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
[Illustration: 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 1.3 × 10²⁰ potassium 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 15,510 minute with emission of measurable gamma rays 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.