important changes in the molecular composition of individual cells within the body.

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]

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.

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.

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.

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.