What happens when x-rays hit you or the patient?

Question 1

What are the 4 ways x-ray photons interact with matter

At what level do these interactions occur?

Answer 1

Transmitted unchanged (go through)
Absorbed (disappear)
Scatter + absorption (partially absorbed, some energy loss)
Pure scatter (no loss of energy)

Occur at atomic level

Question 2

What are the shells of an atom, from innermost to outermost?

Answer 2

K L M N O

Question 3

How is the density of any substance described?

Answer 3

By its atomic number

Question 4

What is the atomic number?

Answer 4

It is the number of protons in the nucleus - the greater the density, the bigger the atom

Question 5

What is the photo electric affect?

Answer 5

Absorption

Question 6

Outline Stage 1 of the photoelectric effect (4 points)

Answer 6

1. X-ray photon (high energy) collides with inner shell electron in patient’s atom
2. Electron that has been hit by photon is ejected from atom (this makes atom unstable)
3. The photon disappears (absorbed) as all the photon’s energy has been used in the collision
4. Atom is left with a vacancy from ejected electron

Question 7

Outline Stage 2 of the photoelectric effect (4 points)

Answer 7

1. Electron from the next outer shell drops in to fill vacancy on the inner shell. As it drops from higher energy level, to lower energy level, energy is omitted.
2. This stage repeats with outermost shell in order to fill the vacancy within the atom as the electrons rearrange.
3. A free electron is captured to stabilise the atom.
4. Energy is given off in the form of heat or light

Question 8

What is the probability of photoelectric interactions proportional to?

Answer 8

Z cubed

Z being the atomic number

Question 9

What is the atomic number (Z) for soft tissue?

Answer 9

7

Question 10

What is the probability of photoelectric interactions for soft tissue?

Answer 10

Z cubed
Z for soft tissue is 7
7 cubed is 343

Question 11

What is the Z value for bone?

Answer 11

12

Question 12

What is the probability of photoelectric interactions for bone?

Answer 12

12 cubed = 1728

Question 13

Why is there more absorption in bone than soft tissues when x-raying our patients, if they have similar atomic numbers?

Answer 13

Because it is proportional to the Z number CUBED

So the probability of photoelectric interactions in bone is much higher than soft tissues

Question 14

Why does an x-ray image appear black, white and grey?

Answer 14

Because of varying levels of absorption of the x-ray photons in the body

Question 15

What element is a good absorber of x-ray photons? What is it’s Z number?

Answer 15

Lead
It’s Z number is 82

This is why it is used for x-ray protection

Question 16

What is the probability of photoelectric interactions inversely proportional to?

Answer 16

Photon energy cubed (keV)

Question 17

Does low kV energy equipment mean more or less absorption of low energy photons?

Answer 17

More absorption of low energy photons

This sis because the lower the energy of the photons, the more they are absorbed

Question 18

What is the Compton effect?

Answer 18

Absorption and scatter

Question 19

Outline the steps involved in the Compton effect (6)

Answer 19

1. Incoming photon interacts with outer shell electron
2. Photon collides with electron, and it is ejected from atom
3. Some energy absorbed from photon during collision
4. Photon hasn’t lost all of its energy, so it is deflected and scattered
5. Unstable atom with vacancy in outer orbit created
6. Free electron captured and atom is stabilized

Question 20

What has happened to the electrons in both the photelectric effect and the compton effect?

Answer 20

An electron has been ejected

Question 21

What is the difference in the photon collision in the compton effect and the photoelectric effect?

Answer 21

The photoelectric effect involves an electron from an inner shell, whereas, the compton effect involves an electron from the outer shell

Question 22

What is the name for the type of radiation that is high-energy enough to remove an electron from an atom?

Answer 22

Ionising

Question 23

What charge does an atom have if an electron has been ejected from it?

Answer 23

It has a positive charge - it is a positive ion.

The electron that has been ejected is the negative ion

Question 24

What is the difference between non-ionising radiation and ionising radiation?

Answer 24

Ionising radiation has enough energy to disrupt the atomic structure of human tissue, whereas, non-ionising radiation does not

Question 25

Describe how tissues may be damaged by x-rays via direct damage

Answer 25

The DNA or RNA in the chromosome takes a direct hit from an x-ray photon which ionises and breaks the bonds between the nucleic acids = point mutation

Question 26

Following DNA damage (e.g. by x-ray photons), what may the subsequent chromosomal effects include? (4)

Answer 26

1. Cell Death > if enough damage is done initially, these cells can be killed outright 2. Abnormal replication > (if nucleic acid bonds are broken, then rejoined together in an abnormal fashion, then these cells replicate, the abnormality will be replicated over and over again) 3. Failure of transference of information 4. Only temporary damage - the DNA being repaired successfully before further cell division

Question 27

If somatic cells are affected by direct damage from x-ray photons, what may be the consequence?

Answer 27

Radiation may induce malignancy

Question 28

If genetic cells are affected by direct damage from x-ray photons, what may be the consequence?

Answer 28

May result in congenital abnormality

Question 29

What factors may affect what actually happens to a cell when irradiated by x-rays? (5)

Answer 29

- The type and number of nucleic acid bonds that are broken - The intensity and type of radiation - The time between exposures - The ability of the cell to repair after damage - The stage of the cell reproductive cycle when irradiated

Question 30

Outline how x-ray radiation may cause damage indirectly

Answer 30

The x-ray photon ionises the water molecule - this produces ions and free radicals which can combine to form toxic substances which damage the DNA

Question 31

What 3 further interactions may happen once water has been ionised from indirect damage?

Answer 31

High energy photon hits water molecule which produces a positive ion (H2O+) and a negative ion (e-) - these ions are more reactive. 1. H2O+ > H+ + OH 2. H2O + e- > H2O - 3. H2O- > H + OH- H & OH are free radicals which combine to form toxic substances like hydrogen peroxide or hydroperoxyl radical > which then damage tissues

Question 32

In dentistry, which type of effects are we most concerned with?

Answer 32

Somatic stochastic effects (somatic = involves body, stochastic = chance, governed by laws of probability....but this does include cancer induction) Every exposure carries possibility of causing damage and no known threshold dose