Biomedical Engineering Reference
In-Depth Information
traditionally given in terms of the exposure (R). Much of the information available
in the literature is presented in this form. Whatever procedures and assumptions
are used in making the shielding design, radiation surveys after installation must
be performed in order to evaluate the results.
Design of Primary Protective Barrier
The attenuation of primary X-ray beams through different thicknesses of various
shielding materials has been measured experimentally. The data have been plotted
to give empirical attenuation curves, which are used to design protective barriers.
It is found experimentally that the primary beam intensity transmitted through a
shield depends strongly on the peak operating voltage but very little on the filtra-
tion of the beam. (The effect of filters on exposure rate is small compared with
that of the thicker shields.) In addition, at fixed kVp, the exposure from transmit-
ted photons at a given distance from the X-ray machine is proportional to the time
integral of the beam current, usually expressed in milliampere-minutes (mA min).
In other words, the total exposure per mA min is virtually independent of the tube
operating current itself. These circumstances permit the presentation of X-ray at-
tenuation data for a given shielding material as a family of curves at different kVp
values. Measurements are conveniently referred to a distance of 1 m from the target
of the tube with different thicknesses of shield interposed.
Attenuation curves measured for lead and concrete at a number of peak voltages
(kVp) are shown in Figs. 15.9 and 15.10. The ordinate,
Q
, gives the exposure of
the attenuated radiation in RmA
-1
min
-1
at the reference distance of 1 m. The
abscissa gives the shield thickness. Figure 15.9 shows, for example, that behind
2 mm of lead, the exposure 1 m from the target of an X-ray machine operating at
150 kVp is
10
-3
RmA
-1
min
-1
. If the machine is operated with a beam current of
200 mA for 90 s, that is, for
200
×
1.5
=
300
mA min, then the exposure at 1 m
10
-3
will be
300
0.3
R behind the 2 mm lead shield. The same exposure results
if the tube is operated at 300 mA for 60 s. The exposure at other distances can be
obtained by the inverse-square law; for example, the exposure per mA min at 2 m
is
10
-3
/2
2
×
=
10
-4
RmA
-1
min
-1
. The 2 mm of lead shielding can be located
anywhere between the X-ray tube and the point of interest.
The amount of shielding needed to provide the primary barrier for an area ad-
joining an X-ray room can be found from the attenuation curves, once the appro-
priate value of
Q
has been determined. In addition to the peak voltage, the value of
Q
in a specific application will depend on several other circumstances:
1. The weekly design goal,
P
,whichisoneofthetwovaluesin
Table 15.1, depending on the area to be protected.
2. The workload,
W
, or weekly amount of use of the X-ray
machine, expressed in mAminwk
-1
.
3. The use factor,
U
, or fraction of the workload during which
the useful beam is pointed in a direction under
consideration.
=
2.5
×
