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.
A linear photoscanner produced these pictures of (A) a normal thyroid, (B) an enlarged thyroid, and (C) a cancerous thyroid.
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.
Screening test for Hyperthyroidism
It is especially important in isotope studies on infants and small children that the radiation exposure be low. By carrying out studies in the whole body counter room, the administered dose can be greatly reduced. The photographs illustrate a technique of measuring radioiodine uptake in the thyroid gland with extremely small amounts of a mixture of iodine-131 and iodine-125. A shows a small television set that is mounted above the crystal in such a way that good viewing requires that the head be kept in the desired position. This helps solve the problem of keeping small children still during a 15-minute counting period. B shows a child in position for a thyroid uptake study.
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[9] 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.
Seven serial scans made with the whole body scanner were put together to provide a whole body scan of this patient with thyroid cancer that had spread to the lung. One millicurie of iodine-131 was administered and the scan made 72 hours later. Note the uptake in the lung. This patient was successfully treated with large doses of iodine-131.
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.
Time-lapse motion pictures of the liver of a 3-year-old girl were made with the scintillation camera 1 hour after injection of 50 microcuries of iodine-131-labeled rose bengal dye. This child was born without a bile-duct system and an artificial bile duct had been created surgically. She developed symptoms that caused concern that the duct had closed. These scans show the mass of material containing the radioactive material (small light area) moving downward and to the right, indicating that the duct was still open.
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.
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”.