radiography, modules 2-5


what is an x-ray, electromagnetic radiation

photons are a type of elctromagnetic radiation. low energy photons make up radiowaves, medium energy photons are visible to use as light.

gamma ray photons and x-ray photons have high energy phtons and are classified as ionising radiation.

gamma rays and x-rays are the same but gamma is naturally occuring whereas x-rays are man made.

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what is an x-ray, interaction

tramsmission - pass straight through matter without touching anything

absorption- the hit matter and all their energy us absorbed

absorption and scatter -hit the matter and are deflected in another direction with no loss of energy.

pure scatter- they are deflected in another direction with no loss of energy

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what is an x-ray, main properties

  • an x-ray is a packet of energy called a photon
  • x-ray beam contains millions of protons
  • they travle in staright lines 
  • they can travel in a vacuum
  • x-rya have a short wavelength and high energy
  • they originate at the atomic level
  • they interact with matter at the atomic level
  • they can blacken film emulsion to create an image
  • they cannot be detected by human senses
  • they can damge human tissue
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what is an x-ray, dangers of ionising radiation

tissue reaction - somatic, deterministic or centainty effects.

certainty effects - resukt from a high dose of radiation. normal body cells are affected and severity of the damage is proportional to the radiation dose. dental radiogrpahy doesn't use radiation this high.

stochastic effects

somatic - chance effects which might result from any dose of radiation. ormal bosy cells affected but severity has no relation to dose. increasinf radiation dose increases chance of damage occuring.

genetic- chamce effects happen to the reproductive cells. if damge to this may affect off spring. can occur at any exposure . unikely in dental radiography

main aim of radiation protection to completely provent the determiistic effects and limit chance of stochastic effects

there is no safe dose of radiation 

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how are x-rays produced, equipment variables

kV (kilovoltage) controls energy of photons and penetrating power of the beam  (quality) - increasing kv will decrease contrast between light and dark parts, "kv kills contrast"

mA (milliamperage) and time control the quantity of x-ray photons produced. increasing mA and time will increase the degree of blackening.

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how are x-rays produced, equipment recommendations

  • operating range 60-70kV
  • 1.5mm thick aluminium filter for less than 70kV and 2.5 mm for above 70 kV
  • DC constant potential output (potential = voltage)
  • may need to select 60kV as opposed to 70 kV to ensure adequate contrast
  • rectangukar collimation
  • maximum diameter of 60mm for circular collinators
  • 200mm fsd or long spacer cone. 300mm in modern equipment 
  • audible and visual warning signals. timer switches must be continually depressed through exposure
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the effects of xrays on human tissue, absorption

the photo-electric effect

x-ray photon collides with an inner shell electron in patients tissue. photons energy absorbed in collision and structure of atom is de-stabilised. as it restabilises it gives off energy in the form of heat and light.

chance of photon hitting inner shell elctron depends on the atoms atomic number cubed. higher the number the more chance photon will be completely absorbed 

commpton effect

x-ray photon collides with an outer shell elctron in patients tissue. x-ray photon loses some of its energy and is deflected. structure of patinets atom is de-stabilised by collision.

both effects result in an unstable atom _positive atom) and a stray electron (negative ion) this process is known as ionisation .

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the effects of xrays on human tissue, how they cau

direct damage 

occurs when x-ray photons hits and ionises thre genetic code in patients cells. (code in DNA or RNA molecules). damage may be severe enough to kill the cell but more likely to result in subtle alteration to the code. te disruption may result in malignancy (somatic cells) or congenital abnormality (reproductive cells).

indirect damage 

x-ray photon hits and ionises other molecules in patinet cells (most commonly water molecules) the resultant ions are highly reactive and damage other parts of the cell. if x-ray ionises a water molecule then hygdrogen peroxide and hygdrogen gas are produced (both toxic).

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the effects of xrays on human tissue

high doses of radiation can cause severe damage or death 

dose in dentistry not high enough to cause deterministic effects so mainly concerned about somatic stochastic effects

cancer is a somatic stochastic effect

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radiation dose and risk, equivalent dose

radiation absorbed dose - total amount of radiation absorbed by the patient. unit is gray (Gy)

equivalent dose 

different types of radiation can cause different levels of damage to a patients cells. equivalent dose takes this into account by giving each type of radiation a weighting factor (WR) according to the amount of damage it does. WR for x-rays is 1. 

unit for equivalent dose is sievert (Sv)

equivalent dose = radiation absorbed dose x radiation weighting factor 

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radiation dose and risk, effective dose

effective dose 

some tissues are more susceptible to radiation damage than others. effective dose takes this into account by giving each tissue a weighting factor (WT) according to its sensitivity to radiation damage. the unit is also the sievert (Sv)

effective dose = equivalet dose x tissue weighting factor

most sensitive to damage  - gonad tissue, red bone marrow and lung tissue. the nearest sensitive affected in dental radiography is the thyroid gland.

effectve dose is the most useful dose 

mSv- milliSievert (one thousandth of a sievert)

uSv - microSievert (one millionth of a sievert)

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radiation dose and risk, effective dose 2

modern equipment lowers dose

  • chest CT scan 6.6 mSv
  • Barium meal 1.5mSv
  • chest x-ray 0.038 mSv
  • intra-oral dental 0.0003mSv

reference dose- maximum absorbed dose allowed for a medical and dental examination. if absorbed dose exceeds this then steps to be taken to reduce it.

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radiation dose and risk, dose limits

there is no set dose limit for patients who have a clear clincial need

yearly limit for dental nurses - 6mSv 

yearly limit for a member of the public, carer or comforter is 1mSv

recomended annual dose for dental staff is 0.3 mSv. exposure can be measured using a personal monitoring device 

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radiation dose and risk, background

modern panorramic x-ray gives same dose as 2 days of natual background radiation 

the younger the patient the greater the risk of fatal cancer arising 

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