A radioactive tracer, it is apparent, corresponds in chemical nature and behavior to the thing it traces. It is a true part of it, and the body treats the tagged and untagged material in the same way. A molecule of hemoglobin carrying a radioactive iron atom is still hemoglobin, and the body processes affect it just as they do an untagged hemoglobin molecule. The difference is that a scientist can use counting devices to follow the tracer molecules wherever they go. [Illustration: _One of the first scans made by a photoscanner. The photorecording (dark bands), superimposed on an X-ray picture for orientation, shows radioactivity in a cancer in the patient’s neck._] It should be evident that tracers used in _diagnosis_—to identify disease or improper body function—are present in such small quantities that they are relatively harmless. Their effects are analogous to those from the radiation that every one of us continually receives from natural sources within and without the body. _Therapeutic_ doses—those given for medical treatment—by contrast, are given to patients with a disease that is in need of control, that is, the physician desires to destroy selectively cells or tissues that are abnormal. In these cases, therefore, the skill and experience of the attending physician must be applied to limit the effects to the desired benefits, without damage to healthy organs. This booklet is devoted to these two functions of radioisotopes, _diagnosis_ and _therapy_; the field of medical research using radioactive tools is so large that it requires separate coverage.[7] DIAGNOSIS Pinpointing Disease Mr. Peters, 35-year-old father of four and a resident of Chicago’s northwest side, went to a Chicago hospital one winter day after persistent headaches had made his life miserable. Routine examinations showed nothing amiss and his doctor ordered a “brain scan” in the hospital’s department of nuclear medicine. Thirty minutes before “scan time”, Mr. Peters was given, by intravenous injection, a minute amount of radioactive technetium. This radiochemical had been structured so that, if there were a tumor in his cranium, the radioisotopes would be attracted to it. Then he was positioned so an instrument called a scanner could pass close to his head. As the motor-driven scanner passed back and forth, it picked up the gamma rays being emitted by the radioactive technetium, much as a Geiger counter detects other radiation. These rays were recorded as black blocks on sensitized film inside the scanner. The result was a piece of exposed film that, when developed, bore an architectural likeness or image of Mr. Peters’ cranium.