Design of diagnostic radiology facilities Flashcards
What annual dose constraint is typically used when determining shielding requirements for diagnostic radiology facilities? Where does this come from?
- An area must be designated as controlled area under IRR17 regulations if special procedures must be followed or if the annual effective dose to a person working in that area is likely to exceed 6 mSv.
- However, 6 mSv is not considered ALARP for diagnostic radiology facilities. 0.3 mSv (=3/10 x 6 mSv) is used instead.
What other dose rate values (IDR, TADR etc.) are used as dose constraints when determining shielding requirements for diagnostic radiology facilities?
- IDR (i.e. that measured on a dose rate meter in real time) = 100 microSv/hr. Issues can be apparent with this constraint for diagnostic radiology facilities due to the short exposure times which can potentially result in a high IDR.
- TADR (time averaged over 1 minute) = 7.5 microSv/hr where employees untrained in radiation safety measures are present.
- TADR8 (time averaged dose rate over an 8 hour working day correcting for maximum workload) = 7.5 microSv/hr.
- TADR2000 (time averaged dose rate over a working year correcting for maximum workload and occupancy) = 0.15 microSv/hr.
What needs to be considered when using effective dose constraints for shielding calculations?
Effective dose is not equivalent to air kerma. It is typically less.
What is the difference between keV and kVp for diagnostic photon beam?
- keV is the mean energy which is typically 1/2 of the kVp.
- kVp is the peak energy.
What information should be gathered\considered to undertake shielding calculations?
- Where x-ray examinations are performed (e.g. floorpan including surrounding areas and their occupation).
- How equipment is to be used (e.g. table x-rays, chest radiography, exposure factors, workload etc).
- Locations of interest (e.g. high occupancy areas, areas of potentially high dose etc.).
What mechanisms of exposure contribute to patient dose in diagnostic radiology?
- Primary beam.
- Scatter.
- Leakage.
What is the scatter factor for general radiography? At what angle does maximum scatter typically occur?
- The air kerma per unit DAP at 1 m for a given angle and kVp.
- 117 degrees.
How is scatter estimated for CT installations?
- Isodose plots provided by the manufacturer. These can be scaled to the relevant size and location of the scanner within the room.
- CT scatter factor (i.e. air kerma per unit DLP).
Where can scatter data for mammography and dental installations be found?
Estimates can be found in the Sutton and Williams shielding book. They are typically expressed in microGy/exposure.
What two methods can be used to estimate tube leakage?
- Leakage to scatter ratios.
- Maximum continuous rating of tube and leakage limit (1 mGy/hr at 1 m) i.e. I = P (max continuous rating in W)/V then leakage in microGy per unit mAs can be determined by dividing the leakage limit (in microGy/s) by this value.
What are some solutions to shield exposure to primary beam?
- Systems of work to ensure no primary beam directed at the console area or doors.
- Extra shielding at certain parts of wall.
- Systems of work to ensure only external walls used where possible.
- Design features on x-ray unit to limit x-ray field size.
Once the total air kerma per unit dose indicator has been determined, what is the next step determining the total expected dose at a location of interest?
- Dose per examination is determined by using DRLs, NRPB dose surveys or other data.
- Dose per year is determined by multiplying by workload (i.e. examinations per year). This information can be obtained from the local department or dose management systems. An ‘uplift’ is typically applied here to account for any changes in practice.
- ISL is used to determine estimated dose at the location of interest.
How are desired transmission and shielding thickness determined given the expected total dose at a location of interest?
- Transmission = Annual dose constraint/Expected total dose at location of interest*occupancy.
- Archer equation used with above and shielding material factors to determine shielding requirements.
What is tertiary scatter? Why is it important to consider in room design?
- Radiation that is scattered back into the x-ray room from the room boundaries and air molecules within the room.
- Could result in increased radiation exposures in a control area in a badly designed room.
What do the Sutton and Williams equations for tertiary ceiling and wall scatter behind a console screen depend on?
- Ceiling: Ceiling height and barrier height.
- Distance from patient to wall.
