Part 4

Graph of Uptake in 1 hour (Percent of administered dose) versus Uptake in 24 hours (Percent of administered dose)]

This simple procedure has been used widely. One difficulty in using it is that its success is dependent upon the time interval between injection and measurement. An overactive gland both concentrates iodine rapidly and also discharges it back to the blood stream as PBI more rapidly than normal. Modifications of the test have been made to compare the amount of iodine-131 that was administered with the amount circulating in the blood as PBI. The system acquires chemical separation of the two forms of iodine from a sample of blood removed from a vein, followed by separate counting. This computation of the “conversion ratio” of radioactive plasma PBI to plasma-total ¹³¹I gives results that are less subject to misinterpretation.

To determine local activity in small portions of the thyroid, an automatic scanner is used. A collimator shields the detector (a Geiger-Müller tube or scintillating crystal) so that only those impulses originating within a very small area are accepted by the instrument. The detector is then moved back and forth slowly over the entire area and the radiation is automatically recorded at definite intervals, creating a “map” of the active area. In cases where lumps, or nodules, have been discovered in the thyroid, the map is quite helpful in distinguishing between cancerous and benign nodules. The former are almost always less radioactive than surrounding tissues.

Fragments of cancerous thyroid tissue may migrate to other parts of the body and grow there. These new cancers are known as metastatic cancers and are a signal of an advanced state of disease. In such a situation even complete surgical removal of the original cancer may not save the patient. If these metastases are capable of concentrating iodine (less than 10% of them are), they can be located by scanning the whole body in the manner that was just described. When a thyroid cancer is discovered, therefore, a doctor may look for metastases before deciding to operate.

Human blood serum albumin labeled with ¹³¹I is used for measurement of the volume of circulating plasma. The procedure is quite similar to that used with radioactive chromium. Iodinated human serum albumin labeled with ¹³¹I is injected into a vein. Then, after allowing time for complete mixing of the sample with the blood, a second sample is counted using a scintillation counter.

For many years, a dye known as rose bengal has been used in testing liver function. About 10 years ago this procedure was improved by labeling the dye with ¹³¹I. When this dye is injected into a vein it goes to the liver, which removes it from the blood stream and transfers it to the intestines to be excreted. The rate of disappearance of the dye from the blood stream is therefore a measure of the liver activity. Immediately after administration of the radioactive dye, counts are recorded, preferably continuously from several sites with shielded, collimated detectors. One counter is placed over the side of the head or the thigh to record the clearance of the dye from the blood stream. A second is placed over the liver, and a third over the abdomen to record the passage of the dye into the small intestine.

Human serum albumin labeled with ¹³¹I is sometimes used for location of brain tumors. It appears that tumors alter a normal “barrier” between the brain and blood in such a manner that the labeled albumin can penetrate tumorous tissues although it would be excluded from healthy brain tissue.

1 Human serum albumin labeled with ¹³¹I 2 Tumor selectively localizes labeled albumin 3 Survey of gamma radiation (Standard points)

The brain behaves almost uniquely among body tissues in that a “blood-brain barrier” exists, so that substances injected into the blood stream will not pass into brain cells although they will pass readily into muscular tissue. This blood-brain barrier does not exist in brain tumors. A systematic scanning of the skull then permits location of these cancerous “hot spots”.

Iron-59

Iron is a necessary constituent of red blood cells, so its radioactive form, ⁵⁹Fe, has been used frequently in measurement of the rate of formation of red cells, the lifetime of red cells, and red cell volumes. The labeling is more difficult than labeling with chromium for the same purposes, so this procedure no longer has the importance it once had.

On the other hand, direct measurement of absorption of iron by the digestive tract can be accomplished only by using ⁵⁹Fe. In achlorhydria the gastric juice in the stomach is deficient in hydrochloric acid, and this condition has been shown to lower the iron absorption. A normal diet contains much more iron than the body needs, but in special cases, sometimes called “tired blood” in advertising for medicines, iron compounds are prescribed for the patient. If ⁵⁹Fe is included, its appearance in the blood stream can be monitored and the effectiveness of the medication noted.

