Javier Rosell

Skin impedance from 1 Hz to 1 MHz

The impedance of skin coated with gel but otherwise unprepared was measured from 1 Hz to 1 MHz at ten sites on the thorax, leg, and forehead of ten subjects. For a 1-cm/sup 2/ area, the 1 Hz impedance varied from 10 k Omega to 1 M Omega , which suggests that the bipotential amplifier input impedance should be very high to avoid common-mode-to-differential-mode voltage conversion. The 1-MHz impedance was tightly clustered about 120 Omega . The 100-kHz impedance was about 220 Omega , which suggests that the variation in skin impedance can cause errors in two-electrode electrical impedance tomographs.<<ETX>>

Magnetic induction tomography: hardware for multi-frequency measurements in biological tissues.

Magnetic induction tomography (MIT) is a contactless method for mapping the electrical conductivity of tissue. MIT is based on the perturbation of an alternating magnetic field by a conducting object. The perturbation is detected by a voltage change in a receivercoil. At physiologically interesting frequencies (10 kHz-10 MHz) and conductivities (< 2 S m(-1)) the lower limit for the relative voltage change (signal/carrier ratio = SCR) to be resolved is 10(-7)-10(-10). A new MIT hardware has been developed consisting of a coil system with planar gradiometers and a high-resolution phase detector (PD). The gradiometer together with the PD resolves an SCR of 2.5 x 10(-5) (SNR = 20 dB at 150 kHz, acquisition speed: 100 ms). The system operates between 20 and 370 kHz with the possibility of extending the range up to 1 MHz. The feasibility of measuring conductivity spectra in the beta-dispersion range of biological tissues is experimentally demonstrated. An improvement of the resolution towards SCR = 10(-7) with an SNR of > or = 20 dB at frequencies > 100 kHz is possible. On-line spectroscopy of tissue conductivity with low spatial resolution appears feasible, thus enabling applications such as non-invasive monitoring of brain oedema.

Biological tissue characterization by magnetic induction spectroscopy (MIS): requirements and limitations

Magnetic induction spectroscopy (MIS) aims at the contactless measurement of the passive electrical properties (PEP) /spl sigma/, /spl epsiv/, and /spl mu/ of biological tissues via magnetic fields at multiple frequencies. Whereas previous publications focus on either the conductive or the magnetic aspect of inductive measurements, this article provides a synthesis of both concepts by discussing two different applications with the same measurement system: 1) monitoring of brain edema and 2) the estimation of hepatic iron stores in certain pathologies. We derived the equations to estimate the sensitivity of MIS as a function of the PEP of biological objects. The system requirements and possible systematic errors are analyzed for a MIS-channel using a planar gradiometer (PGRAD) as detector. We studied 4 important error sources: 1) moving conductors near the PGRAD; 2) thermal drifts of the PGRAD-parameters; 3) lateral displacements of the PGRAD; and 4) phase drifts in the receiver. All errors were compared with the desirable resolution. All errors affect the detected imaginary part (mainly related to /spl sigma/) of the measured complex field much less than the real part (mainly related to /spl epsiv/ and /spl mu/). Hence, the presented technique renders possible the resolution of (patho-) physiological changes of the electrical conductivity when applying highly resolving hardware and elaborate signal processing. Changes of the magnetic permeability and permittivity in biological tissues are more complicated to deal with and may require chopping techniques, e.g., periodic movement of the object.

Transmural versus nontransmural in situ electrical impedance spectrum for healthy, ischemic, and healed myocardium

Electrical properties of myocardial tissue are anisotropic due to the complex structure of the myocardial fiber orientation and the distribution of gap junctions. For this reason, measured myocardial impedance may differ depending on the current distribution and direction with respect to myocardial fiber orientation and, consequently, according to the measurement method. The objective of this study is to compare the specific impedance spectra of the myocardium measured using two different methods. One method consisted of transmural measurements using an intracavitary catheter and the other method consisted of nontransmural measurements using a four-needle probe inserted into the epicardium. Using both methods, we provide the in situ specific impedance spectrum (magnitude and phase angle) of normal, ischemic, and infarcted pig myocardium tissue from 1 kHz to 1 MHz. Magnitude spectra showed no significant differences between the measurement techniques. However, the phase angle spectra showed significant differences for normal and ischemic tissues according to the measurement technique. The main difference is encountered after 60 min of acute ischemia in the phase angle spectrum. Healed myocardial tissue showed a small and flat phase angle spectrum in both methods due to the low content of cells in the transmural infarct scar. In conclusion, both transmural and nontransmural measurements of phase angle spectrum allow the differentiation among normal, ischemic, and infarcted tissue.

