X-ray interaction with matter

Question 1

basic production of radiographic image

Answer 1

source of X-rays - X-ray machine
- production of X-rays

object - teeth and jaws
- interaction of X-rays with matter

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

Interaction with different tissues alters number of photons exiting patient

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

Question 2

4 options for X-ray photons traversing image

Answer 2

pass through unaltered

change direction with no energy loss (scatter)

change direction losing energy (scatter and absorption)

be stopped, depositing all energy within tissue (absorption)

Question 3

scatter

Answer 3

X-ray photons change direction with no energy loss

Question 4

scatter and absorption

Answer 4

X-ray photons change direction and losing energy

Question 5

absorption

Answer 5

x-ray photons be stopped, deposition all energy in tissue

Question 6

effect on photon absorption on image

Answer 6

All photons reach film = black (e.g. air)

Partial attenuation = grey

Complete attenuation = white (e.g. amalgam)

Question 7

structure of atoms

Answer 7

Central nucleus

• Protons (positive charge)
• Neutrons (no charge)

Orbiting electrons (negative charge)

Question 8

atomic number Z

Answer 8

number of protons

Equivalent to no. electrons in neutral atom

Question 9

atomic mass A

Answer 9

number of protons + number of neutrons

Question 10

orbiting electrons

Answer 10

electrons orbit nucleus in “shells” K,L,M,….

maximum number of electrons in orbit greater in outer orbits (2 x n^2)

K shell electrons have highest binding energy (requires more energy to eject electron from shell)

outer shells have lower binding energies

Question 11

2 principles of interactions of diagnostic X-rays in tissue

Answer 11

Photoelectric effect - Absorption

Compton effect – Scatter, and absorption

Question 12

photoelectric effect

Answer 12

energy from X-ray photon is transferred to object

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

photon has energy just higher than the binding energy of electron

X-ray photon disappears

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

electron is ejected (photoelectron)

Atom has “hole” in electron shell: +ve charge
- Not balanced

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

Results in complete absorption of photon energy: photon does not reach film

Question 13

effect of photoelectric absorption

Answer 13

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

Question 14

photoelectric occurence proportional to

Answer 14

Atomic Number3 (Z^3)

1/photon energy3 (1/kV^3)

Density of material
- Noticeable differences between cubes
so Easy to differentiate materials

Question 15

compton effect of X-ray interaction

Answer 15

X-ray photon interacts with loosely bound outer shell electron (not passing through, lower binding energy so lots of energy left in photon)

Photon energy considerably greater than electron binding energy

Electron is ejected taking some of photon energy as kinetic energy: recoil electron

Atom is then positively charged

Question 16

what happens to excess energy in original photon in Compton effect?

Answer 16

Following collision, photon has lower energy (longer wavelength)

Called a scatter photon

Undergoes a change of direction
- Related to how much energy it has lost
Different to incoming energy direction (at least slightly different)

Question 17

following scatter effect (in compton effect)

Answer 17

Atomic stability is regained by capture of free electron as described before (see diagram in Photoelectric effect 2)

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

Question 18

what happens to scattered photons

Answer 18

Scattered photons can travel in any direction

Direction of scatter is affected by energy of scatter photon:

• high energy -> forward direction
• backward direction

Question 19

probability of compton effect occurring

Answer 19

Proportional to density of material (electron density)
- Dense packed electrons – more likely to happen

Independent of atomic number

Not related to photon energy, although forward scatter more likely with high energy photons

Question 20

effect of compoton scattered photons

Answer 20

Scattered photons produced before the image receptor is reached, and scattered backwards, do not reach image receptor and do not contribute to the image

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 image, reducing contrast and image quality

Question 21

reduction of scatter - methods

Answer 21

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)
- Not used with digital receptors

Question 22

absorptions of photons more likely if: (3)

Answer 22

Object traversed has high atomic number

Object traversed is thicker

Photon energy is lower

Question 23

radiographic contrast

Answer 23

Difference in density of light and dark areas of radiograph

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

Contrast is greatest when difference in absorption by adjacent tissues is greatest
- E.g. amalgam and tooth (completely different material) clearer than caries and healthy tooth (same material but poorer state)

Question 24

effect of interaction on dose

Answer 24

Photoelectric absorption results in deposition of all photon energy within tissue:
- Increased patient dose but necessary for image quality

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/ people in the vicinity (stand far away should be find)

Question 25

photoelectric interaction effect on dose

Answer 25

results in deposition of all photon energy within tissue:

- Increased patient dose but necessary for image quality

Question 26

compton interaction effect on dose

Answer 26

deposition of some photon energy within tissue:

• Adds to patient dose but does not give useful information
• May increase dose to operators/ people in the vicinity (stand far away should be find)

Question 27

effect of low kVp on image quality and pt dose

Answer 27

Low tube potential difference (kVp) produces lower energy photons

Photoelectric interactions are increased

Contrast between different tissues increases BUT Dose absorbed by patient is increased

Question 28

effect of high kVp on image quality and pt dose

Answer 28

High tube kVp produces higher energy photons

Photoelectric interactions are reduced

Contrast is reduced but Dose absorbed by patient is reduced

No point reducing dose so much that image is of no diagnostic use
- Wasted dose

Question 29

how to choose kVp

Answer 29

compromise between diagnostic quality of the image and dose

Currently recommended that dental units have kVp 60-70kV

At GDH&S we currently use 70kV for all intra-orals.