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

Although great efforts had been made to improve the physical phantoms used for ca- librating in vivo measurement systems, for technical reasons they can oniy provide a rough representation of human tissue. Substantial corrections mwt therefore be made to calibration factors obtained with such caiibration phantoms for extrapola- tion to a given individuai. These corrections are particularly crucial and delicate in low-energy in vivo measurement when absorption in tissue is significant. To improve caiibration for such special conditions, the posiibility has been raised of using voxe- lised numerical phantoms associated with Monte Carlo computing techniques. In the method described below, a mathematical phantom, consisting of a voxelised repre- sentation derived from scanner images is used, with a specially-designed interface making it possible to not only reconstruct widely-differing contamination confgura- tions 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 "'Am lung measurement was carried out using a tissue equivalent calibration phantom of the type commoniy 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 individuak obtained from maguetic resonance and scanner images. The resulis obtained and the current limitations of this approach in the context are discussed. Developpement de fantomes numeriques voxelises associe au code Monte Carlo MCNP : application a la mesure anthroporadiametrique. Bien que d'importants efforts aient ete realises pour ameliorer la fabrication des fan- tomes physiques servant a l'etalonnage des installations anthroporadiametnques, ils ne peuvent fournir, pour des raisons techniques, qu'une representation plus ou moins grossiere des tissus humains. Par consequent, des corrections significatives doivent etre faites sur les facteurs d'etalonnage obtenus par ces fantomes d'etalon- nage en vue d'une extrapolation a un individu donne. Ces corrections sont particu- lierement cruciales et tres delicates pour la mesure in vivo basse energie tant les absorptions dans les tissus sont significatives. Pour ameliorer l'etalonnage dans ces conditions particulieres, l'utilisation de fantomes numeriques voxelises associant les techniques de calcul Monte Carlo a deja ete evoquee. La methode presentee ici utilise un fantome mathematique, produit sous la forme de voxels reconstruit a partir RESUME