Biomedical Engineering Reference
In-Depth Information
by which ingested materials are assumed to reach the body fluids. The metabolic
rate constants
λ
B
are used by the ICRP for the various chemical elements. The
fraction
f
1
of a stable element that reaches the body fluids after ingestion is given
by
λ
B
λ
SI
+
f
1
=
λ
B
.
(16.7)
Values of
f
1
are specified in the metabolic data of the ICRP (Section 16.12).
As with the lung, each of the four compartments in Fig. 16.8 gives rise to a first-
order differential equation describing the activity changes. Given initial conditions
and the rate of intake
I
(
t
)
into compartment ST, the four equations can be solved
for the activities in each section as functions of time. Activity entering the GI tract
from the respiratory system is included in
I
(
t
)
. Radioactive daughters are included
with the parent in the ICRP calculations. The table in Fig. 16.8 gives the assumed
masses of the sections and their contents and the clearance rates. The activity trans-
ferred to the body fluids as a function of time is given by
λ
B
q
SI
(
t
)
.
When the source organ is a section of the GI tract, the committed equivalent
dose is estimated for the mucosal layer of the walls of each section for penetrating
and nonpenetrating radiations. Other organs are also irradiated by sources in the
contents of the GI tract, and the tract is irradiated by materials located in other parts
of the body. The ICRP has compiled values of
U
S
for various ingested radionuclides
and daughters in the sections of the GI tract. It also gives data for sections of the
GI tract as target organs with the contents of these sections and other organs of the
body as source organs.
In 2007 the ICRP issued its revised dosimetric model of the GI tract, designated
HATM. Publication 100,
Human Alimentary Tract Model for Radiological Protection
,
treats intakes by children as well as male and female adults, with applicability to
both occupational and environmental exposures. Also, like the companion HRTM,
the HATM enables dose calculations to be made for specific target regions consid-
ered important for cancer induction.
16.7
Organ Activities as Functions of Time
The calculation of activities in various organs and tissues of the body as functions
of time after intake is exemplified by computations for the GI tract. We let
I
(
t
)
represent the rate of activity intake (e.g., in Bq s
-1
) of a given radionuclide into the
stomach as a function of time
t
. The rate of change of the activity,
q
ST
(
t
)
,inthe
stomach at any time is then given by the equation
d
q
ST
d
t
=
I
(
t
)-
(16.8)
λ
R
q
ST
-
λ
ST
q
ST
.
Here,
λ
R
is the radioactive decay constant of the nuclide and
λ
ST
is the metabolic
rate constant given in Fig. 16.8 (
λ
ST
=
24
d
-1
). The rate of depletion,
(
λ
R
+
λ
ST
)
q
ST
,is
