Interaction of x rays with matter Flashcards Preview

Year 3 Radiography > Interaction of x rays with matter > Flashcards

Flashcards in Interaction of x rays with matter Deck (28):

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

1. undergo further Compton interactions with tissues
2. undergo photoelectric interactions with tissues
3. 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

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
1. somatic cells
2. stem cells

1. (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

-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



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 ?

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