X-Rays Interaction with Matter

Question 1

what happens when x-ray photons pass from the tube

Answer 1

X-ray photos pass from tube, and some through patient to reach image receptor (IR)

The x-ray beam when it comes out of the patient will not have the same spread of energy levels of x-ray photons it had whenever it went in

Interaction with different tissues alters number of photons exiting patient

Question 2

what happens to variation in numbers of photons

Answer 2

Variation in numbers of photons reaching IR produces radiographic appearance of different tissues

Question 3

what are the different options for x-ray photons transversing tissue

Answer 3

1. pass through unaltered
2. change direction with no energy loss (scatter)
   - The photon comes out and it is going to expose the image receptor but its not really going to be helpful in creating a useful image
3. change direction losing energy (scatter and absorption)
4. be stopped, depositing all energy within tissue (absorption)

Question 4

what is attenuation

Answer 4

Reduction in number of photons (X-rays) within beam

Question 5

why does attenuation occur

Answer 5

Occurs as a result of absorption and scatter

Question 6

what does attenuation affect

Answer 6

Affects number of photons reaching image receptor
This is why we get a varying number throughout the beam of photons reaching the image receptor and this impacts on what the image actually looks like

Question 7

what is the effect of photon absorption on image

Answer 7

• All photons reach film / image receptor
= Black
[Either no material there anyway to interact with or it is through soft tissues where the majority of photons can pass through]

• Partial attenuation
= Grey

• Complete attenuation
= White

Question 8

what does partial attenuation gives

Answer 8

Gives detail and allows us to see the different anatomical features and pick up pathology

Question 9

why does complete attenuation occur

Answer 9

Often due to metal restorations along the crowns of teeth, mostly likely to be amalgam, but it is a material which has completely absorbed the x-ray photon energy

Question 10

explain the structure of the atom

Answer 10

• Central nucleus
○ Protons (positive charge)
○ Neutrons (no charge)

• Orbiting electrons (negative charge)

Question 11

what is the atomic number

Answer 11

• Atomic number
○ Z = number of protons
○ Equivalent to number of electrons in neutral atom

Question 12

what is the atomic mass

Answer 12

• Atomic mass

○ A = number of protons and number of neutrons

Question 13

what are orbiting electrons

Answer 13

Electrons orbit nucleus in “shells”

Question 14

what are the names of the shells

Answer 14

K (closest to the nucleus),
L,
M

Question 15

what is the formula for the maximum number of electrons in orbit

Answer 15

2 x n^2

For L = 2
2x2^2 = 8

Maximum number of electrons in orbit greater in outer orbits

Question 16

what shell of electrons have the highest binding energy

Answer 16

K shell electrons have highest binding energy

Question 17

what is binding energy

Answer 17

(requires more energy to eject electron from shell)

Binding energy is what keeps electrons within their shells

Question 18

what shells have lower binding energies

Answer 18

Outer shells have lower binding energies

Question 19

what are the principle interactions of diagnostic x-rays in tissue

Answer 19

• Photoelectric effect - absorption
[Energy from the x-ray photon is transferred to the material in which the interaction is happening]

• Compton effect - scatter, and absorption

Question 20

what is the photoelectric effect

Answer 20

• X-ray photon interacts with inner shell electron (usually K shell)

• This photon is getting through the outer shells to reach here
• Reaches the K shell and is going to interact with an electron within that shell

• Photon has energy just higher than the binding energy of electron

• This only happens if the photon has higher energy itself than the binding energy of the electron ~ only needs to be a little bit higher
• If it has energy less than this then it won’t happen

• X-ray photon disappears
- The electron then gets knocked out and the x-ray photon effectively disappears

• Most of photon energy used to overcome binding energy of electron, remainder gives electron kinetic energy

• The small amount remaining will give the electron that is getting kicked out some kinetic energy so that itself has an energy value and can go on to have interactions itself but it will no longer be an x-ray photon anymore
• It is then referred as a photo electron (going back to the name of the effect)

• Electron is ejected (photoelectron)
- Obviously this means there is now an empty space on this shell and atoms don’t like this

• Atom has “hole” in electron shell:
○ Positive charge
- There are uneven protons and electrons, we have a positively charged atom

• Ionised atom is unstable
• Electron drops from outer shell, filling void

• Difference in energy between 2 levels is emitted as light / heat
○ Characteristic radiation

