Biomedical Engineering Reference
In-Depth Information
as a function of time. In addition, if the nuclide emits penetrating radiation, other
organs of the body will be irradiated from the decay in a given organ. Using a math-
ematical model for reference man (Fig. 16.1), the ICRP methodology enables one to
calculate the equivalent-dose rates in other organs from disintegrations in a given
organ as functions of time. A complete calculation thus provides the equivalent-
dose rates in every organ and tissue of the body and the effective-dose rate as
functions of time. The committed equivalent doses and the committed effective
dose resulting from the initial intake can be evaluated, and the annual reference
level of intake (ARLI) and derived reference air concentration (DRAC) determined
(Section 14.4). When the original inhaled or ingested nuclide decays into radioac-
tive daughters, contributions of the latter are included in the determination of the
ARLI and DRAC.
We now outline explicitly how the calculation of the committed effective dose for
a radionuclide in the body can be carried out by using the ICRP metabolic models
and reference man. Several concepts will be introduced now and developed further
in the sections that follow. By definition, the committed effective dose is given by
Eq. (14.7) with
τ
=
50 y. We wr i te
E
(50) =
T
w
T
H
T
(50),
(16.1)
where the
w
T
are the tissue weighting factors (Table 14.2), the
H
T
(50) are the com-
mitted equivalent doses, given by Eq. (14.6), and the sum extends over all organs
and tissues
T
of the body. As described in the last paragraph, the committed equiv-
alent dose in a given organ is a result of irradiation by sources in both the organ
itself and in other organs. For a given organ, considered as a target organ
T
,one
computes and adds the contributions to the committed equivalent dose from all
organs, considered as source organs
S
:
=
S
U
S
H
(T
H
T
(50)
←
S).
(16.2)
H
(T
Here
S)
denotes the equivalent dose in
T
(average) per disintegration, or
transformation, of the radionuclide in
S
. The sum extends over all source organs
and tissues
S
of the body, including
T
. The quantity
U
S
denotes the number of
transformations of the nuclide in
S
during the 50-y period of the committed equiv-
alent dose.
Having outlined the calculation of the committed effective dose following the
intake of a radionuclide, we turn now in the next sections to the individual elements
needed for the computations.
←
16.4
ICRP-30 Dosimetric Model for the Respiratory System
The ICRP-30 model for the respiratory system, shown in Fig. 16.4, is divided into
three major parts—the nasal passage (NP), the trachea and bronchial tree (TB), and
