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diagnostic radiology (ionising) > photon interaction processes > Flashcards

photon interaction processes Flashcards

1
Q

what are the 4 mechanisms of photon interactions

A

PE, CS, RS, PP

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2
Q

what happens during photoelectric effect?

A
  1. x-ray photon ejects K-shell electron, transferring all of its energy and leaving a vacancy in its place
  2. an I shell electron with higher energy fills the K shell vacancy, emitting the excess energy as a characteristic photon
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3
Q

photoelectric effect: the photon energy = _____ energy + _____ energy of emitted _____

A

binding, kinetic, photoelectron

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4
Q

a _____ photon is emitted in the photoelectric effect

A

characteristic

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5
Q

characteristic photon energy = difference in __ and __ shell energies

A

K, L

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6
Q

____ ____ is the dominant interaction at low energies

A

photoelectric effect

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7
Q

soft tissue has low/high Z and k-shell binding energies are high/low. metals has low/high Z and k-shell binding energies are high/low

A

low, low, high, high

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8
Q

in soft tissue, characteristic radiation is ____ ____ while in metals, characteristic radiation ____ ____

A

absorbed locally, can escape

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9
Q

two things which can be emitted when electron vacancy filled

A

characteristic x-rays or auger electrons

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10
Q

fluorescent yield formula (omega_k)

A

omega_k=no. of k x-ray photons/no. of k shell vacancies

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11
Q

if omega_k = 1, then __ Auger electrons, if omega_k = 0, then __ Auger electrons

A

no, all

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12
Q

what must be satisfied for fluorescence to occur

A

the energy required to remove a photoelectron from the K shell of an atom must be satisfied, (energy of the absorbed quantum must be greater than the ionization energy)

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13
Q

what is mass attenuation coefficient (mu/rho)

A

probability of photon being removed from the beam by any process

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14
Q

what is photoelectric mass attenuation coefficient (tau/rho)

A

probability of photon being removed from the beam by photoelectric effect

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15
Q

relationship between PE mass coefficient (tau/rho), atomic number Z and photon energy (E)

A

tau_rho proportional to Z^3/E^3

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16
Q

what enables clear imaging of bone

A

relatively high Z and density

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17
Q

mammo: use low/high energy to maximise ____ effect because of no difference in ____ between normal and pathological tissue making differences in ____ subtle

A

photoelectric, density, attenuation

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18
Q

rough mean Z of 1. bone, 2. soft tissue/muscle, 3. fat

A
  1. 14
  2. 7.5
  3. 6
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19
Q

compton scattering is an interaction between an ____ and ____ ____ ____

A

electron, high energy photon

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20
Q

compton scatter: A photon interacts with a ___ ___ ___ in the ___ shell of an atom.
The photon undergoes ____ scatter and ___ the ___ electron from its shell.
The photon is ____ and loses ___.
During this process, the photon imparts ___ ___ to the electron.

A

loosely bound electron, outer, removes, inelastic, recoil, energy, kinetic energy

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21
Q

in compton scatter: the change in photon energy depends on ___ and ___

A

initial photon energy, angle of scatter

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22
Q

change in photon wavelength equation for compton scatter

A

delta lambda = h/m_e*c (1-cos(theta)), where theta = angle through which photon scattered, h = Planck’s constant, m_e = rest mass of electron, c = speed of light

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23
Q

compton scattering: higher photon energy results in a ___ % of energy retained by photon and ___ % of energy taken by electron

A

lower, greater

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24
Q

compton scattering: as photon energy increased, a greater proportion of its energy is retained by the photon or transferred to the electron?

