Biomedical Engineering Reference
In-Depth Information
sity, and relative positions of the source and target organs; and on the specific in-
tervening tissues. The Medical Internal Radiation Dose (MIRD) Committee of the
Society of Nuclear Medicine has made extensive calculations of the specific ab-
sorbed fractions (absorbed fraction per gram of target) for a number of source and
target organs in reference man. Monte Carlo techniques are employed in which the
transport of many individual photons through the body is carried out by computer
codes and the resulting data compiled to obtain the specific absorbed fractions. Cal-
culations have been performed both for monoenergetic photons and for the spectra
of photons emitted by a number of radionuclides.
In general, a radionuclide in S emits several kinds of radiation R, with yields
Y
R
and average energies
E
R
. Multiplication of
Y
R
E
R
by the specific absorbed fraction
gives the average absorbed dose in T per transformation in S contributed by the
radiation R. Expressing the energies in MeV, the ICRP defines the specific effective
energy (SEE) imparted per gram of tissue in a target organ T from the emission of
a specified radiation R in a source organ S per transformation as follows:
AF (T
←
S)
R
Y
R
E
R
w
R
MeV g
-1
.
SEE
(T ← S)
R
≡
(16.39)
M
T
This amount of energy absorbed per gram, weighted by the factor
w
R
, thus rep-
resents the contribution of radiation of type R emitted per transformation of a
radionuclide in S to the equivalent dose in T. To express this contribution in
sieverts, we multiply the SEE by the factor
(1.60 × 10
-13
JMeV
-1
)/(10
-3
kg g
-1
) =
1.60 × 10
-10
Sv (MeV g
-1
)
-1
. Summing over all types of radiation emitted by the
radionuclide, we obtain
3)
10
-10
R
H
(T
SEE (T
S)
R
Sv.
(16.40)
←
S)
=
1.6
×
←
Returning to Eq. (16.2), we multiply by the number of transformations
U
S
in 50 y
and sum over all organs S to obtain
H
T
(50) = 1.6 × 10
-10
S
U
S
R
SEE
(T ← S)
R
Sv
(16.41)
for the committed equivalent dose in T. Finally, the committed effective dose,
Eq. (16.1), can be written
E
(50) = 1.6 × 10
-10
T
w
T
S
U
S
R
SEE
(T ← S)
R
Sv
.
(16.42)
If there are several radionuclides
j
in the body, then one adds the individual contri-
butions given by Eq. (16.42):
U
S
R
S)
R
10
-10
T
w
T
S
SEE (T
Sv.
(16.43)
E
(50)
=
1.6
×
←
j
j
3 Generally, dosimetric results are expressed
with no more than two significant figures for
internal dosimetry. The ARLI and DRAC are
given to only one significant figure.
