Improvements in intraoperative neurophysiological monitoring towards a wireless technology

Résumé

Intraoperative Neurophysiological Monitoring (IONM) is defined as a set of techniques consisting of recording the electrical activity generated by the nervous system structures during a surgical operation. Monitoring of the nervous system activity implies in most cases recording a large amount of electrical signals from the different structures to be controlled and/or stimulated, involving the arrangement of a substantial amount of cabling. Eliminating or diminishing this cabling is seen as a relevant technological step forward that may contribute to increasing ergonomics and easiness for the medical staff, as well as reducing possible sources of noise on the recorded signals in a more cost and time-effective manner. The main motivation of the thesis is the application of wireless technology to this branch of Neurophysiology, developing a wireless system able to pair the characteristics of the current commercial IONM systems. The main body of the thesis, made up of the compendium of three publications, entails the fundamental developments and results of this piece of research. The first one, "A Novel Passive Method for the Assessment of Skin-Electrode Contact Impedance in Intra-operative Neurophysiological Monitoring Systems", presents a new methodology to measure the contact impedance generated between the surface of the recording electrode and the tissue under examination, consisting in a passive approach (without injection of current), that takes advantage of the variation suffered by the measured signal when a resistive load is connected and disconnected in a controlled fashion at the input of the amplification circuit. The results show that it is possible to evaluate the goodness of the connection of the electrodes with this new methodology. The fact that no current injection is utilized leads to the following advantages: (1) It is more suitable for wireless battery-powered devices, since power consumption is reduced. (2) It facilitates the validation of the system according to medical device regulations. Analyzing only the registered signal for the contact impedance measurement (without additional hardware) produces the unwanted effect of long transients because of the load commutation at the input stage. To overcome this adverse effect, publication "A novel approach for the design of fast-settling amplifiers for biosignal detection" presents a new topological approach to obtain a configurable input filter, which can be changed depending on the desired measurement stage. In order to prevent saturation and covering up the wanted AC signal, the biosignal acquisition systems present a high-pass filter to eliminate the DC component produced by the contact potential of the electrodes, normally a RC net with a very low cutoff frequency. It is proposed an adjustable filter that modifies the cutoff frequency and therefore the time constant by means of a voltage control. The results obtained are similar to the ones gathered with the conventional RC filter, with the advantage of a drastic reduction of the time required for the measurement, maintaining the ability of performing the impedance measurement with the same recorded signal. Lastly, the paper "A Quasi-Wireless Intraoperatory Neurophysiological Monitoring System" outlines the design of the prototype developed, consisting of two elements that establish a wireless transparent bridge between the biosignal of interest and the monitoring device: Transmitter and Receiver, that communicate through a bidirectional radio frequency (RF) link. This connection allows sending the signal samples gathered by the Transmitter and modifying the analog input configuration. The Transmitter device is in charge of recording the signals and contains the amplification, filtering, analog/digital conversion and transmission stages. Additionally, it is equipped with the proposed impedance measurement method. The Receiver device is in charge of receiving the information sent by the Transmitter, process it and reconstruct the registered signal so that it can be recorded by conventional IONM systems. Due to the utilization of standard connectors, it is possible to utilize the devices proposed with any current commercial system. Different tests have been carried out for a broad set of monitoring techniques, including signals with different amplitude and frequency ranges. The only significant difference between the original signal and the reconstructed one is a constant delay with a value close to 1.5 ms, which is acceptable for the applications tested. The device developed not only entails a scientific progress due to the novel solutions provided, but it is also presented as an innovative component with commercial potential in the realm of IONM. Proof thereof is the fact that parts of the system proposed have been included in a patent submission that is in course of evaluation. The work described in this dissertation provides a solution to a gap in current commercial technology, consisting of the addition of wireless solutions for an improvement in ergonomics and an increased cost-time efficiency; with the aim of benefiting both the medical staff and the patients involved in surgical procedures and intraoperative monitoring.