Induced Current Bio-Impedance Technique for Monitoring Cryosurgery Procedure in a Two-Dimensional Head Model Using Generalized Coordinate Systems

Abstract

In the noninvasive bio-impedance technique, small amplitude currents are applied to the body and the developing potentials on its surface are measured. This noninvasive technique is used to monitor physiological and pathological processes, which alter the values or the spatial distribution of the electrical impedance inside the human body. A possible application of the bio-impedance technique is monitoring brain cryosurgery procedure--a surgical technique that employs freezing to destroy undesirable tissues. A numerical solver was developed to evaluate the ability of an induced-current bio-impedance system to monitor the growth of the frozen tissue inside the head in simulation. The forward-problem bio-impedance solver, which is based on the finite volume method in generalized two-dimensional (2-D) coordinate systems, was validated by a comparison to a known analytical solution for body-fitted and Cartesian meshing grids. The sensitivity of the developed surface potential to the ice-ball area was examined using a 2-D head model geometry, and was found to range between 0.8 x 10(-2) and 1.68 x 10(-2) (relative potential difference/mm2), depending on the relative positioning of the excitation coil and the head. The maximal sensitivity was achieved when the coil was located at the geometrical center of the model.

Detecting cryoablation with EIT and the benefit of including ice front imaging data

“…It has enjoyed success for many years in applications at or near the body surface (Rubinsky 2000); however, attempts to use cryoablation to kill tissues deep within the body, primarily cancers, were ultimately unsuccessful until the adaptation of imaging methods that could show the physician the extents of the frozen zone during the procedure. These primarily included ultrasound (Onik et al 1991) and MRI (Gilbert et al 1993) but also recently electrical impedance tomography (EIT) (Otten andRubinsky 2000, Hartov et al 2002) and induced-current EIT (Gergel et al 2005). These techniques were vital as the surgeon otherwise had no reliable way of knowing if the frozen region was enveloping the correct volume of tissue: without intra-operative imaging, the frequent result would be either the destruction of excess healthy tissue, the incomplete ablation of the cancer or both.…”