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.