At present time, numerical dosimetry has reached a certain level of maturity and dedicated commercial software are already available. However, fast and accurate characterization of exposure in real condition is still challenging, among other reasons, because the exact posture of the "victim" has to be taken into account. The classical approach is to evaluate the source magnetic field and then to perform a dosimetric computation with a postured phantom. We propose a different approach, based on a change of variable, which takes a not postured phantom to a postured one. We demonstrate that for some rigid transformation this procedure does not end up in a change of tissue conductivity, which is localized in knees, elbows and other articulations, where deformations due to posturing are large. Only the source term will need to be determined by modifying the original one through a suitable transformation.