Interaction of x rays with matter

Question 1

name given to a) pure absorption

b) inelastic scattering

Answer 1

a) pure absorption: photoelectric effect

b) inelastic scattering: Compton effect

Question 2

describe the photoelectric effect

Answer 2

high energy photon hits K shell electron
–> all energy given to K shell electron
–> K shell electron ejected (ionisation), becoming photoelectron
–> photoelectron goes on to have further interactions, acting as a photon
vacancy in K shell is filled by electrons dropping in from outer shells.

Question 3

what happens to excess energy from inner shell electron?

Answer 3

lost as heat or light

Question 4

how is atomic stability achieved?

Answer 4

capture of a free electron

Question 5

what 2 things is the probability of a photoelectric event proportional to? explain each

Answer 5

-Z^3 (atomic number cubed): denser material with higher atomic number increases chance of photoelectric effect because increased chance of photon hitting inner shell electron

1/keV^3 (1/ energy of x ray beam cubed): the lower the energy of the x ray, the higher the chance of the photoelectric effect

Question 6

explain why bone but not soft tissue shows up clearly on an x ray

Answer 6

bone has higher atomic number (is denser) so increased chance of photoelectric effect (Z^3) –> photons are FULLY ABSORBED so none reach radiographic film receptor

Question 7

why is lead used when taking x rays?

Answer 7

it has very high atomic number –> absorbs some electrons before they reach tissue –> maximises absorption –> very bright image on receptor

Question 8

what is an intensifying screen and its function?

Answer 8

screens found in extra-oral cassettes. they use the photoelectric effect, excess energy emitted as light which affects fine emulsion and produces image

Question 9

describe the Compton effect

Answer 9

high energy photons interact with free or loosely bound outer shell electrons

• -> outer shell energy gains energy from photon and is ejected (ionisation, absorption)= RECOIL ELECTRON
• -> remaining photon energy is scattered= SCATTERED X RAY PHOTON

Question 10

which of these effects affects pt dose and why?

Answer 10

Photoelectric effect: photon is fully absorbed, and affects hard tissues more than soft (depends on Z number)
Compton effect does not depend on atomic number; it affects all tissues eqally

Question 11

name another difference between photoelectric effect and Compton effect

Answer 11

Compton effect is more likely with high energy photons (pe effect more likely with low energy)

Question 12

3 options of what the scattered photon may do

Answer 12

1. undergo further Compton interactions with tissues
2. undergo photoelectric interactions with tissues
3. escape the tissues (scattered radiation)

Question 13

overall result of Compton effect interaction

Answer 13

tissue ionisation

Question 14

which effect produces better diagnostic image and why

Answer 14

Photoelectric effect.

Compton effect does not have good discrimination between tissues

Question 15

name 2 methods by which x rays cause damage and which is more common and why

Answer 15

direct
indirect
indirect is more common because we’re 70% water

Question 16

explain the direct method by which x rays cause damage and 4 outcomes

Answer 16

x ray photon OR photoelectron –> break nucleic acid bond –> defective chromosomes which may:

• fail to pass on information
• replicate abnormally
• cause cell death
• cause temporary damage (eg radiotherapy. cells later repair themself)

Question 17

outcomes of direct damage to

1. somatic cells
2. stem cells

Answer 17

1. (somatic cells: radiation induced) malignancy

2. stem cells: congenital abnormality

Question 18

explain indirect damage

Answer 18

ionisation of intracellular O2 –> free radicals –> recombination forms H2O2 –> breaks down proteins eg DNA, RNA, proteins

Question 19

host factors influencing radiation effects 6

Answer 19

• species
• intrinsic resistance
• type/ sensitivity of tissue
• rate of sensitivity of tissue
• rate of cell division
• phase of cell cycle

Question 20

radiation factors influencing effect of radiation

Answer 20

• total dose
• acute v chronic
• local area v whole body

Question 21

2 main categories of biologically damaging effects of radiation

Answer 21

• tissue reaction (deterministic effect): early and late tissue reactions after radiation
• stochastic effects (chance): heritable (genetic) effects, cancer induction

Question 22

in which of these categories is osteoradionecrosis

Answer 22

late tissue reaction

Question 23

is cancer induction (stochastic effect) more likely in younger or older patients? explain

Answer 23

younger. more lifetime ahead so more chance of tumours forming

Question 24

5 effects of radiation in oral cavity

Answer 24

mucositis (atrophic mucosa)
loss of taste
dry mouth
radiation caries 
tooth defects if irradiated while teeth are developing

Question 25

in what site is caries likely when caused by radiation

Answer 25

cervical

Question 26

what is the threshold dose of radiation

Answer 26

there is none

Question 27

how does dose size affect severity of damage induced?

Answer 27

it doesn’t; it only effects probability of damage occurring

Question 28

which biological effect of radiation is more important in diagnostic medicine ?

Answer 28

stochastic

size of dose only affects probability of damage and there is no threshold dose