Interaction of X-Rays with Matter

Question 1

For Diagnostic radiography X-Radation is incident upon the part of the body being examined as we want the radiation to produce an image on the detector, the ability to do this depends on …

Answer 1

how it interacts with the parts encountered, however the situation is much more complex

Question 2

X-Ray beam encounters many different types of body part as it transverses the body - each have differences in …

Answer 2

shape, thickness, atomic number, density and radiosensitivity

Question 3

In radiotherapy you need to give a specific dose of radiation to abnormal (cancerous) tissue however there are healthy organs surrounding cancerous tissue therefore you have to …

Answer 3

restrict the dose to healthy tissues whilst giving a lethal dose to cancerous tissue

Question 4

When a beam of X-Ray or Gamma Ray photons is incident upon an object, which 3 things can occur

Answer 4

Transmission
Absorption
Scattering

Question 5

Describe transmission

Answer 5

Photons are fired but they miss everything, they penetrate without giving up energy, nothing happens, goes straight the patient and hits the detector

Question 6

Describe absorption

Answer 6

photons fired but depending on what they strike in the patient either come to a stop and disappear (absorbed), energy is deposited in medium which absorbs the photons, gives up energy and then disappears

Question 7

Describe scattering

Answer 7

photons fired, when they hit part of the patient they are not absorbed but scattered, the photons change direction and lose energy and some intensity, they give up energy and change direction

Question 8

Secondary radiation is …

Answer 8

the radiation after scattering (starts with primary radiation until scattered)

Question 9

Name the 3 processes that take place when a beam of X/Gamma Rays is incident upon the body

Answer 9

Photoelectric absorption
Compton Scattering
Pair Production (only at greater than or equal to 1.02 MeV - RT)

Question 10

Define Attenuation

Answer 10

the total reduction of intensity of a beam of radiation as it transverses through a medium

Question 11

Define Total Attenuation

Answer 11

the reduction due to absorption + the reduction due to scattering (+ reduction due to pair production - in RT only)

Question 12

Describe photoelectric absorption

Answer 12

this is the process of absorption of an X-Ray beam, an incoming electron must have energy equal to or greater than the binding energy of an inner shell (L or K) electron of the absorber material - if this is the case = energy of the photon is used to eject the inner shell electron, the photon then disappears and the released electron now has kinetic energy equal to the difference between the initial energy of the photon and the electrons binding energy - vacancy remains

Question 13

Explain the electron transition during photoelectric absorption

Answer 13

An electron from an outer shell fills the vacancy left by the ejected electron, therefore a lower energy X-Ray photon is emitted which is also photoelectrically absorbed by adjacent atoms

Question 14

Define the Linear Attenuation Coefficient (u)

Answer 14

The fractional reduction in intensity per unit thickness of the absorber - this tells us how much each mm of absorber reduces the intensity at a given photon energy, does this in terms of what fraction of the original beam is absorbed

Question 15

Describe a problem when using u alone and how its rectified

Answer 15

It doesn’t take into account the density of the material, different latest of the same material can have different densities (e.g. Ice = less dense than water), therefore u/p is used to introduce the materials density

Question 16

Describe the Mass Attenuation Coefficient

Answer 16

defined as the fractional reduction in intensity per unit mass of the absorber, this depends on the absorber’s atomic number and the photons energy, given as t/p

Question 17

t/p is proportional to …. and inversely proportional to …

Answer 17

proportional to atomic number cubed = t/p a Z cubed

inversely proportional to photon energy cubed = t/p a 1/KeV cubed

Question 18

Name the other important feature of the attenuation coefficient- photon energy relationship
An example

Answer 18

it changes abruptly at 1 particular energy = the binding energy of the shell electrons - generally designated as the K/L edge
(e.g. at the K edge, photons with energies slightly greater than the binding energy for the K shell can now interact with the K shell electrons in addition to those in L shell, therefore more electrons in the material that are available for interactions, produces sudden increase in attenuation coefficient at K shell binding energy, theres a similar change in attenuation coefficient at L shell electron binding energy - for most elements this is below 10 KeV and not within useful portion of X-Ray spectrum, photoelectron interactions occur at highest rate when energy of X-Ray photon is just above binding energy of electrons)

Question 19

Describe Compton Scattering

Answer 19

An incident X-Ray photon strikes the outer shell where there’s a loosely bound electron, the incident X-Ray photon ejects the electron and both move off at the same angle (theta) to initial trajectory - the compton electron gains kinetic energy and the scattered X-Ray photon loses photon energy

Question 20

What does the loss of energy experienced by a photon depend on

Answer 20

depends on the angle of scatter, the greater the angle through which the photon is scattered, the greater its loss of energy

Question 21

Give the symbol for mass attenuation coefficient for compton scattering and what its proportional to

Answer 21

o/p - independent of atomic number (Z), proportional to the inverse of the photon energy - o/p a 1/KeV

Question 22

Photoelectric absorption and compton scattering are …

Answer 22

competing processes - each have influence on how much of the X-Ray beam is attenuated

Question 23

Total mass attenuation coefficient (u/p) is …

Answer 23

the sum of the mass attenuation coefficients for the 2 processes - u/p = t/p + o/p

Question 24

Describe with what Pair Production occurs and its significance in DT and RT

Answer 24

occurs with X/Gamma Rays photons whose energy is in excess of 1.02 MeV - significant in RT, relative for Positron Emission Tomography (PET) imaging

Question 25

Describe Pair Production

Answer 25

incoming photons with energy equal to or greater than 1.02 MeV interacts with the field surrounding the nucleus, the incoming photon is converted to an electron and a positron, any energy in excess of 1.02 MeV is shared between these 2 new particles as Kinetic Energy, any KE that the electron has is dissipated locally within a few mm of soft tissue, any KE the positron has is dissipated locally AND when it comes to rest undergoes the reverse formation reaction creating 2 0.51 MeV photons travelling in opposite directions

Question 26

Describe the mass attenuation coefficient for the pair production process and what its proportional to

Answer 26

written as H/p, proportional to atomic number Z and to the photon energy once the threshold or 1.02 MeV has been reached