Development of voxelised numerical phantoms using MCNP Monte Carlo code: Application to in vivo measurement

Abstract

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 humantissue.Substantial corrections must therefore be made to calibration factors obtained with suchcalibration phantoms for extrapolation to a given individual. These corrections are particularly crucialand delicate in low-energy in vivo measurement when absorption in tissue is significant. To improvecalibration for such special conditions, the possibility has been raised of using voxelised numericalphantoms associated with Monte Carlo computing techniques. In the method described below, a mathematicalphantom, consisting of a voxelised representation derived from scanner images is used, with aspecially-designed interface making it possible to not only reconstruct widely-differing contaminationconfigurations and specify associated tissue compositions, but also automatically create an MCNP4b inputfile. After validation of the different sources and geometries, the complete procedure of reconstructionof the phantom and simulation of ²⁴¹Am lung measurement was carried out using a tissue equivalentcalibration phantom of the type commonly used for lung calibration for actinides. The purpose of this workwas to extend the use of this principle to the reconstruction of numerical phantoms on the basis ofphysiological data of individuals obtained from magnetic resonance and scanner images. The resultsobtained and the current limitations of this approach in the context are discussed.