_The Problem_
You are a physician treating a patient who, because of a severe calcium
deficiency, has been suffering from osteoporosis (a softening of the
bones). You think you are on the right track with your treatment, but
you would like to be sure in order to know whether you should continue
the treatment or try something else. You would have your answer if you
knew that the calcium content of his skeleton had stopped decreasing.
How can you determine the amount of calcium in a living human being?
_The Solution_
You know that the usual techniques for determining calcium in the bones
are not very useful. They are either too inaccurate to show that your
patient’s calcium loss has been stopped or can only be used to measure
the calcium content of the bones in his extremities. The latter is not
satisfactory because these few bones may not be representative of the
rest of his skeleton.
Recently, however, there have been reports of neutron activation
analysis of whole persons, in which the calcium content of their bones
has been measured with unusually good reliability. This has been
accomplished by scientists and doctors working at the University of
Washington School of Medicine in Seattle.
You manage to obtain an appointment for your patient and you accompany
him to the hospital for the analysis. There he is placed on a rotating
platform with his head encircled by a plastic helmet and his arms and
legs submerged in a water-filled plastic container. See the photograph
on the next page. The platform is located in a beam of neutrons
emanating from a beryllium target 15 feet away, which is being bombarded
by deuterons from a 22-MeV cyclotron. The purpose of the water is to
surround the bones in that part of the subject’s skeleton with a neutron
moderator equivalent to the body tissue surrounding the rest of his
skeleton. (A neutron moderator slows down the neutrons and thus makes
them more likely to activate the calcium in the bones.) On each side of
the patient, there are two plastic containers permanently filled with a
solution containing a known quantity of calcium. These serve as
standards for the analysis.
The beam of neutrons is turned on for 35 to 40 seconds. It is then
interrupted while platform and patient are rotated 180 degrees. The
irradiation is resumed so that a uniform dose of neutrons bombards the
patient from both front and back.
During the irradiation your patient receives a dose of radiation
equivalent to approximately 10 ordinary chest X rays and one of the
calcium isotopes in his bones (calcium-48) is activated to calcium-49.
The latter has a half-life of only 8.8 minutes and so counting must
begin soon after the irradiation.
[Illustration: Patient]
A patient in position for whole body irradiation with neutrons
generated by an accelerator. His arms and legs are surrounded by
plexiglas containers filled with water and his head is encased in a
plexiglas helmet. On either side of him are containers, which serve as
standards, filled with an aqueous solution of a calcium salt. The
patient is standing on a turntable that is rotated 180 degrees after
half the irradiation is completed so that the dose of neutrons is
uniformly distributed to the front and the back of the patient.
[Illustration: Patient]
A patient in position for whole-body gamma-ray spectrometry. The
detectors are scintillation crystals that produce pulses of light
proportional in intensity to the energy of the gamma ray absorbed in
the crystal. The patient is scanned from head to foot in approximately
12½ minutes at a rate that is varied to compensate for the gradual
decay of the calcium-49 radioactivity during this period. Near the
patient’s head are two calcium standard solutions in plexiglas
containers.
The patient lies down in a padded aluminum box and, only 4 minutes after
the irradiation is concluded, a ring of 4 gamma-ray scintillation
detectors[10] begin to measure the gamma rays emitted by his body. These
detectors, which are each 4 inches thick and 9⅜ inches in diameter, pass
over his body from head to foot. This takes 12½ minutes and since the
calcium-49 is decaying with a half-life of 8.8 minutes, the detectors
are made to scan at a gradually decreasing rate to compensate for the
reduced radioactivity during the later parts of the counting period. The
figure on the next page shows the gamma-ray spectrum for the patient.
Notice the peak corresponding to an energy of 3.1 MeV. Because there are
small contributions to this energy peak from other activated products in
the body, repeat counts are taken later (after the calcium-49 has
decayed) so that these contributions can be measured and subtracted.
Twenty minutes after the irradiation period, the radioactivity of the
calcium standards is measured by the same instrument. The ratio of the
counts from your patient’s body to that of the standards is 0.210; this
serves as an index of the calcium content of his body on this day.
Because of the care taken to make the analysis repeatable, this index is
probably accurate to about 1 or 2%.
Your patient’s disease usually results in a decrease of approximately 3%
of the calcium in his body per year. Thus, by making the same
measurement a year from now, you will be able to tell if your treatment
is a success by noting that the calcium level in your patient’s bones
has stopped decreasing at a dangerous rate.