[Illustration: _The inset picture shows a brain scan made with a positron scintillation camera. A tumor is indicated by light area above ear. (Light area in facial region is caused by uptake in bone and extracellular space.) The photograph shows a patient, completely comfortable, receiving a brain scan on one of the three rectilinear scanning devices in the nuclear medicine laboratory of a hospital._] Mr. Peters, who admitted to no pain or other adverse reaction from the scanning, was photographed by the scanner from the front and both sides. The procedure took less than an hour. The developed film showed that the technetium had concentrated in one spot, indicating definitely that a tumor was present. Comparison of front and side views made it possible to pinpoint the location exactly. Surgery followed to remove the tumor. Today, thanks to sound and early diagnosis, Mr. Peters is well and back on the job. His case is an example of how radioisotopes are used in hospitals and medical centers for diagnosis. [Illustration: _The first whole body scanner, which was developed at the Donner Laboratory in 1952 and is still being used. The lead collimator contains 10 scintillation counters and moves across the subject. The bed is moved and serial scans are made and then joined together to form a head-to-toe picture of the subject._] [Illustration: _The diagram shows a scan and the parts of a scanner. (Also see page 21.)_] In one representative hospital, 17 different kinds of radioisotope measurements are available to aid physicians in making their diagnoses. All the methods use tracer quantities of materials. Other hospitals may use only a few of them, some may use even more. In any case they are merely tools to augment the doctors’ skill. Examples of measurements that can be made include blood volume, blood circulation rate, red blood cell turnover, glandular activity, location of cancerous tissue, and rates of formation of bone tissue or blood cells. Of the more than 100 different radioisotopes that have been used by doctors during the past 30 years, five have received by far the greatest attention. These are iodine-131, phosphorus-32, gold-198, chromium-51, and iron-59. Some others have important uses, too, but have been less widely employed than these five. The use of individual radioisotopes in making important diagnostic tests makes a fascinating story. Typical instances will be described in the following pages. [Illustration: _A differential multi-detector developed at Brookhaven National Laboratory locates brain tumors with positron-emitting isotopes. By using many pairs of detection crystals, the device shortens the scanning time and increases accuracy. (See cover for another type of positron scanner.)_] Arsenic-74