POTASSIUM-40 IN HUMAN BODIES
Data from whole body counters indicate that potassium-40 is the most abundant radionuclide in the human body. Our bodies also contain other naturally radioactive substances but the numbers of atoms usually present are so low (as with radium for instance) that they cannot be detected with whole body counters. Several man-made radionuclides also have found their way into body tissues and organs in quantities that sometimes are large enough to be detected and counted.
Measurement of disintegrating potassium-40 atoms in the tissues of a human body can be used to determine the total amount of potassium (both radioactive and stable) in the body. It is known that potassium-40 makes up 0.0119% of all potassium and that 11% of all disintegrating potassium-40 atoms emit high-energy gamma rays that are measurable by the counter.
The method of determining the amount of potassium in a human subject is to compare the number of gamma rays from a known amount of potassium placed in a phantom, or dummy body, with the number counted from the human subject (see [photo on next page]). Phantoms are artificial bodies, approximately the size, shape, and density of a human body, used for calibrating counters. They are designed so the radioisotopes they contain have similar distribution to the distribution of the isotopes expected in the real body. This is how a test might work:
| Counts per minute from 140 grams of potassium in the phantom | 16,800 |
| Counts per minute with nothing in counter (background) | 12,000 |
| Net counts per minute from 140 grams of potassium | 4,800 |
| Counts per minute with a 77-pound boy in counter | 14,400 |
| Background cunts per minute | 12,000 |
| Net count per minute from boy | 2,400 |
| Calculated amount of potassium in boy | 70 grams |
Figure 7 Phantom used for iodine-131 studies. The radiation spectrum from the thyroid area in the neck is being obtained with a sodium iodide crystal, left.
We can appreciate the sensitivity of whole body counters by comparing the number of gamma rays recorded by the instruments with the total number emitted from the body being counted. The following data, also simplified, illustrate this comparison:
| Total atoms in 70 grams of potassium | 1.08 × 10²⁴[1] |
| Number of atoms of potassium-40 in 70 grams of potassium | 1.3 × 10²⁰ |
| Half-life of potassium-40 in minutes | 6.4 × 10¹⁴ |
| Number of potassium-40 atoms disintegrating | |
| per minute in 70 grams of potassium | 141,000 |
| Number of potassium-40 atoms disintegrating per minute with emission of measurable gamma rays | 15,510 |
| Number of counts recorded | 2,400 |
| Detection efficiency: 2,400 ÷ 15,510 = 15.5% |
Fatty tissues are known to have a low potassium concentration and muscle tissue a higher level. It is therefore apparent that potassium-40 determinations provide a way to indicate the amount of lean muscle in any individual, and indirectly the amount of fat. Estimates of the amount of fat based on the measurement of the specific gravity of the subject, often used in the past, have never been satisfactory. Not only do variable and unmeasurable air spaces change body density, but the process of submerging a person in a tank—to determine his specific gravity by the amount of water he displaces—is a clumsy and uncomfortable one.
Significant variations in potassium content have been found in persons suffering from muscle diseases or malfunctions. For example, a sharp drop in potassium content accompanies the profound muscle weakness that follows diabetic coma. Administration of potassium produces striking improvement in the condition known as familial periodic paralysis.
Whole body counter data from a study of muscular dystrophy and myotonia atrophica patients showed there is a gradual and progressive decrease of body potassium during the unrelenting courses of these diseases. Otherwise healthy children of muscular dystrophy patients, or their brothers and sisters, also may be deficient in potassium. By assisting in muscle research, whole body counters help doctors learn more of how potassium relates to muscle function and muscle health.
Whole body counting is an improvement over potassium determination based on chemical analysis of body fluids. If counters are not used, one way to measure body potassium is to inject a known quantity of potassium-42 (another radioactive form of potassium), wait until this has been uniformly mixed with the potassium already in the body, and then record the radioactivity of a volume of blood serum. From the degree of dilution of the injected potassium-42, the total body potassium can be calculated. This widely used method is uncomfortable for the patient since it involves use of syringes to inject and withdraw fluids. Because about 95% of the body potassium is inside the cells, rather than in fluids between the cells, this method may also be inexact if the mixing process does not continue long enough. (See Radioisotopes in Medicine, another booklet in this series, for a full discussion of medical treatment with radioactive materials.)
Figure 8 A crystal whole body counter “iron room” under construction above, and in use.
