In
health physics
Health physics, also referred to as the science of radiation protection, is the profession devoted to protecting people and their environment from potential radiation hazards, while making it possible to enjoy the beneficial uses of radiation. He ...
, whole-body counting refers to the measurement of
radioactivity ''within'' the human body. The technique is primarily applicable to radioactive material that emits
gamma
Gamma (uppercase , lowercase ; ''gámma'') is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter re ...
rays.
Alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
decays can also be detected indirectly by their coincident gamma radiation. In certain circumstances,
beta
Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bÄ“Ì‚ta or ell, βήτα, vÃta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...
emitters can be measured, but with degraded sensitivity. The instrument used is normally referred to as a whole body counter.
This must not be confused with a "whole body monitor" which used for personnel exit monitoring, which is the term used in
radiation protection
Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Expos ...
for checking for external contamination of a whole body of a person leaving a radioactive contamination controlled area.
Principles
If a gamma ray is emitted from a radioactive element within the human body due to
radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
, and its energy is sufficient to escape then it can be detected. This would be by means of either a
scintillation detector
Scintillation can refer to:
*Scintillation (astronomy), atmospheric effects which influence astronomical observations
*Interplanetary scintillation, fluctuations of radio waves caused by the solar wind
*Scintillation (physics), a flash of light pro ...
or a
semiconductor detector
A semiconductor detector in ionizing radiation detection physics is a device that uses a semiconductor (usually silicon or germanium) to measure the effect of incident charged particles or photons.
Semiconductor detectors find broad applicati ...
placed in close proximity to the body. Radioactive decay may give rise to gamma radiation which cannot escape the body due to being absorbed or other interaction whereby it can lose energy; so account must be taken of this in any measurement analysis. Whole-body counting is suitable to detect radioactive elements that emit
neutron radiation
Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then Neutron capture, react with Atomic nucleus, nuclei of other ...
or high-energy
beta radiation
A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β∠...
(by measuring secondary x-rays or gamma radiation) only in experimental applications.
There are many ways a person can be positioned for this measurement: sitting, lying, standing. The detectors can be single or multiple and can either be stationary or moving. The advantages of whole-body counting are that it measures body contents directly, does not rely on indirect bio-assay methods (such as
urinalysis
Urinalysis, a portmanteau of the words ''urine'' and ''analysis'', is a panel of medical tests that includes physical (macroscopic) examination of the urine, chemical evaluation using urine test strips, and microscopic examination. Macroscopic e ...
) as it can measure insoluble radionuclides in the lungs.
On the other hand, disadvantages of whole-body counting are that except in special circumstances it can only be used for gamma emitters due to self-shielding of the human body, and it can misinterpret external contamination as an internal contamination. To prevent this latter case scrupulous de-contamination of the individual must be performed first.
Whole body counting may be unable to distinguish between radioisotopes that have similar gamma energies. Alpha and beta radiation is largely shielded by the body and will not be detected externally, but the coincident gamma from alpha decay may be detected, as well as radiation from the parent or daughter nuclides.
Calibration
Any radiation detector is a relative instrument, that is to say the measurement value can only be converted to an amount of material present by comparing the response signal (usually counts per minute, or per second) to the signal obtained from a standard whose quantity (activity) is well known.
A whole-body counter is calibrated with a device known as a "phantom" containing a known distribution and known activity of radioactive material. The accepte
industry standardis the Bottle Manikin Absorber phantom (BOMAB). The BOMAB phantom consists of 10 high-density polyethylene containers and is used to calibrate ''in vivo'' counting systems that are designed to measure the radionuclides that emit high energy photons (200 keV < E < 3 MeV).
Because many different types of phantoms had been used to calibrate ''in vivo'' counting systems, the importance of establishing standard specifications for phantoms was emphasized at the 1990 international meeting of ''in vivo'' counting professionals held at the National Institute of Standards and Technology (NIST). The consensus of the meeting attendees was that standard specifications were needed for the BOMAB phantom. The standard specifications for the BOMAB phantom provide the basis for a consistent phantom design for calibrating ''in vivo'' measurement systems. Such systems are designed to measure radionuclides that emit high-energy photons and that are assumed to be homogeneously distributed in the body.
Sensitivity
A well designed counting system can detect levels of most gamma emitters (>200 keV) at levels far below that which would cause adverse health effects in people. A typical detection limit for radioactive caesium (
Cs-137
Caesium-137 (), cesium-137 (US), or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nucl ...
) is about 40 Bq. The Annual Limit on Intake (i.e., the amount that would give a person a dose equal to the worker limit that is 20 mSv) is about 2,000,000 Bq. The amount of naturally occurring radioactive
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
present in all humans is also easily detectable. Risk of death by potassium deficiency approaches 100% as whole-body count approaches zero.
The reason that these instruments are so sensitive is that they are often housed in low background counting chambers. Typically this is a small room with very thick walls made of
low-background steel
Low-background steel, also known as pre-war steel, is any steel produced prior to the detonation of the first nuclear bombs in the 1940s and 1950s. Typically sourced from shipwrecks and other steel artifacts of this era, it is often used for modern ...
(~20 cm) and sometimes lined with a thin layer of lead (~1 cm). This shielding can reduce
background radiation
Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.
Background radiation originates from a variety of sources ...
inside the chamber by several orders of magnitude.
Count times and detection limit
Depending on the counting geometry of the system, count times can be from 1 minute to about 30 minutes. The sensitivity of a counter does depend on counting time so the longer the count, for the same system, the better the
detection limit
The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the ...
. The detection limit, often referred to as the Minimum Detectable Activity (MDA), is given by the formula:
:
...where N is the number of counts of background in the region of interest; E is the counting efficiency; and T is the counting time.
This quantity is approximately twice the Decision Limit, another statistical quantity, that can be used to decide if there is any activity present. (i.e., a trigger point for more analysis).
History
In 1950,
Leonidas D. Marinelli
Leonidas D. Marinelli (28 November 1906 – 13 September 1974) was the American radiological physicist who is best known for founding the field of Human Radiobiology and developing the Marinelli Beaker.
Early life and education
Marinelli ...
developed and applied a low-level gamma-ray Whole Body Counter to measure people who had been injected with radium in the early 1920s and 1930s, contaminated by exposure to atomic explosions, and by accidental exposures in industry and medicine The sensitive methods of dosimetry and spectrometry Marinelli developed obtained the total content of natural Potassium in the human body. Marinelli's Whole Body Counter was first used at Billings Hospital at the
University of Chicago
The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...
in 1952.
[Hasterlik, R. J. and L.D. Marinelli. 1955. Physical Dosimetry and clinical observations on four human beings involved in an accidental critical assembly excursion. Conference on the Peaceful Uses of Atomic Energy, Geneva, Switzerland (June 18.) Vol 11: Biological Effects of Radiation: 25-34. United Nations, N.Y. 1956]
References
External links
{{DEFAULTSORT:Whole-Body Counting
Radiation health effects
Ionising radiation detectors
Radiation protection