Comparison of tissue impedance measurements with nerve fiber architecture in human telencephalon: value in identification of intact subcortical structures.

OBJECT: After establishing the general possibility of real-time impedance monitoring during stereotactic procedures in five postmortem human brains, the authors proceeded to analyze the value of impedance measurements in intact cerebral structures.
METHODS: The authors performed impedance measurements at 24 selected points in the brain of a human cadaver and labeled the points by using 1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI). The texture of the nerve fibers was visualized using confocal laser scanning microscopy, which allowed classification of distinct anatomical regions in the white and gray matter according to their fiber structure (density, direction, structure, and diameter). These regions displayed characteristic differences in impedances according to their fiber structure at frequencies between 8000 and 10,000 Hz. Differences in tissue impedance are not removed, although they are shifted to higher values by fixation with formalin. The reference frequency used in clinical environments (50,000 Hz) is not useful for distinct localization purposes. Finally, the orientation of the electrode in relation to the tissue is a crucial factor.
CONCLUSIONS: It should be possible to draw conclusions about the correct position of the stereotactic needle on the basis of the proposed location, the texture of the fibers (orientation and density), and the orientation of the electrode.