Figure 1: Stretcher type whole body counter with 4 NaI(Tl) scintillation detectors (Bicron 20 cm diam. × 10 mm crystals) for in vivo measurement of medium energy photon emitters (100 - 3000 keV) 
Figure 2: Typical arrangement of 2 phoswich detectors (20 cm diam. 1 mm NaI(Tl) / 50 mm CsI(Tl) crystals) for in vivo measurement of low energy photon emitters such as 210 Pb, 241Am and isotopes of Pu in the lungs 
The IAEA has given guidance on the direct measurement of body content of radionuclides . Advice has also been issued by ICRU  and by Landolt-Börnstein . Direct measurement of body or organ content provides a quick and convenient estimate of activity in the body. It is feasible only for those radionuclides emitting radiation that can be detected outside the body.
Many facilities for the measurement of radionuclides in the whole body or in regions of the body consist of one or a number of high efficiency detectors housed in well-shielded, low-background environments. The geometrical configuration of the detectors is arranged to suit the purpose of the measurement, e.g. the determination of whole-body activity (Fig. 1) or of activity in a region of the body such as the thorax (Fig. 2) or the thyroid. The skull or knees may be used as a suitable site for measurement of radionuclides deposited in the skeleton and some radionuclides deposit preferentially in the liver where they can be detected. In special investigations, or in interpretation of unusual measurements, it may be advantageous to determine the distribution within the body either by profile scanning or by analysis of the relative response of detectors placed at different positions along the body.
Commonly encountered fission and activation products, such as I-131, Cs-137 and Co-60, can be detected with comparatively simple equipment at levels that are adequate for radiological protection purposes. Such simple equipment may consist of a single detector, viewing the whole body or a portion of the body, or, for iodine isotopes, a small detector placed close to the thyroid. The advantage of simple equipment is that it may be operated at the place of work, thereby avoiding the time required to visit a remote whole-body monitoring facility
In principle, the technique can be used for radionuclides that emit: x or g radiation; positrons, since they can be detected by measurement of annihilation radiation; energetic b particles that can be detected by measurement of bremsstrahlung (e.g. Y-90 for Sr-90); and the a‑emitters such as U-235 and Am-241 that can be detected by measurement of their characteristic 186 keV and 60 keV g rays respectively, or a‑emitters such as Pu-238 that can be detected by measurement of its characteristic 13, 17 and 20 keV x rays.
In contrast, high sensitivity techniques are needed for monitoring a few radionuclides at the levels that are required for protection purposes. Examples are the low energy photon emitters such as Pb-210, Am-241 and isotopes of Pu in the lungs (Fig. 2) or in the skull (Fig. 3). In all situations when Pu isotopes are unaccompanied by Am-241 they are not detectable at the levels required for radiation protection purposes. If Am-241 is present then this can provide a valuable tracer for plutonium if the Pu:Am-241 ratio is known.
Up to the mid-1990s most body activity measurement facilities, whether high‑sensitivity or simple systems, used thallium-activated sodium iodide detectors. These have the advantage that crystals of large volume can be manufactured and so provide high efficiency for detection of g-rays. Interpretation of a g-ray energy spectrum obtained from a mixture of radionuclides may, however, raise some difficulties. The components of the spectrum can be resolved by a multiple linear regression analysis technique, but this requires previous calibration of the detection equipment with standard sources of the required radionuclides dispersed in a matrix in such a way as to simulate the distribution and attenuation within the body. The increased availability of high-efficiency germanium detectors has lead to their increasing use, particularly in situations where workers may be exposed to mixtures of unknown g-ray emitting radionuclides. The superior energy resolving power of these detectors simplifies the interpretation of spectra obtained from complex mixtures of radionuclides although calibration is still needed.
The activity present in a wound can be detected with conventional g detectors if the contaminant emits energetic g-rays. In the case of contamination with a-emitting radionuclides, detection is much more difficult since the low energy x-rays that follow the a-decay will be severely attenuated in tissue; this effect is more important the deeper the wound. It is often necessary to localise the active material and this requires a well‑collimated detector. Wound monitors must have an energy discrimination capability if a good estimate is to be made of contamination with mixtures of radionuclides.
The technology on which in vivo measurement systems are based is well-established. Nevertheless, there have been a number of recent developments that offer the promise of improved capabilities. Development work is being carried out on the optimisation of the area and thickness of detectors, with particular emphasis on the use of large detector arrays. Room temperature semiconductor arrays utilising either silicon or the compound semiconductor CdZnTe offer the possibility that bulky liquid nitrogen or electrical cooling systems may no longer be necessary [5-9].
Almost all laboratories continue to use physical phantoms such as the Bottle-Mannikin-ABsorption (BOMAB, Fig. 4) or Lawrence Livermore thorax phantoms (Fig. 5) for activity calibrations, but this approach has significant limitations with respect to the body size, body shape, and radionuclide distribution that can be modelled. These limitations could in principle be overcome using the numerical calibration techniques which have been developed over recent years. Mathematical voxel phantoms are constructed using data from computed tomography (CT) or magnetic resonance imaging (MRI scans on real subjects). Monte-Carlo simulations are then used to model photon transport from the phantom and the detection of photons by a simulated detector [11-13].
Figure 3: Figure 3:
Typical arrangement of 4 HPGe for in vivo measurement of low energy photon emitters such as Pb-210, Am-241 and isotopes of Pu in the skull 
The BOMAB phantom for simulation of homogeneous activity depositions in the whole body 
The LLNL chest phantom for simulation of homogeneous activity depositions in the lungs, tracheobronchial lymph nodes and in the liver