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 controlled speed past the body of the person being studied (Figure 19). To increase its scanning efficiency, the crystal is fitted with a slit and a focusing device. Instruments record the body radioactivity visibly at timed intervals as the crystal moves along the patient’s body. A television screen enables the operator to observe the patient during the counting. [Illustration: 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 and then administering solutions of the tagged salt orally or by injection. 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 Research [Illustration: Figure 20 _Dogs about to be examined in a whole body counter._]