Interaction of xrays

Question 1

Which interactions of X-rays occur at the atomic level?

Answer 1

Photoelectric effect (pure absorption)
Compton effect (inelastic scattering)
Raleigh scattering (elastic scattering)
Pair production

Question 2

What happens in the photoelectric effect?

Answer 2

1. Incoming x-ray photon interacts with a bound INNER shell electron of the tissue atom
2. the INNER shell electron is ejected with considerable energy (now called a photoelectron) into the tissues which undergo further interactions
3. Vacancy now exists in INNER shell = filled by electrons dropping from outer shells (energy loss = light/heat)
4. Captures a free electron to achieve atomic stability

n.b. occurs differently in materials with different atomic number = occurs differently in different tissues

Question 3

How does a photoelectron behave?

Answer 3

Like the original X-ray photon = many similar interactions -> ejecting electrons as it pass through tissues & ionisation interactions within tissue

Question 4

What is the probability of a photoelectric event proportional to?

Answer 4

Atomic number ^3 (z^3)

1/ energy of the X-ray beam ^3 (1/keV^3)

Question 5

How does density of tissue alter the probability of photoelectric effect occurring?

Answer 5

Denser material = greater probability photoelectric effect will occur

= denser material absorbs more photoelectrons = less hits the image receptor = paler patch on film

Question 6

How does kilovoltage of x-rays alter the probability of the photoelectric effect occurring?

Answer 6

Lower kilovoltage = greater probability photoelectric effect will occur

Question 7

How do intensifying screens work?

Answer 7

When exposed to X-rays the screens emit their excess energy as light -> affects film emulsion & produces the image (e.g. DPT)

Question 8

What is the compton effect?

Answer 8

X-ray absorption and scattering

1. Incoming X-ray photon interacts with free/loosely bound OUTER shell electron
2. OUTER shell electron is ejected with some of the energy taken from the photon = recoil electron
3. Remaining energy from photon is scattered (can travel in any direction)

n.b. NOT dependent on atomic number (occurs in all tissues equally) as X-ray photon does not distinguish between one electron and another

Question 9

What does a recoil electron do?

Answer 9

• Always travels in a forward direction
• Has extra energy (behaves like photon & propelled forwards)
• No effect on diagnostic quality of image (increases dose and degrades image) = not good discrimination between tissues on the final radiograph

Question 10

What happens to the proportion of compton interactions with high energy photons?

Answer 10

Proportion of compton interactions is higher with high energy photons

Question 11

What do scattered x-ray photos go on to do?

Answer 11

• Undergo further compton interactions with the tissues & also photoelectric interactions
• Escapes the tissues (scattered radiation)

Question 12

How do x-rays cause damage?

Answer 12

Much of this is theoretical Studies on mice/those exposed to high radiation)
At the moment we dont know any safe dose of radiation
= causes damage to macromolecules (DNA, RNA & proteins)

Question 13

What is direct damage?

Answer 13

Where an x-ray photon / ejected high energy electron breaks nucleic acid bonds = defective chromosomes

Question 14

What is the consequence of defective chromosomes?

Answer 14

Fail to pass on information
Replicate abnormally
Cause cell death
Cause temporary damage

Question 15

What happens when a somatic cell is damaged directly?

Answer 15

Radiation induced malignancy

Question 16

What happens when a stem cell is damaged directly?

Answer 16

congenital abnormality

Question 17

Why is there less of a risk with dental x-rays?

Answer 17

Minimal exposure for dental x-rays
Not aimed at reproductive

BUT if there is not serious problem (i.e. pain/symptoms) with dentition we would rather not risk it and wait until next appointment

Question 18

What is indirect damage?

Answer 18

Ionisation of intracellular water = OH- & H+ = recombine = H+, water & hydrogen peroxide

Question 19

How does hydrogen peroxide damage the cell?

Answer 19

Breaks down the proteins (e.g. DNA)

Question 20

Which factors influence radiation effects?

Answer 20

Host factors
Species of animals
Intrinsic resistance
Type & sensitivity of tissue
Rate of cell division (faster in young)
Phase of cell cycle

Question 21

What are the two biological effects of radiation?

Answer 21

Tissue reactions (deterministic/non stochastic) effects
Stochastic (chance) effects

Question 22

Give examples of early non-stochastic effects?

Answer 22

Reddening of skin
Ulceration
Cataracts

Question 23

Give examples of late non-stochastic effects?

Answer 23

Osteroradionecrosis

Question 24

Give examples of stochastic effects?

Answer 24

Cancer induction & heritable effects

Question 25

List some radiation effects on the oral cavity (with therapeutic doses of radiation:

Answer 25

Mucositis
Loss in taste
Dry mouth
Radiation caries (by gingival margin)
Tooth defects if irradiated while teeth are developing
Atrophic mucosa

Question 26

What is the key thing to remember about stochastic effects?

Answer 26

Size of exposure has no effect on severity of the damage induced, only the probability of it occurring

Question 27

Which type of effects are important in diagnostic medicine?

Answer 27

STOCHASTIC

Question 28

Which heritable effects can occur?

Answer 28

Damage DNA in sperm / egg cells
May cause congenital abnormality -> reduction in IQ of foetus
No threshold dose
Risk estimates based on animal experiments
Dose of 0.5-1Sv double rate of spontaneous mutation

Question 29

When is the most risky time for a foetus to be exposed?

Answer 29

During organogenesis

6-7 weeks