Whole-Body Potassium Counting

Quantitative in vivo measurements of radioactivity in the human body started in the 1940s with the emergence of scintillation detectors. The Whole-Body Counter (WBC) is a device used to identify and measure the radioactive material in the body of human beings and animals resulting from the accumulation of naturally occurring radioactivity and of radiotracers and radioactivity introduced by humans, and from the accidental absorption of radio contaminants.

The WBC uses heavy shielding to exclude natural background radiation from reaching the ultrasensitive radiation detectors and electronic counting equipment. In the most sensitive WBCs, the detectors surround the body and the entire system is kept in room with low background radiation. A second system, the one most frequently encountered, is the chair configuration in which a seated subject is counted with fewer detectors than the former, while a third popular system, containing minimum shielding material, is one with a shadow shield in which a body is scanned by shielded detectors.

Natural potassium (K) is encountered in three isotopic states: 93.1 percent 39 K, 0.0118 percent 40 K, and 6.9 percent 41 K, of which 40 K is radioactive with a half-life (t½) of 1.3 × 109 years. Thus, it is in a secular equilibrium [Page 800]with the stable isotopes and 1 g of K contains about 1.8 × 1018 atoms of 40 K. From the basic laws of physics, 40 K with a decay constant λ (λ = ln2/ t½) will emit about 200 gamma rays at 1.46 MeV per minute per gram of natural K. Because the human body contains 140 g K on average, about 28,000 gamma rays will be emitted per minute. Considering self-attenuation (multiple scatterings) of the gamma rays in the human body, the variable size and weight of humans, and the total detection efficiency of the counting system, this emission rate is adequate for observing and monitoring K in infants.

The uniqueness of in vivo whole body counting is the high specificity of the counted gamma rays to the originative radioisotope. Therefore, there is no ambiguity with elemental identification, and because these systems are totally insensitive to the underlying chemistry, there is a linear relationship between the response and the abundance of the radioisotope of interest. Quantitative analyses can be carried out with WBC systems by first administering to a subject a suitable analog radiotracer (by injection or drinking), counting it, and establishing a conversion factor from counts to grams of the element.

Another, simpler, method is using a series of different sized anthropomorphic-shaped phantoms filled with solutions containing known amounts of the element of interest, as for example, KCl dissolved in water for K phantoms. Attention must be paid to account properly for bony structures in humans, and to the invariance of phantom positioning in the counting space of the counter. In all cases, the validity of the conversion factor must be demonstrated experimentally or by proper modeling. There is an extensive literature on the use of total body potassium (TBK) measured by the WBC in studies of body composition. Ellis (2005) comprehensively discussed them, along with many of the issues mentioned above.

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