Biomedical Engineering Reference
In-Depth Information
Table 15.1
Air-Kerma Shielding-Design Goals,
P
,fromNCRP
Report No. 147
Controlled Areas
Uncontrolled Areas
0.1 mGy wk
-1
0.02 mGy wk
-1
tures, such as concrete walls, provide sufficient secondary barriers; otherwise, ad-
ditional shielding, such as lead sheets, must be added to them.
Generally, structural shielding has been designed in a manner consistent with
limiting the effective dose to an individual outside the X-ray room to 1 mSv wk
-1
in controlled areas and to 0.1 mSv wk
-1
in uncontrolled areas. A controlled area is
one in which access and occupancy are regulated in conjunction with operation of
the facility. Persons working there have special training in radiation protection, and
radiation exposures are monitored. In contrast, individuals are free to come and go
in uncontrolled areas. These design goals adhere to the annual limits of 50 mSv and
5 mSv for occupational and nonoccupational radiation. Since many instruments
used to monitor X radiation are calibrated to measure exposure in roentgen (R), the
shielding design objectives that have traditionally been employed are expressed as
0.1 Rwk
-1
and 0.01 Rwk
-1
. Numerically, an exposure of 1 R produces an absorbed
dose of 8.76 mGy in air (Sect. 12.2). Conversely, a 1-mGy absorbed dose in air is
equivalent to 0.114 R.
In 2004 the National Council on Radiation Protection and Measurements issued
Report No. 147,
Structural Shielding Design for Medical X-Ray Imaging Facilities
.In
this Report, the Council recommends that
air kerma
,
K
(Sect. 12.10), be the quantity
used for making X-ray shielding calculations. It specifies that an instrument read-
ing in R can be divided by 114 to obtain the air kerma in Gy. The recommended
design goal for occupational exposure is also revised. The cumulative effective-dose
limit (Table 14.4) implies an average annual limit of 10 mSv.
In the design of new fa-
cilities
, the Council recommends one-half this value, or 5 mSv y
-1
,andaweekly
design goal of
P
=
0.1
mGy air kerma. Using one-half also accomplishes adher-
ence to a monthly equivalent-dose limit of 0.5 mSv to a worker's embryo or fetus.
For uncontrolled areas, the recommended design goal is
P
=
0.02
mGy wk
-1
air
kerma, corresponding to the annual 1 mSv shown in Table 14.4. Report No. 147
“is intended for use in planning and designing new facilities and in remodeling
existing facilities.” Installations designed before publication of Report No. 147 and
meeting previous NCRP requirements need not need be reevaluated. The design
goals of the Report are summarized in Table 15.1.
Our discussion of X-ray shielding design will be directed toward the levels for in-
dividuals given in Table 15.1. In what follows we shall present a prototype example
that illustrates how elements of an X-ray shielding calculation can be performed
and put together into a final design. As will be pointed out, some important details
will be simplified and will not specifically follow NCRP Report No. 147. For the
transmission of X rays through different materials, we shall use measured data as