• Outer voids filled by “free” electrons
○ Might be another photo electron from another atom

• The atom is now neutral again
• Results in complete absorption of photon energy: photon does not reach film

Question 21

what is characteristic radiation

Answer 21

Difference in energy between 2 levels is emitted as light / heat

This characteristic radiation will often be in the form of light or heat
In the form of light it is really useful to us

spare energy that becomes available

Characteristic to element and to the shells

Question 22

what is the effect of photoelectric absorption

Answer 22

• Results in complete absorption of photon, preventing any interaction with active component of image receptor:
  ○ image appears white if all photons involved
  ○ Grey if some photons not involved
• Occurrence proportional to:
  ○ Atomic number^3 (Z3)
  ○ 1/photon energy^3 (1/kV^3)
  ○ Density of material ~ how closely packed together the atoms are

Question 23

what is the atomic number (Z) and cubes of soft tissue

Answer 23

7

7^3 = 343

Question 24

what is the atomic number (Z) and cubes of bone

Answer 24

12

12^3 = 1728

Question 25

what is the atomic number (Z) and cubes of aluminium

Answer 25

13

13^3 = 2197

Question 26

what is the atomic number (Z) and cubes of calcium

Answer 26

20

20^3 = 8000

Question 27

what is the atomic number (Z) and cubes of gold

Answer 27

79

79^3 = 493039

Question 28

what is the atomic number (Z) and cubes of lead

Answer 28

82

82^3 = 551368

Question 29

what does relatively small differences in atomic number result in

Answer 29

relatively small differences in Z result in large differences in photoelectric absorption

Question 30

what does the photoelectric mechanism give

Answer 30

We get good differentiation of tissues because of this mechanism

Question 31

what is the Compton effect

Answer 31

• X-ray photon interacts with loosely bound outer shell electron
- photon isn’t getting through the outer shells

• Photon energy considerably greater than electron binding energy
- This means because this shell has a much lower binding energy that frequently there will be quite a lot of energy left in the photon after it has caused that electron to come out of it’s shell

• Electron is ejected taking some of photon energy as kinetic energy: recoil electron
- Electron gets ejected and gets some kinetic energy from the photon

• Atom is then positively charged
- This positively charged atom wants to become neutral again

Question 32

what happens to excess energy in original photon

Answer 32

• Following collision, photon has lower energy (longer wavelength)
• Called a scatter photon
• Undergoes a change of direction
• The change in direction is actually related to how much energy it has lost

Question 33

what happens following scatter events

Answer 33

• Atomic stability is regained by capture of free electron as described before
- Still an element of characteristic radiation produced

• Recoil electron can interact with other atoms in tissue

• Scatter photon, dependent on energy and position of bound electron involved, can be involved in more compton or photoelectric interactions
- Scatter photon has implication for radiation dose - not just for the patient but potentially to other people as well

Question 34

what happens to scattered photons

Answer 34

Scattered photons can travel in any direction

direction of scatter is affected by energy of scatter photon:
> high energy = forward direction [but not exactly the same path as the original photon]
> backward direction = low energy

Full range of directions between 2 extremes dependent on energy

Question 35

what is the probability of the Compton effect occurring

Answer 35

• Proportional to density of material (electron density)
○ If you have a material that is very dense with lots of electrons packed closely together it is more likely that this effect will happen because it is more likely that the incoming photon will bump into one of these outer electrons

• Independent of atomic number
○ Can’t calculate this based on the atomic number

• Not related to photon energy, although forward scatter more likely with high energy photons
○ Tends to happen more with higher energies and there will be more forward scatter with higher energy photons

Question 36

what is the effect of Compton scattered photons

Answer 36

• Scattered photons produced before the image receptor is reached, and scattered backwards, do not reach image receptor and do not contribute to the image
○ The image is not degraded by this scatter photons

• Scattered photons produced beyond image receptor, and scattered back towards it, may reach image receptor producing darkening:
○ As their path is randomly altered they do not contribute useful information to the image
○ Results in fogging of the image, reducing contrast and image quality
○ These photons are not good for the image they will cause fogging of the image = increase in darkness
○ These photons degrade the image

Question 37

is scatter useful for image formation

Answer 37

no

Question 38

wha are the reduction of scatter methods

Answer 38

• Reduction of area irradiated (collimation) and therefore volume irradiated, will reduce the number of scattered photons produced as well as reducing patient dose
• Lead foil within film packet prevents back scattered photons from oral tissues reaching film (in addition to absorbing some of the energy in the primary beam)

