Radiation Shielding DesignThe purpose of radiation shielding is to limit radiation exposure to employees and members of the public outside the x-ray room to an acceptable level, as determined by legislation.
Radiation shielding design can be calculated for both existing buildings and off the plan. The shielding requirements are calculated using NCRP Report No. 147 “Structural Shielding for Medical Imaging Facilities”, 2004 and present the minimum shielding required in terms of mm thickness of lead. All shielding recommendations can be confirmed post installation by inspection.
In the preparation of radiation shielding needs for an x-ray facility a number of factors need to be taken into consideration, namely:
- The direction and energy of the x-ray beams
- The amount of radiation involved, i.e. use factor
- The distance between the radiation source and the operator as well as the distance between the radiation source and members of the public.
- Composition of existing walls or other barriers.
- Maximum permissible exposure limits as determined by legislation. This is currently 20mSv/year for occupationally exposed persons and 1mSv/year for members of the public. Where both members of the public and operators (staff) are likely to be exposed, it is the lower of these two values that will be used in calculations.
The ALARA principle
As well as the above factors used in shielding calculations, the ALARA principle is also applied. The ALARA (As Low As Reasonably Achievable) principle, as recognised by legislative bodies, states that in all situations every effort must be made to ensure that radiation exposure to people is kept as low as reasonably achievable, economic and social factors being taken into account.In order to calculate shielding requirements, RadTest Australia will require a floor plan showing:
- X-ray room(s) with location of x-ray table, vertical bucky and control panel Dimensions of room(s)
- Constitution of surrounding walls – e.g. stud/plaster, brick, etc
- Areas adjacent to x-ray room(s) – e.g. toilet, office, passage, etc
- For external walls, distance to next building (if applicable)
- If multi-storey, constitution of ceiling and floor
- The maximum kVp of x-ray unit and (estimate of) average mAs per exposure and an estimate of average number of patients per week and average number of exposures per patient