A

when the incident photon has a relatively low energy
(energies below 25 eV)

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17
Q

the odds of a proton being absorbed by photoelectric absorption depend on what factors?

A
  • the energy of a photon
  • the chemical elements in the absorbing material
  • the number of protons in the atom (atomic number)
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18
Q

what is the relationship between the likelihood of photoelectric absorption and the atomic number of atoms?

A

the likelihood of photoelectric absorption increases as the cube of the atomic number, Z^3

(photoelectric absorption is more likely to occur in materials that have many atoms with relatively large values of Z because of this relationship)

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19
Q

an example of a metal that is a good absorber of X-ray photons

A

lead (Z = 82)

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20
Q

describe the attenuation of an X-ray beam in human tissues

A
  • air: negligible
  • bone: significant due to relatively high density (atom mass of Ca)
  • soft tissue (ex: muscle) : similar to water
  • fat tissue: less important than water
  • lungs: weak due to density
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21
Q

what does a higher kVp value result in?

A

it will make the X-ray beam more penetrating, cause less difference in attenuation, and lowers contrast
(of x ray image)

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22
Q

how can bone and lung structures be simultaneously visualized?

A

higher kVp reduces photoelectric effect

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23
Q

what is the Compton Effect?

A
  • an interaction with the OUTER electrons which are not tightly bound to an atom
  • the photon collides with an electron and gives some of its energy to it
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24
Q

what are the outcomes of the different types of collisions of photons in the Compton Effect?

A
  1. if collision is head on - the photon has its direction of travel reversed and so it loses the maximum amount of energy
  2. if collision is only a glancing one - the energy given to the recoil electron will be much less
  3. if a single photon undergoes several collisions - it loses some energy each occasion and eventually is absorbed by the photoelectric effect
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25
Q

what does the actual loss of energy as a result of the Compton Effect depend on?

A

the angle through which the photon is scattered

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26
Q

the Compton Effect is the dominant effect for which photon energies?

A

above 200 keV and up to 2 MeV

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27
Q

between what energies can both the photoelectric and Compton Effect occur?

A

60 keV and 90 keV

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28
Q

explain why only the thickness of an absorber and its density are important for photon absorption at higher energies by the Compton Effect?

A
  • photons in the Compton effect interact with electrons as though they were not bound to an atom
  • meaning only the total number of electrons in a block of material matters, regardless of arrangement (thickness)
  • the Compton Effect distinguishes between materials through their different densities
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29
Q

why is the Compton Effect relatively insensitive to variations in anatomy compared to the photoelectric effect?

A

because most soft tissues have very similar densities, which makes the Compton Effect invalid in this case

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30
Q

is contrast more affected by chemical composition or differences in density? explain

A

since the photoelectric absorption effect plays the dominant role, chemical composition affects contrast in imaging

31
Q

what are some differences between the photoelectric effect and the Compton effect?

A
  • the photoelectric effect is a low-energy phenomena, and the photons that interact with electrons vanish as soon as they give their energy to them
  • the Compton effect, on the other hand, is a mid-energy phenomenon in which photons contact electrons and are scattered
  • photoelectric is mainly responsible for image contrast, Compton contributes to artifacts in the images
32
Q

what is the pair production effect?

A

another method of absorption, less important than the Compton and photoelectric effects, that only applies to very high-energy photons

33
Q

what happens during pair production?

A

if the photon has sufficient energy, it can be absorbed by an atomic nucleus in the absorber, which results in the production of a positron and an electron

34
Q

how much energy is needed to produce the pair of particles (positron and electron) in pair production? what happens if energy is higher than needed?

A
  • 1.02 MeV
  • (if photon has more than 1.02 MeV, then excess simply increases the velocity of the electron and positron)
35
Q

what happens to the positron produced from pair production?

A

it will not live very long because if it meets an electron it will combine with it to produce two photons of 0.51 MeV

36
Q

what are photons produced from the combining of a positron and electron known as?

A

annihilation radiation

37
Q

pair production is the dominant effect for which photon energies?

A

energies above 5 MeV

38
Q

what photon energies are each effect best suited for?

A

photoelectric effect - low energies
Compton Effect - mid energies
pair production - very high energies

39
Q

which effect is a case of energy being converted into mass?

A

pair production effect

40
Q

what is another way to reduce the intensity of radiation?

A

moving away from the source (point source) decreases the intensity of radiation

41
Q

what happens if the radiation from a source can spread in all directions?

A

its intensity will fall off in inverse proportion to the distance squared

42
Q

since ionizing radiation cannot be detected directly, how else can we detect it?

A

we rely on the radiation interacting with another material and producing an effect in which we can detect

43
Q

who is the father of radiology and first discovered X-rays?

A

Wilhelm Conrad Röntgen

44
Q

who first detected radiation and discovered radioactivity?

A

Henri Becquerel

45
Q

what are the two methods used by Henri Becquerel to detect ionizing radiation?

A

film & electroscope

46
Q

if a person is being exposed to radiation, why do we need to know the received dose rate ASAP?

A
  1. so that we can calculate the accumulated dose
  2. give a warning if the dose rate is very high
47
Q

what is a dosimeter?