Question 39

how does reducing the area irradiated help to reduce the scatter

Answer 39

○ Reduce the area of the beam and the area of the patient irradiated
○ So it is the smallest area that is compatible with the diagnostic outcomes
○ Reducing the area = reduces the volume of tissue which therefore will reduce the volume of scatter radiation produced which also helps to reduce patient dose = positive thing to do
○ This is done with every technique which we use

Question 40

how does lead foil within a film packet help to reduce scatter

Answer 40

○ Not used with digital receptors
§ They don’t have the lead foil because they are more sensitive to x-rays so we are using a lower radiation dose anyway and there is a reduced exposure time combined with collimation and so on
§ There already is quite a bit going on already to try and reduce the scatter

○ If scatter happened in the tissues beyond the film packet and came back to the film packet, the lead foil would be sufficiently thick and powerful to absorb those scatter photons and stop them from degrading the image

○ Some of the energy in the primary beam would also be absorbed, but not all of it

○ Lead foil had a pattern to it so if someone by mistake put the film packet in back to front and the beam went through the lead before reaching the film this pattern would show up and produce a lighter image

Question 41

when is absorption of photons more likely

Answer 41

• Object traversed has high atomic number
○ Linked with photoelectric effect

• Object traversed is thicker
○ When a material doesn’t fully absorb the x-ray energy (like amalgam or gold does) like enamel or dentine…?
○ If they are thicker you will get more x-ray absorption

• Photon energy is lower
○ Get more absorption

Question 42

what is radiographic contrast

Answer 42

Difference in density of light and dark areas of radiograph

Something that is measurable
Ability to see a sharp junction between materials

Question 43

what image has high contrast

Answer 43

Image showing both light and dark areas with clear borders has high contrast

Question 44

when are we most likely to get high contrast

Answer 44

Most likely to get high contrast when adjacent materials are very different to each other:
§ The distinction between an amalgam &the rest of the tooth is very clear
§ Whereas the distinction between caries and the rest of the tooth is not actually that clear ~ this is because they are not very different tissues, it is just that one of them has lost some of its mineral content

Question 45

when is contrast greatest

Answer 45

Contrast is greatest when difference in absorption by adjacent tissues is greatest

Question 46

what does photoelectric absorption result in

Answer 46

Photoelectric absorption results in deposition of all photon energy within tissue:
○ Increased patient dose but necessary for image quality
○ We accept the dose - it is a necessary by-product that is going to help our diagnosis

Question 47

what does Compton scatter result in

Answer 47

Compton scatter results in deposition of some photon energy within tissue:
○ Adds to patient dose but does not give useful information
○ May increase dose to operators
§ This is because some of it may come out of the patient
§ This would only increase dose to the operator if the operator is standing very close to the patient ~ if you are standing where you are meant to stand then you shouldn’t be getting any dose from this
§ If you cannot leave the actual room where the x-ray is being taken then what you would do is use distance to protect you and also not stand in the actual path of the x-ray beam
○ Unfortunately cannot prevent it happening

Question 48

what is the effect of low kVp on image quality and patient dose

Answer 48

• Low tube potential difference (kVp) produces lower energy photons
○ For example 50kV

• Photoelectric interactions are increased
○ Very good for contrast

• Contrast between different tissues increases
○ BUT dose absorbed by patient is increased
○ We can reduce that dose by increasing the kV

Question 49

what is the effect of high kVp on image quality and patient dose

Answer 49

• High tube kVp produces higher energy photons
○ For example 60/70kV

• Photoelectric interactions are reduced
• Contrast is reduced

• Dose absorbed by patient is reduced
○ Dose is very strongly affected by photoelectric absorption

• No point reducing dose so much that image is of no diagnostic use
○ Might just think why not just make the kV higher to bring the dose down for the patient? this notably reduces the contrast
§ If you have the contrast so poor that you cannot diagnose things then that is a wasted dose as it has given no benefit to the patient

Question 50

what does choosing a kVp potentially compromise

Answer 50

Chosen kVp is a compromise between diagnostic quality of the image and dose

Question 51

what kVp should dental units do

Answer 51

Currently recommended that dental units have kVp 60-70kV