Resistor mesh model of a spherical head: part 1: applications to scalp potential interpolation.

A resistor mesh model (RMM) has been implemented to describe the electrical properties of the head and the configuration of the intracerebral current sources by simulation of forward and inverse problems in electroencephalogram/event related potential (EEG/ERP) studies. For this study, the RMM representing the three basic tissues of the human head (brain, skull and scalp) was superimposed on a spherical volume mimicking the head volume: it included 43 102 resistances and 14 123 nodes. The validation was performed with reference to the analytical model by consideration of a set of four dipoles close to the cortex. Using the RMM and the chosen dipoles, four distinct families of interpolation technique (nearest neighbour, polynomial, splines and lead fields) were tested and compared so that the scalp potentials could be recovered from the electrode potentials. The 3D spline interpolation and the inverse forward technique (IFT) gave the best results. The IFT is very easy to use when the lead-field matrix between scalp electrodes and cortex nodes has been calculated. By simple application of the Moore-Penrose pseudo inverse matrix to the electrode cap potentials, a set of current sources on the cortex is obtained. Then, the forward problem using these cortex sources renders all the scalp potentials.

Resistor mesh model of a spherical head: part 2: a review of applications to cortical mapping.

A resistor mesh model (RMM) has been validated with reference to the analytical model by consideration of a set of four dipoles close to the cortex. The application of the RMM to scalp potential interpolation was detailed in Part 1. Using the RMM and the same four dipoles, the different methods of cortical mapping were compared and have shown the potentiality of this RMM for obtaining current and potential cortical distributions. The lead-field matrices are well-adapted tools, but the use of a square matrix of high dimension does not permit the inverse solution to be improved in the presence of noise, as a regularisation technique is necessary with noisy data. With the RMM, the transfer matrix and the cortical imaging technique proved to be easy to implement. Further development of the RMM will include application to more realistic head models with more accurate conductivities.

Ex vivo discrimination between normal and pathological tissues in human breast surgical biopsies using bioimpedance spectroscopy.

Ex vivo bioimpedance data measured on normal and cancerous female breast tissues are reported. They clearly show that the electrical properties of normal tissues, surrounding tissues, and carcinoma are different. These differences lie in the conductivity, in the characteristic frequency (frequency of the maximum of the imaginary part of the bioimpedance), and also in the shape of the Bode plots. Modeling using an R-S-Zcpe model is reported as well as indexes extracted from the real and imaginary parts of the bioimpedance. Even if a classification of the different types of tissues remains a difficult task and leads to much less precise diagnosis than microscopic examination, the electrical behavior of mammary tissue could be used to develop a noninvasive technique for early breast cancer detection.

A multifrequency serial EIT system.

A multifrequency (1 kHz-1 MHz) serial electrical impedance tomography (EIT) system has been developed. It is based on 16 active electrodes and can be extended up to 32. Each active electrode can be programmed for current driving and for measuring either the injected current or the voltage difference between adjacent electrodes, and includes calibration facilities. Real and imaginary parts of the impedance are obtained by applying a parametric identification method (extended Prony), but other techniques are easily adaptable. Image reconstruction is carried out using the Sheffield filtered back-projection algorithm. Characteristic frequency images are under development and should be of great interest to distinguish between normal and tumorous tissues.

Bioelectrical impedance techniques in medicine. Part I: Bioimpedance measurement. Second section: impedance spectrometry.

Electrical impedance spectrometry is an important application field of bioimpedance measurements. After introducing the electrical properties of biological tissues, this part presents instrumental aspects and applications of electrical impedance spectrometry. The main instrumental constraints encountered in spectrometric electrical impedance measurements are reviewed, focusing on low-frequency applications. Examples of impedance cells and probes are presented and several instrumental setups operating in the frequency and time domain are described. Some examples of applications are presented, including in vitro characterization and modeling of normal tissues, in vitro and in vivo characterization of cancerous tissues, and assessment of tissue perfusion/ischemia levels.

In vitro tissue characterization and modelling using electrical impedance measurements in the 100 Hz-10 MHz frequency range.

In vitro electrical impedance spectrometry was performed on tissue samples excised from sheep. Measured data have been processed to reduce dispersion in measurements and to provide criteria useful for tissue comparison. Two electrical models are proposed for tissues exhibiting a one-circle impedance locus and a two-circle impedance locus. Measurement results and electrical parameters of tissues and models fitted to experimental data are presented. Model sensitivity to parameter variations is discussed.

Closed loop control of PaCO2 during ECC.

