Impedance Measurements and Tissue Characteristics of the Human Brain: Real-Time Impedance Monitoring During Stereotactic Procedures

Impedance measurements in intact cerebral structures of human cadaver brains were performed to analyze the potential of this method to verify the position of the needle during a stereotactic intervention. Experiment 1: Impedance measurements were performed at 24 selected points in human cadaver brains. The points were labeled using 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. These regions displayed characteristic differences in impedances according to their fiber structure at frequencies between 8000 and 10,000Hz. The reference frequency used in clinical environments (50,000Hz) is not useful for distinct localization purposes. Finally, the orientation of the electrode in relation to the tissue is a crucial factor. Experiment 2: Using a vector network analyzer, transmission and reflection coefficients were measured from 500MHz to 18 GHz in four formalin-fixed human brains. The positions of the electrodes were marked, and the tissue was histologically stained to visualize the myelo- and the cytoarchitecture as well as the nerve fiber orientation at the electrodes. The profiles of the transmission coefficients showed characteristic minimum peaks. In order to describe these peaks, a mathematical function was fitted. Parameters derived from digital image processing were used to characterize the myelo- and cytoarchitecure of the tissue at the electrodes. A multiple regression model, with the frequency at the transmission peak minimum as a dependent variable and two tissue characteristics at the two electrodes as independent variables, showed a multiple regression coefficient of 0.765. A neural network model was able to estimate the frequency at the transmission peak minimum from the tissue characteristics at the electrode. Conclusions: The measurements of dielectric properties are well suited to differentiate distinct intracerebral structures. The method could be used for online monitoring of the needle's position during a stereotactic intervention in neurosurgery.