A

transferred to electron

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25
Q

how does the probability of compton interaction vary with energy

A

decreases with increasing energy

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26
Q

what happens in compton interaction at low energies

A

most of the energy is retained by the photon

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27
Q

compton scattering: at higher energies, even though scatter is less likely, why can it have effect on image quality

A

more penetrating photons resulting from scatter which may escape more easily and reach detector

28
Q

compton mass attenuation coefficient (sigma/rho):

approx ____ in diagnostic range

proportional to ___ if E > 100 keV

and is independent of __

A

constant

1/E

Z

29
Q

there are N_0 ( ___ ___ ) atoms in A ( ___ ___ ) grams, so the number of atoms per gram = __ / __.

there are __ electrons per atom

therefore, the equation for electron density, defined as the number of electrons per gram = _______

A

avagadros no, atomic mass, N_0/A

Z

(N_0 Z)/A

30
Q

disadvantages of compton scatter (2)

A
  1. staff dose
  2. reduces image contrast
31
Q

formula for backscatter factor

A

backscatter factor = dose at P with scatter/ dose at P without scatter

32
Q

typical backscatter factors

A

1.2-1.3

33
Q

is undercouch or overcouch tube preffered for reducing backscatter

A

undercouch

34
Q

what happens during elastic scattering

A
  1. the whole atom absorbs the recoil
  2. “bound” electrons resonate and photon frequency
  3. electrons re-radiate energy at same frequency and energy of photon
  4. scattering in forward direction
35
Q

what is the k-absorption edge effect

A

where the photon energies exceed the k shell binding energy so there is an abrupt increase in the photoelectric effect

36
Q

how can k edge be used in imaging

A

maximise absorption in detector

by using image detector with high absorption
matches energy of the k-edge approx. to peak of x-ray spectrum

37
Q

what benefit from using k edge in imaging

A

fewer photons for given optical density meaning lower dose to patient

38
Q

contrast agents are __ and __

A

iodine, barium

39
Q

iodine Z=__, k-edge=__

A

53, 33 kev

40
Q

why is iodine used as contrast agent

A

high atomic number

41
Q

example application of iodine as contrast

A

angio

42
Q

how does iodine work as contrast agent in angiography

A

greater absorption
means greater difference between transmitted intensity through the iodine in the blood vessel and soft tissue

43
Q

pair production: ___ __ ___ hits ___ producing __ __ pair

A

high energy photon
nucleus
e- e+

44
Q

energy equivalent of e- and e+ rest mass

A

m(e) c^2 = 511 keV = 0.511 MeV

45
Q

pair production: how much energy needed to produce electron positron pair

A

2 x rest mass energy of electron = 1.02 MeV

46
Q

pair production: why must both positron and electron be produced

A

to conserve charge because the incident photon is neutral

47
Q

pair production: how do both electrons and positrons lose energy

A

by interactions with other electrons

48
Q

pair production: __ positron eventually __ with a __ __

A

slow
annihilates
free electron

49
Q

product of annihilation: two ___, each with energy of __ __ emitted in __ __

A

photons
0.511 MeV
opposite directions

50
Q

is pair production important in dr

A

no because not possible <1.02 Mev and dr is up to 150 kev

51
Q

what applications is it used

A

nm: positron emitting radionuclides for pet scanning
rt: important interaction

52
Q

what is the energy and z dependence of elastic interactions (eta)

A

1/E^2
Z^2

53
Q

what is the energy and z dependence of photoelectric effect (tau)

A

1/E^3
Z^3

54
Q

what is the energy and z dependence of compton scatter (sigma)

A

for 10-100keV, constant dependence of energy, for >100 keV, varies with 1/E
independent of Z

55
Q

what is the energy and z dependence of pair production (pi)

A

E-1.02
Z

56
Q

how to approximate mass energy coefficient (mu/rho) for mixtures and compounds

A

weight by the fraction by weight

57
Q

how are neutrons classified

A

according to energy
fast, thermal and intermediate

58
Q

how do fast neutrons lose energy

A

via elastic and inelastic scattering

59
Q

neutron interactions: fast neutrons losing energy - more energy is lost in __ __ __ materials

A

low atomic number

60
Q

how may thermal neutrons be captured

A

by other nuclei

61
Q

__ __ results in production of radioactive material

A

neutron capture

62
Q

what does neutron capture result in the production of

A

radioactive material

63
Q

where can neutron activation occur

A

in heads of high energy linacs

64
Q

what material for neutron shielding and why

A

boron
has low atomic number

65
Q
A

diagnostic radiology (ionising) (7 decks)

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