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

5.2.4 Primary radiation
A barrier is required to attenuate the primary beam to a level that complies with the dose constraint. Primary
barriers are typically required in general radiographic rooms, dedicated chest rooms and rooms where there
is a combination of radiography and fluoroscopy. For mammography, fluoroscopy, CT and DXA – the entire
primary beam is normally incident on the face of the image detector which acts as a primary beam stop.
Therefore for these types of rooms only secondary radiation needs to be considered.
The use factor (U) is the fraction of the primary beam workload that is directed toward a given barrier (NCRP,
2004). It is used in the NCRP methodology for primary radiation shielding calculations and the value will
depend on the type of installation and the barrier being assessed. Different barriers may have different use
factors. For example, the workload presented for the radiographic room (Chest Bucky) in Table 5.4 is directed
entirely at the wall behind the Bucky, and the use factor, U = 1 for this wall. In addition, this workload only
contributes secondary radiation to the other barriers in the room. However, it should be noted that a more
complex situation can arise with other configurations.
Primary beam use factors published by the NCRP are reproduced in Table 5.6. The ceiling and operator’s
control screen are generally considered to have U = 0. U is also zero for installations where only secondary
radiation needs to be considered such as image intensifier and mammography systems.
Table 5.6: Use factors from NCRP for radiographic room (based on NCRP, 2004)
Barrier Use factor (U) Workload distribution
Floor 0.89 Rad Room (floor or other barriers)
Cross-table wall 0.09 Rad Room (floor or other barriers)
Unspecified wall (other than chest
bucky wall or cross-table wall)
0.02 Rad Room (floor or other barriers)
Chest Bucky wall 1.00 Rad Room (Chest Bucky)
To calculate the shielding required in the primary barrier, the barrier transmission factor must be determined.
This is the ratio of the air kerma behind a barrier of thickness, x, to the air kerma at the same location with
no barrier. In the following sections, an overview of the BIR methodology and equations are presented.
Detailed equations for employing the NCRP methodology are available (NCRP, 2004). A comparison of both
methods is presented in Example 1 in Section 5.3.
BIR methodology for primary radiation
The BIR recommends that one of two methods is used to calculate primary shielding requirements, and the
method used will depend on the clinical situation. The film dose method is used where the X-ray beam is
entirely intercepted by the patient and the entrance surface dose method is used where this is not the case.
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The Design of Diagnostic Medical Facilities where Ionising Radiation is used