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
[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.
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.)
[Illustration: Figure 8 _A crystal whole body counter “iron room” under
construction above, and in use._]
[Illustration: Figure 8, below]