6. How do you limit the dose to patients and yourself? Flashcards
LOs
- Explain the dose limitation principles of justification, optimisation, and limitation
- Give details of the radiation limits for patients and dentists and their staff
- Describe the practical radiation protection measures used for patients, including those women who may be pregnant
- Describe the practical radiation protection measures required for dentists and their staff within the dental surgery
Classification of ionising radiation effects
Aims of radiation protection
- Prevent DETERMINISTIC (certainty) effects
+ guaranteed to occur
+ have threshold dosage
+ have rules & have guidelines based on scientific evidence to prevent magnitude exceeding threshold dosage - Limit probability of stochastic effects to acceptable levels
+ determine level of risk
+ physical methods of dose reduction
what is an acceptable level of risk
- CANNOT be quantified
- different for everyone
- for x-rays patients do not choose to be exposed to ionising radiation
- dentist and Dr has to decide what is acceptable hence needs to know benefits and magnitude of risk
how are radiation dose limits decided?
- X-rays = dangerous hence governed by law to ensure that the risk to various individuals is as small as possible
- based on recommendations by the International Commission on Radiological Protection (ICRP)
(gather research)
ICRP 3 principles on dose limitation
- JUSTIFICATION
- No practice shall be adopted unless there is a net positive benefit - OPTIMISATION
- All exposures shall be kept as low as reasonably practicable - LIMITATION
- The equivalent dose to individuals does not exceed the limit set by the commission
Importance of X-ray examinations directly associated with illness
/
Number + type of radiographs taken is responsibility of clinician
Based on:
No set dose limits
Number + type of radiographs taken is responsibility of clinician
Based on:
- correct assessment of indications
- expected yields ( How much info is obtained )
- how the results will influence diagnosis and treatment
- clinician should have adequate knowledge of physical properties + biology effects of ionising radiation
How is the dose to patients limited ?
Equipment = X-ray generating and image receptors
Clinical judgement
Radiographic technique
What is the X-ray equipment requirements for dose limitation to patients ?
70 kV
Aluminium filtration
Adjustable kV , mA and time
- short exposure times
DC-constant potential
Long fsd - 200mm
Rectangular collimation
Critically examined and acceptance test
What are the image receptor requirements for dose limitation to patients ?
Fast film ( E/F speed )
Rare earth intensifying screens
Digital - solid state or phosphor plate
Benefit of having a higher kV
- Penetrating power of beam is increased
- Less photons stop in patient
- Lowers dose
- Produces more photons in continuous spectrum but increases the energy of the photons
Benefit of using aluminium filtration
Removes lowest energy photons from the beam
Harmful protons cannot pass through aluminium filter
Benefit of adjustable kV, mA and time
Able to optimise dose
Increase diagnostic yield
Ability to vary contrast and degree of film blackening
Benefit of having DC-constant potential
Ensures X-ray production is kept at a peak
Overall energy of photon bean is higher and shorter exposure times
Benefit of rectangular collimation
Reduces size of beam
Same size as film packet
Reduces area of face/body being irradiated
Benefit of a long fsd
Emerging beam is narrower
Less divergence
Reduces area of patient that is being irradiated
Current dental X-ray generating equipment recommendations
Operating range = 60-70 kV
1.5mm aluminium filter
Adjustable kV , mA and exposure time
DC - constant potential
Rectangular collimation = 40 x 50 mm
200 mm fsd
Exposure timer switch has to continuously be depressed
Benefit of using faster intra oral film
Speed E or F
Shorter exposure times
Benefit of using indirect action film + rare earth intensifying screens
Fewer photons required to produce green light which affects green sensitive emulsion to create visual image
Faster than traditional calcium tungstate screens
Benefit of using digital sensors
Solid state
DC-constant potential
Selection criteria for clinical judgement whether to use X-ray
Descriptions of clinical conditions
Derived from patient signs + symptoms
Identify patients who are likely to benefit from a particular radiographic technique
Evidence based recommendations
Radiographic techniques used for dose limitation
Use image receptor holders with beam aiming devices to produce geometrically accurate images
Take minimum number of images
Avoid retakes
Chemically process films under optimum conditions
- manual or automatic
Optimise digital images to maximise diagnostic yield
Adhere to strict quality assurance procedures
Use appropriate protective lead aprons
- thyroid shield
- thyroid has own weighting factor
- radiosensitive structure
Dose limitation requirements for pregnant patients
- Only take radiographs if clinically necessary
- Check justification
- Avoid periodic health checks
- Reassure and utilise all routine radiation safety measures
- Could offer protective lead apron
+ not essential
+ psychological benefit
Physical methods of dose limitation form members of dental team
All staff must know risks to own health created by exposure to X-rays
All female staff should know risk of pregnancy + X-ray
All staff to know safety precautions
Golden rules
Golden rules
2 metres away from X-ray machine + patient / behind a protective lead screen
Never stand in line of main beam
Always stand out of controlled area
Never hold X-ray film packet or image receptor in patients mouth
