In a Hospital
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.
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.
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.