Phosphorus-32

The phosphate ion is a normal constituent of the blood. In many kinds of tumors, phosphates seem to be present in the cancerous tissue in a concentration several times that of the surrounding healthy tissue. This offers a way of using phosphorus-32 to distinguish between cancer cells and their neighbors. Due to the fact that ³²P gives off beta rays but no gammas, the counter must be placed very close to the suspected tissue, since beta particles have very little penetrating power. This fact limits the use of the test to skin cancers or to cancers exposed by surgery.

Some kinds of brain tumors, for instance, are difficult to distinguish visually from the healthy brain tissue. In such cases, the patient may be given ³²P labeled phosphate intravenously some hours before surgery. A tiny beta-sensitive probe counter then can be moved about within the operative site to indicate to the surgeon the limits of the cancerous area.

Sodium-24

Normal blood is about 1% sodium chloride or ordinary salt. This fact makes possible the use of ²⁴Na in some measurements of the blood and other fluids. The figure illustrates this technique. A sample of ²⁴NaCl solution is injected into a vein in an arm or leg. The time the radioisotope arrives at another part of the body is detected with a shielded radiation counter. The elapsed time is a good indication of the presence or absence of constrictions or obstructions in the circulatory system.

1 ²⁴NaCl solution injected 2 Blood carries ²⁴NaCl to both legs 3 High reading—good circulation Site of constriction 4 Low reading—poor circulation

The passage of blood through the heart may also be measured with the aid of sodium-24. Since this isotope emits gamma rays, measurement is done using counters on the outside of the body, placed at appropriate locations above the different sections of the heart.

1 ²⁴Na intravenously injected To lungs, To body, From arm, From right lung, From left lung 2 Geiger counter detects radiations from ²⁴Na 3 Ink writing recorder shows route of ²⁴Na

Technetium-99^{m}

Because of its short half-life of six hours, technetium-99^{m} is coming into use for diagnosis using scanning devices, particularly for brain tumors. It lasts such a short time it obviously cannot be kept in stock, so it is prepared by the beta decay of molybdenum-99. A stock of molybdenum is kept in a shielded container in which it undergoes radioactive decay yielding technetium. Every morning, as the technetium is needed, it is extracted from its parent by a brine solution. This general procedure of extracting a short-lived isotope from its parent is also used in other cases. We shall see later that radon gas is obtained by an analogous method from its parent, radium.

Thulium-170 and Gamma Radiography

For years it has been recognized that there would be many uses for a truly portable device for taking X-ray pictures—one that could be carried by the doctor to the bedside or to the scene of an accident. Conventional X-ray equipment has been in use by doctors for many years, and highly efficient apparatus has become indispensable, especially in treating bone conditions. There is, however, a need for a means of examining patients who cannot be moved to a hospital X-ray room, and are located where electric current sources are not available.

A few years ago, a unit was devised that weighed only a few pounds, and could take “X-ray pictures” (actually gamma radiographs) using the gamma rays from the radioisotope thulium-170. The thulium source is kept inside a lead shield, but a photographic shutter-release cable can be pressed to move it momentarily over an open port in the shielding. The picture is taken with an exposure of a few seconds. A somewhat similar device uses strontium-90 as the source of beta radiation that in turn stimulates the emission of gamma rays from a target within the instrument.

Still more recently, ¹²⁵I has been used very successfully in a portable device as a low-energy gamma source for radiography. The gamma rays from this source are sufficiently penetrating for photographing the arms and legs, and the necessary shielding is easily supplied to protect the operator. By contrast with larger devices, the gamma-ray source can be as small as one-tenth millimeter in diameter, virtually a point source; this makes possible maximum sharpness of image. The latest device, using up to one curie of ¹²⁵I, weighs 2 pounds, yet has adequate shielding for the operator. It is truly portable.

If this X-ray source is combined with a rapid developing photographic film, a physician can be completely freed from dependence upon the hospital laboratory for emergency X rays. A finished print can be ready for inspection in 10 seconds. The doctor thus can decide quickly whether it is safe to move an accident victim, for instance. In military operations, similarly, it becomes a simple matter to examine wounded soldiers in the field where conventional equipment is not available.

Tritium