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 is comprised 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.
The reason that these instruments are so sensitive if that they are often housed in low background counting chambers. Typically this is a small room with very thick walls made of 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.
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).