Occupational radiation doses and shielding
Several studies have evaluated scattered radiation from pencil-beam and fan-beam DXA systems and the occupational dose to operators [34, 53, 54]. The time-averaged dose rates at 1 m from the central axis of the imaging table range from about 0.01 µSv/h to about 5 μSv/h, depending on the DXA model. Scatter radiation from fan-beam systems is higher than that from pencil-beam systems. The annual occupational dose at 1 m from a fan-beam system can reach 1.5 mSv [34]. With regard to occupational doses, the ICRP recommends a limit on the effective dose of 20 mSv/year, averaged over 5 years, with a further provision that the effective dose should not exceed 50 mSv in any single year. Although the dose limit of 20 mSv/year is much higher than the expected annual occupational doses from DXA, the use of a protective lead shield may be necessary occasionally for fan-beam systems in a confined space to ensure that the operator dose is as low as possible [34].
Special dose limits apply for the conceptus after a pregnant employee declares pregnancy. The ICRP and European Commission recommend that the individual developing in utero should be protected by the application of a dose limit of 1 mGy during the remainder of pregnancy, once it has been declared [55, 56]. The exclusion of pregnant workers from DXA examinations on the basis of radiogenic risks from occupational DXA exposure cannot be justified on scientific grounds. However, the scatter radiation can exceed the exposure limits for pregnant workers, especially for fan-beam systems. For this reason, radiation protection measures should always be taken to ensure that the conceptus dose will be kept below 1 mGy during the declared pregnancy. Moreover, the use of a personal radiation meter at waist level is recommended to monitor radiation exposure.
An important requirement in limiting the risk of radiation exposure in the workplace is the correct design of the room in which the imaging device has been installed. Measurements performed by Larkin et al. [34] showed that the scatter from fan-beam DXA systems can exceed the limits for public exposure i.e. 1 mSv/year [19]. In these cases, additional structural shielding might be required, especially when the distance from the imaging table to the adjacent wall is less than 1 m. However, other parameters should also be taken into account such as the workload, the material of the walls, the location of the operator and the location and use of rooms that adjoin the imaging room.