Whole body counting
Encyclopedia
In Health Physics
Whole body counting, refers to the measurement of radioactivity within the human body. The technique is only applicable to radioactive material that emit gamma
rays, although in certain circumstances, beta emitters can be measured also.
The term is also relevant when referring to practices within a morgue. The whole body count within a morgue setting literally refers to counting whole bodies.
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 and not does rely on indirect methods (such as urinalysis
) and that it can measure insoluble radionuclides in the lungs.
On the other hand, disadvantages of whole body counting are that it can only be used for gamma emitters, except in special circumstances, and it can misinterpret external contamination as an internal contamination.
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 < 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) (Kramer and Inn 1991). 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.
) 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, present in all humans and clearly not harmful as we've all got it, is also easily detectable.
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 perhaps lined with a thin layer of lead (~1 cm). The background reduction
inside the chamber will be several orders of magnitude.
. The detection limit, often referred to as the Minimum Detectable Activity (MDA), is given by the following formula
Where N is the number of counts of background in the region of interest, E = counting efficiency, and T = 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).
Health physics
Health physics is a field of science concerned with radiation physics and radiation biology with the goal of providing technical information and proper techniques regarding the safe use of ionizing radiation...
Whole body counting, refers to the measurement of radioactivity within the human body. The technique is only applicable to radioactive material that emit gamma
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
rays, although in certain circumstances, beta emitters can be measured also.
The term is also relevant when referring to practices within a morgue. The whole body count within a morgue setting literally refers to counting whole bodies.
Principle
A gamma ray is emitted from a radioactive element within the human body. If its energy is sufficient that it can escape the body before being absorbed or have any other interaction where it can lose energy, it can be detected. Usually either a scintillation detector or a semiconductor detector would be used for such purposes.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 and not does rely on indirect methods (such as urinalysis
Urinalysis
A urinalysis , also known as Routine and Microscopy , is an array of tests performed on urine, and one of the most common methods of medical diagnosis...
) and that it can measure insoluble radionuclides in the lungs.
On the other hand, disadvantages of whole body counting are that it can only be used for gamma emitters, except in special circumstances, and it can misinterpret external contamination as an internal contamination.
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 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 < 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) (Kramer and Inn 1991). 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-137Caesium-137
Caesium-137 is a radioactive isotope of caesium which is formed as a fission product by nuclear fission.It has a half-life of about 30.17 years, and decays by beta emission to a metastable nuclear isomer of barium-137: barium-137m . Caesium-137 is a radioactive isotope of caesium which is formed...
) 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, present in all humans and clearly not harmful as we've all got it, is also easily detectable.
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
Low-background steel is steel produced prior to the end of World War II. Following Atomic bombing of Hiroshima and Nagasaki, and the nuclear weapons testing during the Cold War, world background radiation levels increased substantially...
(~20 cm) and perhaps lined with a thin layer of lead (~1 cm). The background reduction
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
inside the chamber will be 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 limitDetection limit
In analytical chemistry, the detection limit, lower limit of detection, or LOD , is the lowest quantity of a substance that can be distinguished from the absence of that substance within a stated confidence limit...
. The detection limit, often referred to as the Minimum Detectable Activity (MDA), is given by the following formula
Where N is the number of counts of background in the region of interest, E = counting efficiency, and T = 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).