Although great efforts had been made to improve the physical phantoms used for calibrating
in vivo measurement systems, for technical reasons they can only provide a rough representation of human
tissue.
Substantial corrections must therefore be made to calibration factors obtained with such
calibration phantoms for extrapolation to a given individual. These corrections are particularly crucial
and delicate in low-energy in vivo measurement when absorption in tissue is significant. To improve
calibration for such special conditions, the possibility has been raised of using voxelised numerical
phantoms associated with Monte Carlo computing techniques. In the method described below, a mathematical
phantom, consisting of a voxelised representation derived from scanner images is used, with a
specially-designed interface making it possible to not only reconstruct widely-differing contamination
configurations and specify associated tissue compositions, but also automatically create an MCNP4b input
file. After validation of the different sources and geometries, the complete procedure of reconstruction
of the phantom and simulation of 241Am lung measurement was carried out using a tissue equivalent
calibration phantom of the type commonly used for lung calibration for actinides. The purpose of this work
was to extend the use of this principle to the reconstruction of numerical phantoms on the basis of
physiological data of individuals obtained from magnetic resonance and scanner images. The results
obtained and the current limitations of this approach in the context are discussed.