  Study of the transfer of nutrients and other substances from an expectant mother’s body to that of her unborn child is one of the most challenging areas of biological research. A team of scientists headed by N. S. MacDonald has used a whole body counter at the University of California at Los Angeles to study one aspect of this problem by comparing the concentrations of radioactive materials in newborn infants, in babies who are born dead, and in tissues of the mothers-to-be. In these studies the scintillation crystal was placed directly beneath a plastic bassinet holding the babies. Twenty-eight infants, 6 to 24 hours old, were counted for 45 minutes each. The only radionuclide found was the ever-present potassium-40. The bodies of seven stillborn babies were counted for at least 10 hours each. More kinds of radionuclides were found than in the living babies, although the large counting time may have affected the results. The same counting techniques were used with placental tissues from mothers of three of the stillborns. The placenta is the organ that nourishes an unborn child and through which substances from the mother’s bloodstream are exchanged with those in the baby’s blood. The graphs in Figure 17 show data from this experiment and illustrate the method of interpreting whole body counter data. When the counts per minute at each band of gamma-ray energy recorded from the placental tissues (b) were subtracted from corresponding values from the stillborns (a), it was found (c) that the placentas contained more of the isotopes ruthenium-103, ruthenium-106, and zirconium-95 than did the babies that had been nurtured by these placentas. The babies’ bodies contained more niobium-95 and potassium-40 than the placentas. Niobium-95 is produced by the radioactive disintegration of zirconium-95. This suggested that zirconium-95 atoms do not pass readily through the placenta, but, after they have decayed to niobium-95, they pass into the baby’s bloodstream easily. Actually, the gamma-ray energies of zirconium-95 and niobium-95 are so similar that the counter cannot distinguish between them. The two isotopes, however, were separated chemically, and whole body spectra were prepared from samples of the pure elements. The spectrum (d) of pure zirconium-95 subtracted from that of pure niobium-95 was strikingly similar to the spectrum of “stillborn baby minus placenta” on the graphs. Cesium-137 was added to the synthetic spectrum to provide a reference mark at the 0.660-Mev point. This revealed that the ratio of cesium-137 to potassium-40 is lower in babies than in adults. Figure 17 Results of experiment studying transfer of nutrients from an expectant mother to her unborn child. Research on Body ProcessesRadioactive tracer atoms, either natural or purposefully built into molecules of vital materials like proteins, are revealing how these substances function in the body to produce Figure 18 A multidetector positron scanner to record radiations with opposed pairs of detection crystals. Scanning devices are commonly used for noting the fate of tracer isotopes in medical diagnosis. Two types of adaptations enable whole body counters to locate accumulations of radioactive materials in specific organs or small portions of the body. At the National Institutes of Health, Bethesda, Md., one counter is fitted with three rows of six 12-by-12-inch plastic blocks. Each block has four photomultiplier tubes to collect the scintillations from the crystal. The rows are curved so as to be equally distant from the patient’s body. The current pulses from each of the 18 blocks can be fed individually into the pulse sorter, counter, and recorder. Thus the kinds and numbers of gamma rays from the sector of the patient adjacent to any block can be studied individually. Similarly, the path and speed of the administered materials can be followed by taking recordings from the blocks sequentially. At the U. S. Naval Hospital in Bethesda, Md., a whole body counter is fitted with a crystal that can be moved at Figure 19 The U. S. Naval Hospital whole body counter, showing the moving crystal, left, and instruments, including the television screen used by the operator. The moving crystal makes it possible to use this whole body counter for scanning. The role of iron in preventing one form of anemia has been clarified by using iron-59 as a tracer. Persons suffering from chronic infections or such blood diseases as leukemia and polycythemia vera have been checked for the amount of iron carried by their red blood cells. Cobalt-60 atoms have been substituted for stable cobalt in molecules of vitamin B-12 so that the way the body makes use of this vitamin can be studied. Similarly, the body’s use of sodium can be studied by labeling sodium chloride with sodium-22 Whole body counters used in tracer studies cause a minimum of inconvenience for the patient. Their sensitivity permits use of smaller quantities of radioactive material than is required with small scanning instruments. Those are unusual jobs for whole body counters, however. Scanners or other types of instruments are used more typically in following tracer isotopes. Animal ResearchFigure 20 Dogs about to be examined in a whole body counter. How do dogs accumulate fallout isotopes in their bodies? This question was answered effectively by placing dogs in whole body counters and comparing the count from radioactive strontium-emitted gamma rays originating in their bodies with the count from a masonite phantom dog containing a known amount of radioactive strontium-85. It was found that female dogs increase their strontium retention while they are nursing newborn puppies. Strontium is much like calcium, which is a major component of milk. One dog measured had broken a leg in a fight. The counter showed above-average strontium accumulation for this dog, and it was conjectured that strontium, a “bone-seeking” element, had followed calcium to the point of bone repair and new bone growth. Figure 21 A wild deer under a whole body counter. Figure 22 Graph of whole body counter survey of four different animal species, showing differences in their retention of orally administered radioactive zinc-65. Figure 22 shows how animals differ in their retention of orally administered zinc-65, as revealed by a whole body counter. It is apparent that counters can be used to determine the differences in the metabolism of different animal species used for research. Standard data developed in this way can serve to reduce error that may occur if results from one species are used for interpretation of data for another species, such as man. A University of Illinois project to breed meat animals with a high lean-to-fat ratio has been aided by whole body counters. The tendency to deposit fat seems to be inherited, and breeding stock with low fat content can be selected, using “muscle-seeking” potassium-40 to show the proportion of muscle in each potential parent. The Illinois counter is unique in being large enough to examine an adult steer (Figure 23). A similar counter at Cornell University has been used to study animals infested with internal parasites, comparing them with parasite-free animals. The counter revealed that a positive relationship exists between the level of parasite infestation and loss of iron-59-labeled blood from the digestive tract. The possibility of using this method to evaluate parasite-killing drugs is being considered. The Cornell counter is kept clean by covering the animals with plastic sheeting. The same counter also can serve human patients, who are positioned in a wheeled hospital stretcher. (See Figure 1D.) |
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