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“ In relation to any particular source within a practice, the magnitude of individual doses , the number of people exposed, and the likelihood of incurring exposures where these are not certain to be received should be kept as low as reasonably achievable , economic and social factors being taken into account . This procedure should be constrained by restrictions on the dose to individuals ( dose constraints ), or the risks to individuals in the case of potential exposures (risk constraints), so as to limit the inequity likely to result from the inherent economic and social judgements.”
ALARA as low as reasonably achievable . ALARP as low as reasonably practicable
“ The exposure of individuals resulting from the combination of all the relevant practices should be subject to dose limits, or to some control of risk in the case of potential exposure. These are aimed at ensuring that no individual is exposed to radiation risks that are judged to be unacceptable from these practices in any normal circumstances. Not all sources are susceptible of control by action at the source and it is necessary to specify the sources to be included as relevant before selecting a dose limit.”
Prevent deterministic effects
Limit risk of stochastic effects to acceptable level .
Beware - can vary between different sets of same manufacturer and model!
more filtration, higher kV, less pulses/second
less filtration, low kV, more pulses/second .
Automatic brightness control Brightness controlled mainly by adjustment of kVp Brightness controlled by simultaneous adjustment of kVp and mA The tube current is maximised at 3 mA Tube heating kV x mA (99% energy goes to heat, typically ¼ kW over few mm 2 ) X-ray intensity kV 2 x mA Penetrating power with kV Contrast with kV
Medical and Dental Guidance Notes A good practice guide on all aspects of ionising radiation protection in the clinical environment
240 pages, £20 (discount for bulk purchase!)
Buy from http://www.ipem.org.uk/publications/pubs-list2.htm#protection
View at http://www.ipem.org.uk/publications/ IRR99.html
“ an essential reference book for all those working with ionising radiation in medical or dental practice, including medical and dental staff, radiographers, scientific and technical staff, and their employers .”
Absorbed dose weighted for relative radio-sensitivity of different organs
Units are Sieverts (Sv)
Tissue or organ w T Gonads 0.20 Red bone marrow 0.12 Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesphagus 0.05 Thyroid 0.05 Skin 0.01 Bone surfaces 0.01 Remainder 0.05 e.g. if gonads alone received 2 Gy to tissue, E = 0.20 x 2 = 0.4 Sv .
Risk of inducing cancer or hereditary disease is proportional to Effective Dose