what happens when X rays hit you or your patient?

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recap - what are X rays?

• X rays are a type of electromagnetic radiation 

• they are an ionising radiation source 

• they are at the higher end of the EM spec 

interaction of X rays with matter

there are two types of interactions that can occur at the atomic level - what are they?

1. photoelectric effect 

2. Compton scatter 

photoelectric effect

• this is the effect that gives us contrast in our final images 

• the electrons that are furthest from the nucleus have the weakest attraction 

• the photoelectric effect is what happens when an X ray interacts with in inner shell electron 

• X ray will come into the atom, interact with an inner shell electron 

• because the electron is highly attracted to the nucleus, it requires all of the Xray’s energy to eject that electron from the atom 

• the X ray becomes fully absorbed - and completely removed from the X ray beam 

• contributes to contrast on the final image → the more Xrays that are removed from the beam - the less X rays are going to hit the detector, the more RADIOPAQUE / WHITER the image will look

• the more X rays hit the detector - the darker, or more radiolucent the image will be  

• ion is formed

atoms with a higher atomic number …

• more shells of electrons

• more photoelectric interactions will be taking place 

• more X rays will become absorbed 

• less X rays hit the detector 

• the whiter / more radiopaque the image will become 

photoelectric effect - consequence

• greater atomic number - more photoelectric interactions - more radiopaque 

• contributes to contrast 

• because the electron is ejected from the atom - it has a net positive charge - IONISING 

• BUT contributes to dose 

what is Compton scatter ?

• scatter occurs when an X ray interacts with an outer shell electron 

• because outer shell electrons are further from the nucleus 

• they have less strong attraction to the nucleus 

• therefore less energy is required to eject one of these outer shell electrons from the atom 

• only part of the Xrays energy is used to eject the electron 

• electron is still ejected - ionising  STILL 

• part of the Xray’s energy was required to eject the atom - X ray is redirected with less energy in a different direction - scatter 

Does Compton scatter still contribute to dose?

yes - because the atom still becomes ionised

what is an issue with Compton scatter?

• because the X ray is diverted after knocking an electron off - it hits the X ray detector at a different location than the correct anatomical point - reduces image quality and reduces contrast 

• contributes to dose but doesn’t contribute to image quality 

we would prefer to have more photoelectric interactions instead

what are the two main ways X rays affect molecules? (DNA molecules)

if atoms become ionised - the molecules that they are a part of can also become ionised 

1. direct effect 

2. indirect effect 

direct effect of X rays on DNA molecules 

• direct effect of X rays on DNA molecules 

• minority of DNA damage 

Indirect effect of X rays on DNA molecules

• interactions between X rays and water molecules 

• water molecules become ionised - break down , they produce free radicals 

• free radicals go on to damage the DNA 

• majority of DNA DAMAGE

Interaction of X rays with cells - if DNA within a cell becomes damaged - what are the two potential outcomes?

1. cell death 

2. cell mutation 

interactions of X rays with tissues

this depends on whether the X ray causes mutations or cell death 

deterministic effects

dependent on cell death

Stochastic effects (chance)effects

dependent on cell mutations

what is a deterministic effect?

• ‘radiation burns’

• effects that will definitely occur once a threshold dose is passed

• (skin reddening, hair loss, cataracts)

• caused by cell death

• higher above the threshold dose that you go, the more severe the effects become 

• before the threshold dose - it is impossible for these effects to occur 

• in diagnostic imaging - the doses are so low, that it is impossible for these effects to occur 

when do deterministic effects actually occur?

• radiotherapy - when they have a cancer - inducing cell death in cancer cells

• there will be collateral damage to healthy cells 

• erythema to skin, hair loss, death of salivary gland cells - dry mouth 

what are stochastic/chance effects?

• may be induced by any dose of radiation - depends on the laws of probability

• there is no safe dose

• radiation induced cancers → if body cannot apoptose the mutated cell → malignancy may form

• damage to cells based on mutations 

• severity of the outcome is not dose dependent

• probability of outcome is dose dependant

deterministic vs stochastic effects

note axis

dental imaging is far below threshold dose

3rd type of effect - stochastic genetic effects - these only occur if the reproductive organs are affected