A
  • an instrument that measures ionizing radiation
  • comprises an electrometer (a measuring assembly) and one or more detector assemblies
48
Q

how is monitoring equipment calibrated?

A

in terms of mrad/h or μGy/h

49
Q

how are low levels of dose rate measured?

A
  • using Geiger-Muller (G-M) tubes
  • scintillation counters
50
Q

what are other ways dose rate can be measured? what advantages do they hold?

A
  • using an ionization chamber

advantages:
- more accurate
- less affected by radiation energy
- can measure high dose rates which would saturate other monitors

51
Q

what are some instruments that measure dose/dose rate and can be worn on the body?

A
  • pocket dosimeters
  • G-M tubes with a dose rate alarm
  • solid-state detector and scintillation counter systems (available but expensive and not suitable to routine dose measurements)
52
Q

what is TLD?

A
  • thermoluminescent dosimetry
  • used by the cheapest and most common personal monitors
53
Q

what are the advantages of film dosimeters?

A

a film badge as a personal monitoring device:
- is very simple, therefore not expensive
- provides a permanent record
- is very reliable
- is used to measure & record radiation exposure due to gamma rays, x-rays, and beta particles

54
Q

what are the disadvantages of film dosimeters?

A
  • usually cannot be read on site (need to be sent away for developing)
  • for one-time use only
  • less than 0.2mSv exposures of gamma radiation cannot be accurately measured
55
Q

what are the advantages of TLD dosimeters?

A
  • able to measure a greater range of doses in comparison with film badges
  • doses from TLDs may be easily obtained
  • can be read on site instead of being sent away
  • easily reusable
56
Q

what are the disadvantages of TLD dosimeters?

A
  • each dose cannot be read out more than once
  • the readout process effectively “zeroes” the TLD
57
Q

what are electronic personal dosimeters?

A
  • high-range, alarming, active dosimeters
  • designed to be worn by occupational radiation workers in planned exposure situations
  • to measure personal dose equivalence for regulatory compliance
  • displays dose AND dose rate
  • high level of radiation sensitivity
58
Q

what are some qualities of electronic dosimeters?

A
  • display dose AND dose rate
  • sensitive to high levels of radiation
59
Q

what is an electric current?

A

a flow of electrons or ions

60
Q

how can an electric current flow in the air?

A

if some of the atoms in the air are ionized, then free electrons are produced and an electric current can flow

61
Q

how does lightning occur?

A

the very high potential gradient between the cloud and the ground is sufficient to ionize the air and allow current to flow

62
Q

what happens in an ionizing chamber? what allows current to flow through the chamber?

A
  • when the chamber is exposed to ionizing radiation: (1) positive and negative ions are produced (2) electrons in the air are freed
  • the electrons fill the chamber and allow a current to flow
  • a potential is applied across metal plates
  • the positively charged ions are attracted to the negative plate
  • the negatively charged ions are attracted to the positive plate
  • this allows current to flow through the chamber
63
Q

what is the current measured by? why is that needed?

A
  • a sensitive ammeter
  • because the currents to be measured are often of the order of 10^-9 A
  • which is equal to 6 x 10^9 electrons per second (quite difficult to measure)
64
Q

what are the uses of an ionization chamber?

A
  • used to measure the ionizing radiation output of therapeutic and diagnostic ionizing radiation generators
  • used to make accurate measurements of patient radiation dose
65
Q

what is a G-M tube?

A
  • a very sensitive form of ionization chamber (so sensitive it can detect single ionizing particles which enter the tube)
  • differs from an ionization chamber as it is filled with a gas (such as argon or neon) rather than air
66
Q

what is the pressure of the gas inside a G-M tube?

A

about one-fifth of atmospheric pressure

67
Q

what happens in a G-M tube?

A
  • incident ionizing radiation will produce free electrons within the tube
  • these electrons will be attracted towards the central electrode (held at a positive potential)
  • the electrons are accelerated by the potential
  • this allows them to gain sufficient energy to cause further ionization, causing a chain reaction
  • when all hit the central anode, they can cause photons to be emitted
  • these photons can cause more ionization in the gas of the chamber
68
Q

what is the result of G-M tubes? how is it measured?

A
  • ionization in the gas of the chamber results in the original incident ionizing radiation to produce about 10^5 electrons in the chamber
  • it is measured as a pulse of current lasting about 1 μs
69
Q

how is the G-M tube operation much simpler than an ionization chamber?

A

the G-M tube operation does not deal with the movement of the positive ions in the tube (since they travel much slower than electrons)

70
Q

what is the “dead time” of the G-M tubes? why is it important?

A
  • the time it takes the tube to recover from the recorded pulse
  • it is important as it limits the number of events which can be recorded each second
71
Q

which instrument that measures radiation can measure ALL types of radiation (gamma, beta, alpha)?

A

G-M tubes

72
Q

the use of G-M tubes is not recommended in which case?

A

for diagnostic radiology

73
Q

what are two main difficulties in diagnostic radiology for the use of G-M tubes/counters?

A
  • response time of several seconds
  • a strong energy dependence at low photon energies

MED-107 Medical Physics II: Medical Imaging and Radiotherapy (16 decks)

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