What Do We Mean by Tracer Atoms?
When a radioisotope is used as a tracer, the energy of the radiation triggers the counting device, and the exact amount of energy from each disintegrating atom is measured. This differentiates the substance being traced from other materials naturally present.
This is the first photoscanner, which was developed in 1954 at the University of Pennsylvania and was retired from service in 1963. When gamma rays emitted by a tracer isotope in the patient’s body struck the scanner, a flashing light produced a dot on photographic film. The intensity of the light varied with the counting rate and thus diseased tissues that differed little from normal tissue except in their uptake of an isotope could be discerned.
With one conspicuous exception, it is impossible for a chemist to distinguish any one atom of an element from another. Once ordinary salt gets into the blood stream, for example, it normally has no characteristic by which anyone can decide what its source was, or which sodium atoms were added to the blood and which were already present. The exception to this is the case in which some of the atoms are “tagged” by being made radioactive. Then the radioactive atoms are readily identified and their quantity can be measured with a counting device.
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.
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]