interaction of photons and matter

Question 1

what your diagnosis?

Answer 1

broken wing

Question 2

what are photons

Answer 2

photons include gamma rays and x-rays
-gamma rays: ejected from a nucleus (radioactive decay)
-X-rays: arise from transitions between shells of an atom or from interactions between electrons and nuclei

Question 3

what is diagnostic and therapeutic radiation energies?

Answer 3

-diagnostic imaging involves photons with energies in the keV range (thousands of eV’s)
-radiation therapy usually involves photons with energies in the MeV range (millions of eVs)

Question 4

as a photon beam enters matter (ex: lead sheet, patient) there are two possible outcomes for each photon:

Answer 4

1. penetrate without interaction (no energy is deposited)
2. undergo an interaction with atoms (energy is deposited)

Question 5

what is the difference between transmitted and attenuated photons?

Answer 5

-when photons do not undergo an interaction they are transmitted through the matter (come out the other side)
-when photons undergo an interaction, they do not pass through the matter, resulting attenuation of the beam (reduction in intensity)

Question 6

the transfer of energy from a photon to matter consists of 2 steps:

Answer 6

1. interaction of the photon with an atom, causing an electron or electrons to be set in motion
2. transfer of energy from the electron to the medium through ionization

Question 7

do photons transfer energy directly to matter?

Answer 7

no
-first to an electron (classified as indirectly ionizing radiation)

Question 8

what is exponential attenuation?

Answer 8

-if a photon undergoes an interaction, it is attenuated (removed from the beam)
-the attenuation is exponential, which means that for a given thickness of material a certain percentage of the photons are attenuated

Question 9

what is an example of exponential attenuation?

Answer 9

-every 1 cm of muscle removes 10% of the photons in a 500 keV x-ray beam
-this means that 10% of the photons undergo an interaction with atoms in that 1 cm layer of muscle

Question 10

what is the universal attenuation curve?

Answer 10

-percent transmission of intensity as a function of material thickness
-a typical x-ray beam with a spectrum of energies will not follow exponential behavior exactly (not a single value of μ because composed of a spectrum of energy)

Question 11

what is the half-value layer?

Answer 11

-the thickness that attenuates a photon beam to 50% is called the half-value layer (HVL)
-described the beam in terms of ability to penetrate a material
-from 120 to 400 kV energy photons, HVL is described in mm Cu, for <120 kV energy photons, HVL is described in mm Al
-HVL in metals is not used to describe high energy therapy beams (MeV energy photons) because penetration in water (soft tissue), not metal, is relevant

Question 12

what are the 3 main interactions that photons can undergo in material, resulting in their attenuation?

Answer 12

-photoelectric effect (PE)
-compton (incoherent) scattering (CS)
-pair production (PP)

Question 13

what is the total attenuation?

Answer 13

-the sum of the individuals attenuation for each process (PE+CS+PP)
-the number of photons attenuated by each process varies in a different way with photon energy E and atomic number Z of the matter

Question 14

when does the photoelectric interaction occur?

Answer 14

occurs when a photon’s energy is transferred to an atom and an electron (‘photoelectron’) is ejected
-photoelectrons have kinetic energy equal to the difference between the incident photon energy and the binding energy of the ejected electron
-the photon is’ absorbed’ by the atom and only an electron leaves the interaction
-electrons deposit their energy very rapidly, so only travels about 1 cm in human tissues

Question 15

what about the empty shell in a photoelectric interaction?

Answer 15

-characteristic x-rays will be produced because an orbital electron is ejected
-because they are low energy x-rays (equal to the differences in shell energies) they are usually absorbed within the material within a very short distance, and are not considered important
-this is particularly true for soft tissue, which is composed of low Z material (H, C, O) and therefore has low K shell binding energies (H=0.14 keV, C=2.8 kev)

Question 16

what is the likelihood of attenuation by photoelectric interaction?

Answer 16

inversely proportional to E^3 of the incoming photon
-directly proportional to Z^3 of the material
-E (incoming photon energy)
-Z (atomic number of material)

Question 17

why is photoelectric interactions important for diagnostic imaging?

Answer 17

photoelectric interactions are important at the lower energies used for diagnostic imaging (keV range)
-therefore, the likelihood of attenuation during diagnostic imaging is highly dependent on the Z of the material
-a high Z material will attenuate many more diagnostic x-rays than a low Z material

Question 18

what is the photoelectric interaction summary?

Answer 18

-the incoming photon is absorbed, and an electron (‘photoelectron’) is ejected
-the likelihood of photoelectric interaction is proportional to 1/E^3 and Z^3
-important process at lower photon energies (up to 100 keV) which are used for diagnostic imaging (ex: radiography, computed tomography)

Question 19

when does Compton (incoherent) scattering happen?

Answer 19

occurs when a photon’s energy is transferred to an atom and an electron (“Compton electron”) is ejected
-the photon is not absorbed by the atom, but also leaves the interaction (“scattered” because its direction is changed)
-the energy of the incoming photon is shared between the Compton electron and the scattered photon

Question 20

what is the likelihood of attenuation of compton scattering?

Answer 20

-inversely proportional to E
-independent of Z

Question 21

what is compton scattering used for in imaging?

Answer 21

-compton scatter (and photoelectric) occurs at the energies used for diagnostic imaging (keV range)
-compton scattering is by far the most important process at the higher energies used for radiation therapy (MeV range)

Question 22

why do we use MeV beams for radiation therapy?

Answer 22

-one of the reasons we use MeV beams for radiation therapy of people and animals with tumors is that all tissues will attenuate x-rays in a similar way, regardless of Z

Question 23

what is Compton (incoherent) scattering summary?

Answer 23

-the incoming photon is scattered and an electron is ejected
-the scattered photon and the compton electron share the energy of the incoming photon
-the scattered photons need to be considered when workers are in the room during patient radiographs
-the likelihood of compton scatter is proportional to 1/E and independent of Z
-the most important process at higher photon energies (MeV range) which are used for radiation therapy

Question 24

what the pair production interaction?

Answer 24

-an interaction between a photon and a nucleus where the energy of the photon is transformed into an electron and a positron
-the energy of the incoming photon is shared between the electron and the positron
-the photon is ‘absorbed’ by the atom

Question 25

what leaves the interaction (pair production)?

Answer 25

-the positron leaves the interaction, and when it stops, it interacts with a free electron
-the positron and electron disappear (annihilate) and two 0.511 MeV photons are created

Question 26

when is pair production very important (commonly used)?

Answer 26

only becomes important for very high energies (>10 Mev)
-pair production cannot occur unless the incoming photon has an energy > 1.022 MeV
-even though the incoming photon is absorbed by the atom, scattered photons are creates because the positron results in two photons when it annihilates
-the likelihood of pair production increases with increasing Z

Question 27

what is the relative importance of interactions in soft tissue (water) summary?

Answer 27

Question 28

what is total attenuation of a photon beam?

Answer 28

depends on Z of material and energy of photon beam
-a cobalt 60 therapy machine produces two gamma rays of 1.16 MeV and 1.33 MeV (average energy is 1.25 MeV)
-an x-ray machine produces a spectrum of photon energies, but can be roughly considered as a monoenergetic beam of peak kilovoltage E/3
-a linear accelerator produces a high energy x-ray beam and is named based on the maximum photon energy (ex: a 6 MV linac has a maximum photon energy of 6MeV and an average photon energy of 2 MeV)