Whole-body counting

Principles

If a gamma ray is emitted from a radioactive element within the human body due to radioactive decay, and its energy is sufficient to escape, then it can be detected by means of either a scintillation detector or a semiconductor detector placed close to the body. Radioactive decay may give rise to gamma radiation, which cannot escape the body due to being absorbed or other interactions through which it can lose energy. Any measurement analysis must take this into account. Whole-body counting is suitable to detect radioactive elements that emit neutron radiation or high-energy beta radiation (by measuring secondary x-rays or gamma radiation), but only in experimental applications.

Whole-body counting can take place while a person is sitting, standing, or lying down, depending on the particular equipment setup used for the measurement. 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, and does not rely on indirect bio-assay methods (such as urinalysis) as it can measure insoluble radionuclides in the lungs.

However, there are some disadvantages to whole-body counting. Aside from special circumstances, it can only be used to detect gamma emitters due to self-shielding of the human body. It can also misinterpret external contamination as an internal contamination; to prevent this, a person must be rigorously decontaminated before the measurement. 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, meaning that 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 radioactivity 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 accepted industry standard is 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

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) is about 40 Bq. The annual limit on intake–the amount that would give a person a dose equal to the worker limit, which is 20 mSv–is about 2,000,000 Bq. A counting system can also easily detect the amount of naturally occurring radioactive potassium present in all humans; 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 (≈20 cm) and sometimes lined with a thin layer of lead (≈1 cm). This shielding can significantly reduce background radiation inside the chamber, which increases the sensitivity of the instruments.

Count times and detection limit

Depending on the counting geometry of the system, counting can take between 1 and 30 minutes. The sensitivity of a counter does depend on counting time; for a single counting system, longer counting times have better detection limits. The detection limit, often referred to as the minimum detectable activity (MDA), is given by the formula: :MDA = \frac{2.707+4.65\sqrt{N}}{ET} ...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. This can be used as a trigger point for more analysis.

History

In 1950, Leonidas D. Marinelli developed and applied a low-level gamma-ray whole body counter to measure people who had been exposed to radiation, including people who had been injected with radium in the early 1920s and 1930s, people contaminated by exposure to nuclear testing and other 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 in 1952.

References

References

1. (2021-03-09). "Whole-body counter".
2. ''Operational Monitoring Good Practice Guide - The Selection of Alarm Levels for Personnel Exit Monitors.'' Industry Radiological Protection Coordination Group, NPL, UK, Dec 2009.
3. Oliver Meisenberg, Werner Buchholz, Klaus Karcher, Patrick Woidy, Udo C. Gerstmann: ''[https://www.sciencedirect.com/science/article/abs/pii/S0969806X20304680 Measuring the internal activity of the neutron emitter ²⁵²Cf in-vivo: Basics and potentials based on measurements in phantoms.]'' [[Radiation_Physics_and_Chemistry]] 176, 2020, article no. 109087.
4. [https://web.archive.org/web/20070927024522/http://hps.org/hpssc/N13_35_1999.html Bottle Manikin Absorber phantom] (web archive)
5. Kramer GH and Inn KGW. "A Summary of the Proceedings of the Workshop on Standard Phantoms for ''In-Vivo'' Radioactivity Measurement". ''Health Physics'' 61(6) (1991), pp.893-894.
6. Marinelli, L.D. 1956. The use of Na-T1 crystal spectrometers in the study of gamma-ray activity in vivo: A summary of developments at the Argonne national laboratory. Brit. Journ. of Radiol. Supplement 7 (Nov.): 38-43. (London Brit. Inst. Of Radiology)
7. Berlman, I.B. and Marinelli L.D. 1956. “Twin” scintillation fast neutron detector. Rev. Sci. Instr. 27(10) (June 25): 858-859
8. Miller, C.E., and L.D. Marinelli. 1956. The gamma-ray activity of contemporary man. Science, 124 (3212) (July 20): 122-123
9. Berlman, I.B. and Marinelli L.D. June 25, 1956. “Twin” scintillation fast neutron detector. Rev. Sci. Instr. 27(10): 858-859
10. Gustafson, P.F., L. D. Marinelli, and E. A. Hathaway. 1957. A case of accidental puncture contaminated with thorium-227: Studies on elimination and residual body activity. Radiology 68(3) (March): 358-365
11. Marinelli, L.D. Nov. 1958. Radioactivity and the human skeleton. The Janeway Lecture. Am. J. Roentgenol. & Ra. Therapy and Nuclear Medicine, 80(5):729-739
12. L.D. Marinelli (with Supplement by H.A. May). 1961. The use of low-level gamma scintillation spectrometry in the measurements of activity in human beings. Radioactivity in Man. Ed. H. Meneely, C. C. Thomas, Springfield, IL: 16-30
13. May, H.A. and L.D. Marinelli. 1962. Sodium iodide systems: Optimum crystal dimensions and origin of background. Proceedings of the Symposium on Whole Body Counting, June 12–16, 1961. International Atomic Energy Agency, Vienna: 15-40
14. 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