Interaction of x rays with matter Flashcards
name given to a) pure absorption
b) inelastic scattering
a) pure absorption: photoelectric effect
b) inelastic scattering: Compton effect
describe the photoelectric effect
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.
what happens to excess energy from inner shell electron?
lost as heat or light
how is atomic stability achieved?
capture of a free electron
what 2 things is the probability of a photoelectric event proportional to? explain each
-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
explain why bone but not soft tissue shows up clearly on an x ray
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
why is lead used when taking x rays?
it has very high atomic number –> absorbs some electrons before they reach tissue –> maximises absorption –> very bright image on receptor
what is an intensifying screen and its function?
screens found in extra-oral cassettes. they use the photoelectric effect, excess energy emitted as light which affects fine emulsion and produces image
describe the Compton effect
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
which of these effects affects pt dose and why?
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
name another difference between photoelectric effect and Compton effect
Compton effect is more likely with high energy photons (pe effect more likely with low energy)
3 options of what the scattered photon may do
- undergo further Compton interactions with tissues
- undergo photoelectric interactions with tissues
- escape the tissues (scattered radiation)
overall result of Compton effect interaction
tissue ionisation
which effect produces better diagnostic image and why
Photoelectric effect.
Compton effect does not have good discrimination between tissues
name 2 methods by which x rays cause damage and which is more common and why
direct
indirect
indirect is more common because we’re 70% water
explain the direct method by which x rays cause damage and 4 outcomes
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)
outcomes of direct damage to
- somatic cells
- stem cells
- (somatic cells: radiation induced) malignancy
2. stem cells: congenital abnormality
explain indirect damage
ionisation of intracellular O2 –> free radicals –> recombination forms H2O2 –> breaks down proteins eg DNA, RNA, proteins
host factors influencing radiation effects 6
- species
- intrinsic resistance
- type/ sensitivity of tissue
- rate of sensitivity of tissue
- rate of cell division
- phase of cell cycle
radiation factors influencing effect of radiation
- total dose
- acute v chronic
- local area v whole body
2 main categories of biologically damaging effects of radiation
- tissue reaction (deterministic effect): early and late tissue reactions after radiation
- stochastic effects (chance): heritable (genetic) effects, cancer induction
in which of these categories is osteoradionecrosis
late tissue reaction
is cancer induction (stochastic effect) more likely in younger or older patients? explain
younger. more lifetime ahead so more chance of tumours forming
5 effects of radiation in oral cavity
mucositis (atrophic mucosa) loss of taste dry mouth radiation caries tooth defects if irradiated while teeth are developing
in what site is caries likely when caused by radiation
cervical
what is the threshold dose of radiation
there is none
how does dose size affect severity of damage induced?
it doesn’t; it only effects probability of damage occurring
which biological effect of radiation is more important in diagnostic medicine ?
stochastic
size of dose only affects probability of damage and there is no threshold dose
