ruimei ma, ph.d., seiichi yasumura, ph.d., f. avraham .../67531/metadc697243/...measuring the same...
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IN VIVO NEUTRON ACTIVATION FACILITY AT BROOKHAVEN NATIONAL LABORATORY
Ruimei Ma, Ph.D., Seiichi Yasumura, Ph.D., and F. Avraham Dilmanian, Ph.D.
Medical Dept., Brookhaven National Laboratory, Upton, NY 11973
ABSTRACT
Seven important body elements, C, N, Ca, P, K, Na, and C1, can be
measured with great precision and accuracy in the i n vivo neutron
activation facilities at Brookhaven National Laboratory. The
facilities include the delayed-gamma neutron activation, the
prompt-gamma neutron activation, and the inelastic neutron
scattering systems. In conjunction with measurements of total
body water by the tritiated-water dilution method several body
compartments can be defined from the contents of these elements,
also with high precision. In particular, body fat mass is derived
from total body carbon together with total body calcium and
nitrogen; body protein mass is derived from total body nitrogen;
extracellular fluid volume is derived from total body sodium and
chlorine; lean body mass and body cell mass are derived from
total body potassium; and, skeletal mass is derived from total
body calcium. Thus, we suggest that neutron activation analysis
may be valuable for calibrating some of the instruments routinely
used in clinical studies of body composition. The instruments
that would benefit from absolute calibration against neutron
activation analysis are bioelectric impedance analysis, infrared
interactance, transmission ultrasound, and dual energy x-ray/
photon absorptiometry.
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I. INTRODUCTION
Measurements of in vivo body composition are vital to research in
growth and development’, the aging process’, malnutrition’, and
diseases, such as osteo~~orosis4 and the acquired immunodeficiency
syndrome (AIDS)’ . The current widespread interest in the study of body composition indicates the importance of having a rapid,
reliable, and relatively inexpensive way af determining it.
However, most instruments used in clinical. research have not been
calibrated against absolute standards.
The in vivo neutron acti.vation (IVNA) faci.lities at Brookhaven
National Laboratory (BNI,) have been used to measure body
composition f o r such researches for more than 20 yearsc-”. With
our experience in designing and making phantoms, Monte Carlo
simulation, and in.model.ing body composition, we can readily and
accurately calibrate other instruments against IVNA.
11. NEUTRON ACTIVATION FACILITIES AT BNL
A. The delayed-gamma neutron activation (DGNA) facility
The DGNA system consists of a whole body c:ounter (WBC) and a
whole-body neutron activation system that uses ‘38P~Be neutron
sources. The WBC consists of a special counter room containing 32
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disclosed, or r e p e n t s that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.
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NaI(T1) crystals that provide high sensitivity and high
resolution for detecting radionuclides. Each of the 32 crystals
is 4" (W) x 4'' (H) x 18'' (L) with 16 detectors above and 16 below
the patient's bed. The entire room is shielded by four feet of
low-radioactivity concrete, and a 4-in steel, 1/8-in lead and
1/16-in aluminum liner. The absolute photopeak efficiency of the
WBC is about 20% for a uniform line source of I 3 l I (364.5 keV)
placed axially on the bed. This arrangement gives a statistical
fluctuation of 1.7% in y-ray counts for a 10-minute measurement
of a 1 nCi I3 'I line source. The detection limit of WBC is
approximately 1 nCi of radioactivity in the body.
The WBC is used to measure total body potassium (TBK) by counting
the radioactive isotope 40K. An anthropomorphic hollow phantom,
filled with a solution containing a known amount of K, is used to
calibrate the WBC for TBK measurements. Recently, we upgraded the
WBC data acquisition system to provide faster and easier
collection, storage, display, and analysis of data''.
To investigate the total body content of other elements, such as
Ca, Na, C1, and P, the person is activated in the whole-body
neutron activation facility, then counted in the WBC. The
measurements of total body calcium in the DGNA facility
originally were calibrated with an anthropomorphic hollow phantom
filled with solutions containing predetermined amounts of Ca6.
However, 99% of the total Ca in the human body is contained
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within the skeleton and is not homogeneously distributed.
Recently, an artificial skeleton was designed, constructed, and
placed in a bottle mannequin absorber pharntom12 to better
represent this distribution; now, the DGNA system is calibrated
with this phantom containing the artificial This
change has improved the accuracy of the measurements. Also, the
simple geometry of this phantom and the artificial skeleton
allows us to simulate thie neutron activation process using a
Monte Carlo code, so ena.bling us to calibrate the system for
people larger and smaller than the phantoms used as standards.
