Développement de l'unité centrale d'un système d'acquisition simultanée d'électroencéphalogrammes et de données de tomographie d'impédance électrique

Electrical impedance tomography (EIT) is a non-invasive technique for imaging changes in the electrical conductivity of tissues within a body section from impedance measurements performed with surface electrodes. The Institut de génie biomédical (IGB) at École Polytechnique de Montréal has been involved in the development of this technique since 1987. Recent projects of the IGB include developing a system for monitoring epileptic patients that are candidates for surgery. This system is designed to acquire EIT and electroencephalographic (EEG) data simultaneously. The rationale for combining EIT and EEG data is to improve the accuracy with which areas of the brain responsible for triggering epileptic seizures can be localized. Accurate localization of these foci allows, in many cases, to cure the patient from epilepsy by surgically removing these areas of the brain. Although various clinical tests and medical imaging procedures are presently used to localize epileptic foci, confirmation of their positions often require a direct electrophysiological investigation with implanted electrodes. This is a highly invasive procedure that carries a significant risk of hemorrhage. The system we are developing could eventually replace direct investigations with intracranial electrodes by a simple noninvasive imaging technique that can be deployed at relatively low cost. The combined EIT-EEG data acquisition system has four components: 1) an array of 24 scalp electrodes, 2) a scan head module containing the front-end circuits required to measure the impedance of cerebral tissues and record EEG signals, 3) a base station based on a field programmable gate array (FPGA) that performs all digital signal processing and system control operations, and 4) a computer that receives the data from the base station and reconstructs images of brain conductivity changes. The primary goal of this master's project was to develop the base station module. This was achieved by performing the following tasks. 1. We developed the configuration code for the FPGA of a base station module previously built by the IGB. The code was written in VHDL, a high-level language, in order to facilitate maintenance and portability to future systems. 2. The base station module was tested at the board and system level by measuring three performance indicators: a) spurious-free dynamic range (SFDR), b) signal to noise ratio (SNR), c) overall accuracy, and d) thermal drift. 3. The performance indicators measured on the new system were compared to those of an older system that uses discrete specialized chips for signal processing. 4. Preliminary in vivo tests were done with the new system.