Biomedical subjects
Publications and source records attributed to J Jossinet.
Over the past 150 years the study of the electrical properties of various biological tissues has been undertaken by researchers from a wide variety of scientific backgrounds. This has, unfortunately, led to the existing range of confusing and misunderstood terminology/concepts. Some of the most important are presented and explained.
The electrode/electrolyte interface impedance can be represented by the parallel combination of a non-faradaic pseudocapacitance and a faradaic, charge transfer resistance. The non-linearity of the overall electrode/electrolyte interface impedance is largely due to that of the faradaic resistance which is derived from the Butler-Volmer equation. As the charge transfer resistance dominates the interface impedance at low frequencies, it is in this region that non-linearities are first observed. The voltage limit of linearity has been investigated and found to increase gradually for higher frequencies. Although relatively linear compared with the charge transfer resistance, the non-faradaic impedance becomes non-linear at large applied voltage amplitudes and dominates the high-frequency non-linear behaviour of the overall interface impedance. Mid-frequencies are affected by a combination of the faradaic and non-faradaic non-linearities.
A method is described by which bioelectrical spectroscopy could be combined with multi-frequency electrical impedance tomography (EIT) enabling a Cole-Cole plot to be drawn for any selected pixel in the image for tissue characterization. To demonstrate the principles of the method, multi-frequency EIT measurements were simulated with a numerical model. As an example, porcine myocardium, immersed in an electrolytic tank, was simulated for a frequency range of 20 kHz to 2 MHz. The method could have an important application in transplant surgery for screening organs for viability prior to transplant.
The characterization of tissue by means of electrical impedance tomography requires accurate impedance measurements at relatively high frequencies. The present study describes the design and fabrication of broad-band, active, current electrodes for in vivo measurements. This approach eliminates the lead capacitances of the sourcing electrodes. The value of the resistors used were computed according to a protocol taking into account the constraints of a given application and the values and tolerances of the available components. Additional selection of the components further reduced the variability in circuit performance. The current sources designed according to the method described are usable at frequencies higher than 1 MHz, for differential load impedances up to 2000 omega. The parallel output conductance for each active electrode is 0.85 microS +/- 2%. The design method is valid for either adjacent or diametric drive, and can be applied to various applications by modifying the shape of the electrode.
The equivalent series resistance and reactance of the electrode-electrolyte interface impedance have both been used to detect the onset of signal amplitude induced nonlinearity. Using a theoretical model, it is shown that the choice of the most sensitive indicator depends on the phase angle of the "polarization" impedance and on the applied frequency.
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In this the second of a series of papers on the nonlinearity of the electrode-electrolyte interface impedance, the wealth of experimental observations which exists in the literature on AC impedance nonlinearity is physically interpreted. The interface impedance is well represented by the parallel combination of a constant phase angle impedance and a charge transfer resistance. The charge transfer resistance is the major source of the observed nonlinearities. As a result, the current limit of linearity, iL, increases with frequency such that iL is proportional to omega beta. The series resistance, Rs, of the interface impedance initially increases with applied signal amplitude, reaches a maximum and then decreases. The series reactance, Xs, decreases monotonically with signal amplitude.
The design and implementation of high performance differential voltmeters for semi-parallel data acquisition are described. The general requirements and specific conditions encountered in electrical impedance tomography (EIT) for accurate measurements are analysed. The major parameters are common-mode rejection and bandwidth. A specific implementation of the voltmeters, with separate DC supplies and independent signal references, is described. This arrangement, in which each voltmeter follows the input signal, automatically cancels any common-mode voltage present at the input. The signal is fed to the remainder of the instrumentation through a transformer. The use of a reduced number of components contributes to the minimisation of the inter-channel variations. Furthermore, the geometrical distribution of the voltmeters around the object minimises the length of electrode wires, also reducing the input capacitance. The number of modular voltmeters and DC/DC converters is 32. The common-mode rejection of these voltmeters is greater than 72 dB in the frequency range 3.6-560 kHz. In conclusion, the proposed solution ensures a minimisation of common-mode errors and enables the use of a 250 kHz frequency.
The electrode-electrolyte interface impedance may be simplistically modelled by an equivalent circuit comprising a resistance, RTOTAL, in series with an empirical, constant phase angle impedance, ZCPA. This pseudo-capacitance can be thought of as representing empirically the non-faradaic, double layer capacitance in the presence of specific adsorption and surface roughness effects. RTOTAL is the sum of the lead and electrolyte resistances. Depositing a thin layer of silver chloride on silver electrodes can yield improved electrical performance characteristics (potential and impedance) when used in conjunction with a chloride gel. An electrolytically deposited AgCl layer tends to have a rough surface profile thus leading to an increase in the effective interface area. This gives rise to a decrease in RTOTAL and ZCPA, both of which are desirable. Unfortunately AgCl is a relatively poor conductor. Increasing layer thickness causes RTOTAL to increase, thus adversely affecting the inter-electrode impedance at high frequencies. Electrode systems for use in electrical impedance tomography therefore require only relatively thin layers of AgCl.
