Biomedical subjects
Publications and source records attributed to B Rigaud.
Three parametric modelling approaches based on the Cole-Cole model are introduced. Comparison between modelling only the real part and modelling both the real and imaginary parts is carried out by simulations, in which random and systematic noise are considered, respectively. The results of modelling the in vitro data collected from sheep are given to reach the conclusions.
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.
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.
Measurement accuracy is a key point in impedance imaging and is mainly limited by factors that take place in the acquisition system. This part is a review of hardware solutions developed in acquisition systems for electrical impedance tomography (EIT). The general principles of EIT along with the changes that have taken place in the last decade, in terms of measurement strategy, and a certain number of definitions are introduced. The major hardware error sources that occur in the front end of EIT systems are presented. A review of the various alternatives published in the literature that are used to drive current, including current and voltage approaches, and the main solutions recommended in the literature to overcome the key point drawbacks of voltage measurement systems, including voltage buffers, instrumentation amplifiers, and demodulators, are provided. Some calibration procedures and approaches for the evaluation of the performance of EIT systems are also presented.
In several areas of clinical medicine, electrical impedance tomography could offer significant advantages over existing methods. These advantages have been supported by preliminary studies or by validation studies, which are described. The suggested applications are reviewed in this section. They mainly concern developments in impedance variations on brain, lung (neonatal, edema, emphysema), and heart; changes in blood volume, gastrointestinal system (gastric emptying, gastroesophageal reflux, pharyngeal transit time); pelvis (pelvis congestion); and thermal mapping in hyperthermia and breast (tissue characterization). The conductivity information at one frequency in a pixel is insufficient to take into account the very complex physiological mechanisms that underlie the observed impedance changes. To gain a better understanding of these mechanisms, research is currently being carried out on imaging of the imaginary part, parametric imaging, spectroscopic imaging, and 3D imaging, which are developed at the end of this section.
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.
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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.
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.
The main experimental problems (temperature, electrode, polarisation, non-uniformity of electric field, non-homogeneity of biological material) of impedance measurements of biological tissue are discussed. Minimisation of the inherent sources of experimental error is proposed.
In order to characterise small samples of tissue by complex impedance measurements between 10(-6) and 20 MHz, an experimental set-up is proposed. This paper describes the methodology used and presents some preliminary results.
The concepts of the point spread function (PSF) and its Fourier transform, the modulation transfer function (MTF) are introduced to evaluate an impedance imaging system. The effects of some practical factors which should be taken into account in calculating the MTF are analysed. Experimental results from the groups in Sheffield, Barcelona and Toulouse are presented.
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