Neuronavigation systems are usually subject to inaccuracy due to intraoperative changes like brain shift or tumor resection. In order to correct for these deformations a biomechanical model of the brain is proposed. Not only elastic tissues, but also fluids are modeled, since an important volume of the head contains cerebrospinal fluid, which does not behave like soft tissues. Unlike other approaches, we propose to solve the differential equations of the model by means of the boundary element method, which has the advantage of only considering the boundaries of the different biomechanically homogeneous regions. The size of the matrix to invert is therefore drastically reduced. Finally, our method is assessed with sequences of intraoperative MR images, showing better performances for the elastic/fluid model than for the purely elastic one.