Needle position determined by tissue impedance

Abstract

In this project we have shown that small needle electrodes can be used to characterize tissue with high spatial resolution and determine the anatomical position of a needle for clinical use. Invasive needle electrodes have different designs for a wide range of applications. We studied the electric properties of some commercial available needle electrodes. Fundamental knowledge about impedance and current distribution in monopolar needle electrode systems were gathered. Needle electrodes were then used to measure different tissue models in-vitro and in anesthetized pigs. Data gathering was done by monopolar measurements of complex impedance. Tissue impedance and polarization impedance, sensitivity zone and capacitive properties of the electrodes were determined. In the measurements the small electrode areas gave considerable electrode polarization effects, but the impedance of small volumes adjacent to the needle tip also was reflected. In agreement with the analysis, only a few millimetres movement of the needle between tissues of different properties could give substantial changes in the measured impedance. These findings were used to develop a medical device prototype for the determination of needle position during insertion. © Springer-Verlag 2007.