A prototype for the control of blood gases during extracorporeal circulation (ECC) has been built. It is composed of a CDI300 continuous gas blood analyzer, a programmable gas blender and an IBM PC. The air blender is composed of three mass flow controllers (air, oxygen and total flow rate). The microcomputer commands these controllers in order to obtain the desired mixture of air and oxygen. The system acquires the data sent by the blood gas analyzer every 6 seconds and commands the gas blender in order to maintain the desired arterial dioxide partial pressure (PaCO2) in the arterial line. The first experimental results on sheep show that if blood gas parameters are stable when beginning the closed loop control, then the desired PaCO2 value may be obtained. Otherwise, the value of PaCO2 under control is slightly different but a stable value is obtained after 10 minutes. More experiments must be done in order to establish the real limits of such a system and optimize the gain of the control system.

Tissue characterization by impedance: a multifrequency approach.

Two experimental set-ups for in vitro characterization of electrical bio-impedance are described. The first one, based on a commercially available instrument, operates in the frequency range 1 Hz-10 MHz. The second one uses an identification process and operates in the frequency range 1 Hz-1 MHz. Some results are presented and discussed in the context of multifrequency electrical impedance tomography.

Experimental acquisition system for impedance tomography with active electrode approach.

An experimental system for impedance tomography has been constructed. The acquisition system uses 16 multifunctional active electrodes, each including a current source and a voltage buffer. Images of active and reactive parts of different target impedances in a phantom filled with liquid have been obtained. The system performance has been compared with those of other systems using either a mesh phantom or rods as point sources used for the determination of the modulation transfer function.

Automatic modules for extracorporeal circulation control.

Maintenance of a constant level in the oxygenator during surgical extracorporeal circulation (ECC) is of paramount importance. We propose a device able to regulate the pump speed accordingly to the level in the oxygenator, and an automatic clamp to control venous return from the patient. The level regulator permits pump speed control and also stops the pump when a prefixed low level is reached (during ECC). Linearization of the level sensor has been made in order to optimize the PI regulator. Simulation results are presented and the regulator is evaluated in a clinical environment. The automatic clamp provides a good solution for reproducible and easy transitions in venous return. When coupled with the level regulator, the pump speed will increase automatically from zero to the correct equilibrium speed, with variable duration (from 30 seconds to 5 minutes) chosen by the operator. The same technique is used for stopping the ECC. These two devices are, in fact, modules, making up part of an evolutive device for ECC automation. The modules can exchange data with a supervising microcomputer.

Evaluation of extracorporeal blood heat exchanger devices.

An accurate and easy to use test circuit has been developed for the performance evaluation of the heat exchangers used in cardiac surgery during extracorporeal circulation. The water part of the heat exchanger is fixed at a flow rate of 15 l/min and at a temperature of 40 degrees C. In the blood side, the flow rate is varied from 1 to 6 l/min and the temperature is 30 degrees C. The use of a precise thermometer (precision 0.01 degrees C) and of a microcomputer permits efficiency with a good reproducibility. The results obtained show a real improvement in the efficiency of the new extracorporeal blood heat exchangers devices.

[An electronic visual aid using a luminance amplifier in night vision disorders]. / Aide visuelle électronique par amplificateur de luminance dans les altérations de la vision nocturne.

The efficiency of a light amplifier system for the improvement of night vision has been tested on 30 normal subjects and on subjects suffering from night blindness including 36 patients with pigmentary retinopathy, 18 myopic patients with night blindness and 12 patients with senile cataracts. In all cases, the scotopic vision presented a great improvement and it reached up to half of photopic vision. The use of the light amplifier would certainly provide a considerable improvement for subjects suffering from night blindness and mostly for those suffering from pigmentary retinopathy. But two conditions are necessary for this: miniaturization of the device and absence of major macular lesions in the patient.

[Free-living Amoebae in fresh water. Study of the water supply of the town of Poitiers (France) (author's transl)]. / Recherche des amibes libres dans l'eau. Etude du réseau d'alimentation en eaux de la ville de Poitiers.

46 out of 76 samples of water originating from the water supply of the town of Poitiers, and concerning each stage of it, were found positive to free-living amoebae. 3/4 of the isolated strains belonged to the genus Acanthamoeba only 2 to the genus Naegleria, and the remaining to the genus Hartmanella. All the strains are insensible to 5-Fluoro-cytosin and Amphotericin B at concentrations compatible with human use. No experimental pathogen power could be demonstrated by inoculation to laboratory mice.