In previous studies, the authors have shown the importance of impedance recording by stereotactic procedures for brain tumor detection. In this study they report the principles and the first results of an attempt to obtain the impedance value of any given point inside the brain by an external way. The experimental device is mainly made of a set of 128 electrodes, surrounded by an electrode guarding monitored by an operational amplifier the purpose of which is to made the electric field uniform and to enable a focused measure of the impedance by each of the 128 electrodes. The effect of this electrode guarding is demonstrated, and the present sensitivity of the prototype is shown on various obstacles. conductive or non-conductive; the influence of an impedance barrier on this sensitivity is the project of an impedometric scanner is described, and the problems due to the non-linear propagation of electric currents and to the anisotropic properties of the brain tissue are evaluated.