Measuring the reactive component of the bio-impedance enables a full characterisation of the frequency response of a tissue. The amplitude of the reactive component is relatively small in the frequency range generally used in electrical impedance tomography (EIT). Its measurement is therefore more sensitive to errors. At higher frequencies, the amplitude of this component increases, which increases the signal-to-noise ratio. The stray capacitance, however, also increases and the front-end circuit must be designed carefully. The purpose of the present study is to show the feasibility of the collection of data at relatively high frequencies; 31.25 and 250 kHz were used. Both the real and reactive components were used to reconstruct images from capacitive targets. This study suggests that it may be possible to use multifrequency systems to determine the parameters of frequency loci and therefore tissue characterisation.
The use of a micro-computer improves the performance of an impedance cardiograph. The impedance signals and the ECG are real-time processed and four analog signals are displayed on the screen. A beat-to-beat estimation of cardiac output and several indexes are computed and displayed. User's interface is by interactive screen menus. Some facilities permit data identification, storage and post-processing. The software has been designed in such a way as to be adaptable to each specific application and to enable the development of new routines for cardiac signal processing. The signals recorded in healthy patients are in agreement with the results of other similar studies.
A system is described which can be used to collect data from 16 electrodes around a breast. The electrodes are actuated by pneumatic jacks and will accommodate breast diameters between 70 and 170 mm. The system could accommodate 32 electrodes.
The basic aim of bio-electrical multi-electrodes systems is to map the distribution of electrical parameters of the interior of the body by means of external measurements. Ideally, a perfect imaging system would be able to explore individually each point of the medium. In practice each recorded value results from the contributions of many points. For example, in classical tomography, each measurement is the integral along the line between source and detector. The reconstruction algorithm then extracts the individual value for each point of the image. The properties of electrical fields result in multi-path current flow in a medium: a given measurement not only involves the points on the line linking the electrodes, but also other points in the medium. The purpose of this work was to study the distribution of sensitivity in a model. The body was simulated by an insulating cylindrical tank of 19 cm diameter filled with water. Sixteen carbon electrodes of 6 mm diameter were placed regularly around the tank in a transverse plane. The frequency used was 20 kHz. The sensitivity at each point of the model was studied by means of a 21 mm diameter sphere of stainless steel plunged into the tank. Qualitatively the results confirmed that the sensitivity is high in the proximity of the electrodes and lower far from them. Furthermore, it is clear that the sensitivity out of the plane containing the electrodes is far from negligible. The data also allowed us to compare different electrode configurations and select those exhibiting the better sensitivity for interior points.(ABSTRACT TRUNCATED AT 250 WORDS)
Bio-electrical impedance measurements have been widely used for the study of body tissues. Apart from recordings of biophysical signals (respiration, perfusion, cardiac output, red cell settling, etc.) measurements of specific resistivity of a tissue provide information about its pathological state. Mapping electrical parameters will give more detailed information. The interpretation of recorded data and the design of equipment, both require a preliminary knowledge of the values encountered under normal and pathological conditions. The purpose of the technique described here is to determine the complex resistivity of breast tissue samples in vitro at frequencies between 0.5 kHz and 1 MHz. The equipment is described and the calibration procedure explained. A calculation of the final error interval is given. Characteristic spectra of modulus and phase angle recorded in normal and pathological breast tissue are shown. The technique can however be used for other body tissues, typical applications being fundamental tissue studies and a determination of the most suitable frequencies for use with impedance measuring devices. In clinical practice it could contribute to the determination of intra- and extracellular volume, the monitoring of transplanted organs and the examination of surgically treated tumours, and to any technique based on tissue characterization.
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A Doppler flowmeter and the necessary modifications for implantation are described in detail. Since only part of the electronics was implanted a phase-locked loop had to be introduced in order to keep the flow measurement directional. The proper working of the apparatus is demonstrated in vitro and in vivo. As an example the result of flow studies in the aorta and the pulmonary artery after homotransplantation of the lung in dogs are given.
Electrical Impedance Tomography (EIT) is an imaging technique based on multiple impedance measurements using electronically multiplexed surface electrodes. The present study describes an EIT prototype tomograph which uses two frequencies of applied signal: 31.25 kHz (within the frequency range originally used in early EIT studies) and 250 kHz. The use of the latter frequency was made possible as a result of several technical innovations including the use of separate groups of electrodes for current injection and for differential voltage measurement and the use of parallel input channels distributed around the object. In vitro images were successfully obtained using the system at the above two frequencies. It is concluded from this study that Electrical Impedance Tomography can be used at sufficiently high frequencies to enable bio-electrical tissue characterisation.