Part 3

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.

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.

Arsenic-74

Brain tumors tend to concentrate certain ions (charged atoms or molecules). When these ions are gamma-ray emitters, it is possible to take advantage of the penetrating power of their gamma rays to locate the tumor with a scanning device located outside the skull.

Arsenic-74 and copper-64 are isotopes emitting positrons, which have one peculiar property. Immediately after a positron is emitted from a nucleus it decays, producing two gamma rays that travel in exactly opposite directions. The scanning device has two detectors called scintillation counters, one mounted on each side of the patient’s head.

The electrical circuitry in the scanner is such that only those gamma rays are counted that impinge simultaneously on both counters. This procedure eliminates most of the “noise”, or scattered and background radiation.

Chromium-51

Because chromium, in the molecule sodium chromate, attaches itself to red blood cells, it is useful in several kinds of tests. The procedures are slightly complicated, but yield useful information. In one, a sample of the patient’s blood is withdrawn, stabilized with heparin (to prevent clotting) and incubated with a tracer of radioactive sodium chromate. Excess chromate that is not taken up by the cells is reduced and washed away. Then the radioactivity of the cells is measured, just before injection into the patient. After a suitable time to permit thorough mixing of the added material throughout the blood stream, a new blood sample is taken and its radioactivity is measured. The total volume of red blood cells then can be calculated by dividing the total radioactivity of the injected sample by the activity per milliliter of the second sample.

In certain types of anemia the patient’s red blood cells die before completing the usual red-cell lifetime of about 120 days. To diagnose this, red cells are tagged with chromium-51 (⁵¹Cr) in the manner just described. Then some of them are injected back into the patient and an identical sample is injected into a compatible normal individual. If the tracer shows that the cells’ survival time is too short in both recipients to the same degree, the conclusion is that the red cells themselves must be abnormal. On the other hand, if the cell-survival time is normal in the normal individual and too short in the patient, the diagnosis is that the patient’s blood contains some substance that destroys the red cells.

When chromium trichloride, CrCl₃, is used as the tagging agent, the chromium is bound almost exclusively to plasma proteins, rather than the red cells. Chromium-51 may thus be used for estimating the volume of plasma circulating in the heart and blood vessels. The same type of computation is carried on for red cells (after correction for a small amount of chromium taken up by the red blood cells). This procedure is easy to carry out because the radioactive chromium chloride is injected directly into a vein.

An ingenious automatic device has been devised for computing a patient’s total blood volume using the ⁵¹Cr measurement of the red blood cell volume as its basis. This determination of total blood volume is of course necessary in deciding whether blood or plasma transfusions are needed in cases involving bleeding, burns, or surgical shock. This ⁵¹Cr procedure was used during the Korean War to determine how much blood had been lost by wounded patients, and helped to save many, many lives.

For several years, iodine-131 has been used as a tracer in determining cardiac output, which is the rate of blood flow from the heart. It has appeared recently that red blood cells tagged with ⁵¹Cr are more satisfactory for this measurement than iodine-labeled albumin in the blood serum. It is obvious that the blood-flow rate is an extremely important physiological quantity, and a doctor must know it to treat either heart ailments or circulatory disturbances.

In contrast to the iodine-131 procedure, which requires that an artery be punctured and blood samples be removed regularly for measurement, chromium labeling merely requires that a radiation counter be mounted on the outside of the chest over the aorta (main artery leaving the heart). A sample of labeled red blood cells is introduced into a vein, and the recording device counts the radioactivity appearing in the aorta as a function of time. Eventually, of course, the counting rate (the number of radioactive disintegrations per second) levels off when the indicator sample has become mixed uniformly in the blood stream. From the shape of the curve on which the data are recorded during the measurements taken before that time, the operator calculates the heart output per second.

Obstetricians caring for expectant mothers use red cells tagged with ⁵¹Cr to find the exact location of the placenta. For example, in the condition known as placenta previa, the placenta—the organ within the uterus by which nourishment is transferred from the mother’s blood to that of the unborn child—may be placed in such a position that fatal bleeding can occur. A radiation-counting instrument placed over the lower abdomen gives information about the exact location of the placenta. If an abnormal situation exists, the attending physician is then alert and ready to cope with it. The advantages of chromium over iodine-131, which has also been used, are that smaller doses are required, and that there is no transfer of radioactivity to the fetal circulation.

Still another common measurement using ⁵¹Cr-labeled red blood cells is the determination of the amount and location of bleeding from the gastrointestinal tract (the stomach and bowels). The amount is found by simple measurement of chromium in the blood that appears in the stools. To find the location is slightly more complicated. The intestinal contents are sampled at different levels through an inserted tube, and the radiation of the samples determined separately.

Finally, gastrointestinal loss of protein can be measured with the aid of ⁵¹Cr-labeled blood serum. The serum is treated with CrCl₃ and then injected into a vein. In several very serious ailments there is serious loss of blood protein through the intestines. In these conditions the ⁵¹Cr level in the intestinal excretions is high, and this alerts the doctor to apply remedial measures.

Cobalt-60

Vitamin B₁₂ is a cobalt compound. Normally the few milligrams of B₁₂ in the body are stored in the liver and released to the blood stream as needed. In pernicious anemia, a potentially fatal but curable disease, the B₁₂ content of the blood falls from the usual level of 300-900 micromicrograms per milliliter (ml) to 0 to 100 micromicrograms per ml. The administration of massive doses of B₁₂ is the only known remedy for this condition.

If the B₁₂ is labeled with radioactive cobalt, its passage into the blood stream may be observed by several different methods. The simplest is to give the B₁₂ by mouth, and after about 8 hours study the level of cobalt radioactivity in the blood. Cobalt-60 has been used for several years, but recently cobalt-58 has been found more satisfactory. It has a half-life of 72 days while ⁶⁰Co has a 5.3-year half-life. This reduces greatly the amount of radiation to the patient’s liver by the retained radioactivity.

Iodine-131

Like chromium-51, iodine is a versatile tracer element. It is used to determine blood volume, cardiac output, plasma volume, liver activity, fat metabolism, thyroid cancer metastases, brain tumors, and the size, shape, and activity of the thyroid gland.

Because of its unique connection with the thyroid gland, iodine-131 is most valuable in measurements connected with that organ. Thyroxin, an iodine compound, is manufactured in the thyroid gland, and transferred by the blood stream to the body tissues. The thyroxin helps to govern the oxygen consumption of the body and therefore helps control its metabolism. Proper production of thyroxin is essential to the proper utilization of nutrients. Lowered metabolism means increased body weight. Lowered thyroid activity may mean expansion of the gland, causing one form of goiter.

Iodine-131 behaves in the body just as the natural non-radioactive isotope, iodine-127, does, but the radioactivity permits observation from outside the body with some form of radiation counter. Iodine can exist in the body in many different chemical compounds, and the counter can tell where it is but not in what form. Hence chemical manipulation is necessary in applying this technique to different diagnostic procedures.

The thyroid gland, which is located at the base of the neck, is very efficient in trapping inorganic iodide from the blood stream, concentrating and storing the iodine-containing material and gradually releasing it to the blood stream in the form of protein-bound iodine (PBI).

One of the common diagnostic procedures for determining thyroid function, therefore, is to measure the percentage of an administered dose of ¹³¹I that is taken up by the gland. Usually the patient is given a very small dose of radioactive sodium iodide solution to drink, and two hours later the amount of iodine in the gland is determined by measuring the radiation coming from the neck area. In hyperthyroidism, or high thyroid gland activity, the gland removes iodide ions from the blood stream more rapidly than normal.

Oral dose ¹³¹I