The Design of Diagnostic Medical Facilities where ... - ResearchGate

Traditionally, an RPA may specify the lead requirements for an installation, rounded up to the nearest 0.5
mm. In practice, as lead is not available in increments of 0.5 mm, it is preferable to specify the code required,
to avoid errors arising from inappropriate rounding up or down later. Lead sheet is manufactured to BS EN
12588 standard and has a ± 5% tolerance on the thickness (BIR, 2000). Lead products compliant with the
above standard should be utilised.
6.1.2 Concrete and concrete blocks
High-density solid concrete (2350 kg/m 3 ) is a commonly used shielding material. It may take the form of
poured concrete or solid concrete blocks. Its transmission properties at a density of 2350 kg/m 3 are given in
Appendix C. The lead equivalence of concrete and other shielding materials is also given in Appendix C for
a range of kVp (BIR, 2000). Solid concrete blocks may have internal voids, so care needs to be taken in their
use. Lightweight concrete blocks are also available (2200 kg/m 3 ) and if used the thickness of concrete will
have to be scaled accordingly. Likewise using concrete of this density, with the transmission curves in
Appendix C, will require an appropriate adjustment in the shielding thickness read from the curves. Concrete
blocks may be used in combination with other materials such as barium plaster. In practice, approximately
69 mm of high density solid concrete is equivalent to 1 mm lead at 70 kVp; similarly at 70-125 kVp 150 mm
concrete is approximately equal to 2 mm lead (WHO, 1975).
Hollow concrete blocks are not generally suitable for shielding except with low energy or low dose applications
(mammography, dental radiology, DXA scanning) or when used in combination with other shielding materials.
They should be used with caution, as attenuation coefficients can be difficult to obtain.
6.1.3 Barium plaster
Barium plaster is a gypsum plaster incorporating barytes aggregates, which may be used in combination with
concrete blocks. Its absorption properties with respect to X‐rays are greatly enhanced by the presence of a K
absorption edge in barium. A thickness of up to 25 mm can be applied to a surface (BIR, 2000). Applying
barium plaster so that it will be well finished to the thickness required, and crack free, requires a plasterer
with the appropriate skill. It must be applied in several coats and takes a considerable period to dry.
The composition and the attenuation properties of barium plaster manufactured by British Gypsum (Thistle
X‐ray) have been altered since the publication of BIR (2000). The net effect has been to reduce its lead
equivalence by between 15% and 30%. Revised values have been published by Williams (2005).
There are no published coefficients for secondary transmission through barium plaster. Coefficients for
primary transmission may be used for energies below 100 kVp (BIR 2000). However above this, the attenuation
properties are unknown. Barium plaster is therefore unsatisfactory for shielding high kVp facilities such as CT
and high kVp chest units, and alternatives should be considered until more complete data are available.
The Design of Diagnostic Medical Facilities where Ionising Radiation is used 75