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

The primary beam will be attenuated by components that it passes through, and this should be estimated
based on the geometry of the exposure. The three clinical radiography geometries which should be considered
are:
1) Table radiography => attenuation in the cassette plus table.
2) Lateral radiography => attenuation in cassette only.
3) Vertical bucky radiography => attenuation in cassette plus vertical bucky.
Data on the transmission of X‐rays through these components has been published by the BIR. The combined
equivalence of the cassette and table assembly is given as 0.8 mm lead or 75 mm concrete (60 – 125 kVp).
The lead equivalence of a film cassette, or cassette plus grid are given as approximately 0.2 mm lead (60 – 125
kVp). This level of attenuation is quite low and the beam may not always be fully collimated to the cassette.
Therefore the attenuation due to the cassette is often ignored for practical purposes (BIR, 2000).
5.2.5 Secondary radiation – scatter & leakage
Secondary radiation is, in practice, the most ubiquitous radiation type for which shielding is provided. It is a
combination of scattered radiation (generally from the patient) and leakage (from the tube housing). The
former is frequently the dominant component. For shielding calculations, the patient may be regarded as the
source of scattered radiation. The amount of scatter increases with the field size and the thickness of the part
of the patient irradiated. It is also dependent on the spectrum of the primary beam and the scattering angle.
The scattered radiation is generally present throughout the room and decreases with the distance from its
source.
The housing of a diagnostic X‐ray tube is lined with lead and is designed so that the leakage radiation in 1
hour is less than 1 mGy at 1 m from the focal spot, averaged over any area of 100 cm 2 (IEC 2008). Although
the leakage radiation component is small, it is highly filtered and consequently is much harder and more
penetrating than the primary beam. It has been shown that for energies below 100 kVp, the contribution
from leakage radiation is negligible (BIR, 2000). However, at energies of 100 kVp and above, transmission
curves for secondary radiation rather than primary should be used as they take this into account and assume
greater penetration due to the leakage component.
The maximum scatter kerma is often a strong function of the Dose Area Product (DAP). It will vary with kVp
and the maximum value at a boundary 1 m from the patient can be calculated using the following equation
which gives the maximum scatter factor per unit DAP (BIR, 2000):
S max
= [(0.031 x kVp) + 2.5] μGy (Gy cm 2 ) -1
Equation 5.3
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The Design of Diagnostic Medical Facilities where Ionising Radiation is used