B. The prompt-gamma neutron activation (PGNA) facility
The PGNA system has undergone several upgrading since it was
built in the late 1970~;~” in particular the system underwent
extensive upgrading in 1991 to improve its reliability and the
precision of the measurements of total body nitrogen (TBN)”.
The facility uses a 75.5, Ci “‘PuBe neutron source positioned
under the patient’s bed, and two 15.2 cm x’ 15.2 cm NaI(T1)
detectors positioned close to the patient’s sides. Measurements
are taken, with the pers’on in both the supine and prone
positions. At each position, the patient’s bed passes over the
source along the direction of the body axis in a step-and-shoot
method, in which measurements are recorded. at five of 20-cm steps
of 200 s each. A 2.5 cm-high tank of D 2 0 positioned immediately
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underneath the patient's bed acts as a premoderator, increasing
the amount of epithermal neutrons at the entrance to the
patient's body. The system uses total body hydrogen (TBH) as an
internal standard', i.e. it measures the ratio of counts from the
10.8 MeV y line of I4N neutron capture events to the counts from
the 2.23 MeV y emission of the 'H neutron capture reaction. The
TBH assessment includes measurements of total body water and
body-composition modeling.
The PGNA system is calibrated with three bottle mannequin
absorber phantoms1* of different sizes15 filled with same
solution. We found that the ratio of the nitrogen to hydrogen
counts can be approximated by a linear function of phantom
volume. Corrections for body size thus were derived, and used for
TBN measurements of people of different sizes.
C. The inelastic scattering (INS) facility
Because of the high carbon content in fat and because a high
fraction of TBC is in the body fat, the INS method is considered
the most direct way to measure total body fat. The INS system at
BNL uses a pulsed D-T Zetatron generator to produce 14 MeV
neutrons at a 10 kHz repetition rate. The 4.4 MeV y-rays emitted
from the patient are detected in two 15.2 cm x 15.2 cm
NaI(T1) detectors positioned on both sides of the person. A
plastic scintillator is used to count the total number of
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neutrons produced by the! generator during the measurements.
Patients are measured from the shoulder to the knee in both
supine and prone positions on a bed that traverses the neutron
source. Data collection and the motion of the bed are controlled
by a computer.
The INS system is calibrated with an anthropomorphic hollow
phantom filled with a solution with predetermined amount of Cll.
DO Summary of the precisions and doses f o r elemental
measurements at BNL:
Table 1. summarizes the precisions and radiation doses fo r
elemental measurements performed at BNL. The precision of the
elemental measurements is estimated based on repeated
measurements with human subjects (TBK) and with phantoms
containing the amount of the element of interest close to that in
Reference ManI6. The radiation doses are estimated for a typical
measurement in each of the three facilities using a quality
factor of 20 for neutrons.
111. CALIBRATION OF BOD'Y COMPOSITION MEASUREMENTS
Although all of the devices used for routine clinical
measurements of body composition are calibrated against standards
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provided by the manufacturers, few, if any, of them are based on
absolute elemental composition, and for many of these techniques,
human measurements are the only proper method of evaluating the
system performance. Furthermore, although the precision of most
instruments is quite good when measuring the standard, the
accuracy of some of them is subject to errors. These errors may
be in part, due to inadequate corrections for differences in body
size and shape. Also, instruments from the same manufacturer when
measuring the same compartment at different sites yield results
that are statistically different.
Since IVNA is based on direct elemental measurements, body
elemental composition can be determined with high precision and
accuracy. Thus, IVNA can be used as the standard to calibrate
body composition instruments now widely used in clinical
settings. In fact, the BNL IVNA facilities have already been used
for several inter-method calibrations, mostly with the dual
photon x-ray absorptiometry
ACKNOWLEDGMENTS
This research was supported by the Department of Energy under
contract DE-AC02-76CH00016, and by the National Institutes of
Health grant 1-Pol-DK42618.
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Key Words: In vivo neutron activation, body composition, phantom,
calibration
Table 1. Summary of precisions and radiation doses for the elemental measurements performed at BNL.
Elements Facility Precision (%) Radiation Dose
K WBC 1.0 ~ 0 . 0 1 (from
(men)
calibration)
) 575 1 Ca DGNA 1 .5
Na DGNA 1 .6
c1 DGNA 1 . 7
P DGNA 2 . 5
N PGNA 2.2 -I
80
C INS 3.0 50