Sensitivity maps and system requirements for magnetic induction tomography using a planar gradiometer.

We evaluated analytically and experimentally the performance of a planar gradiometer as a sensing element in a system for magnetic induction tomography. A system using an excitation coil and a planar gradiometer was compared against a system with two coils. We constructed one excitation coil, two different sensing elements and a high-resolution phase detector. The first sensor was a PCB square spiral coil with seven turns. The second sensor was a PCB planar gradiometer with two opposite square spirals of seven turns, with a distance between centres of 8 cm. Theoretical sensitivity maps were derived from basic equations and compared with experimental data obtained at 150 kHz. The experimental sensitivity maps were obtained measuring the perturbation produced by a brass sphere of 12 mm in empty space. The advantage of using a gradiometer is that it can be adjusted to give a minimum signal for homogeneous objects, while increasing the sensitivity to local perturbations of the conductivity. Results show that a system using a planar gradiometer as detector has less demanding requirements for the electronic system than a system using simple coils.

Bioelectrical impedance vector analysis in haemodialysis patients: relation between oedema and mortality

In this work, bioelectrical impedance vector analysis (BIVA) method is used in a sample of haemodialysis patients in stable (without oedema) and critical (hyperhydrated and malnutrition) states, in order to establish the relation between hyperhydration (oedema) and mortality. The measurements obtained were single frequency (50 kHz), tetrapolar (hand-foot) complex impedance measurements (vector components are: resistance R and reactance Xc). The impedance components were standardized by the height H of the subjects, (R/H and Xc/H) to obtain de impedance vector Z/H, that is represented in the RXc plot (abscise R/H, ordinate Xc/H). Measurements were performed on a sample of 74 patients (30 men and 44 women, 18-70 year, body mass index (BMI), 19-30 kg m(-2)) at the Saturnino Lora University Hospital in Santiago de Cuba. The 46 stable patients comprised 28 men and 18 women; the 28 critical patients 16 men and 12 women. The reference population consisted of 1196 healthy adult subjects living in Santiago de Cuba (689 men and 507 women, 18-70 year, BMI 19-30 kg m(-2)). We used the RXc plot with the BIVA method to characterize the reference population using the 50%, 75% and 95% tolerance ellipses. Students t-test and Hotellings T2-test were used to analyse the separation of groups obtained by means of clinical diagnosis and those obtained by BIVA. We obtained a significant difference (P < 0.05) in R/H, Xc/H and phase angle (PA) in men as in women between the location of Z/H vectors in the RXc graph and the separation made by the doctors between stable and critical patients. Critical (hyperhydrated) patients were located below the inferior pole of the 75% tolerance ellipse, whereas stable patients were within the tolerance ellipses. Some cases classified as stable by the clinic were classified as hyperhydrated by BIVA with 100% sensitivity and 48% specificity. In conclusion, the BIVA method could be used to classify patients by hydration state and to predict survival. Advantages of the method are its simplicity, objectivity and that it does not require the definition of patient dry weight.

A wide-band AC-coupled current source for electrical impedance tomography.

A current source suitable for application in electrical impedance tomography (EIT) is described. The first stage of the commercially available current-feedback amplifier AD844 constitutes a current-conveyor implementation and allows the construction of wide-bandwidth current sources, thus avoiding the mismatching and temperature-induced problems that arise in discrete realizations. The lack in gain accuracy of this circuit is overcome by the inclusion of its input buffer in an operational amplifier (op amp) feedback loop. Saturation problems that appear when placing a DC-blocking capacitor between the source and the electrode are solved by a DC feedback that maintains DC voltage at the output near to 0 V without reducing the output impedance of the source. Two AC-coupled current sources, in both inverting and non-inverting configurations, are described and their possible applications to EIT are listed.

Errors in prolonged electrical impedance measurements due to electrode repositioning and postural changes

Long-term electrical impedance measurements are affected by specific errors. Electrode failure, changes in its impedance due to aging, and postural changes are among the most important. We analyse errors due to electrode replacement and body postural changes. Electrode replacement errors can cause impedance changes up to 5% of basal value. This is one of the most important factors in data reproducibility. Body postural changes also contribute to impedance variations. We have proposed the use of a reference position to carry out impedance measurements as the one that shows the smallest impedance sensitivity to postural changes. In general, we observed that this is achieved with arms and legs slightly separated from the body. We propose the use of a ratio of impedance at two different frequencies to discern the origin of impedance changes, whether from physiological phenomena or postural errors.

Multi-frequency static imaging in electrical impedance tomography: Part 1. Instrumentation requirements.

Static images of the human body using electrical impedance tomography techniques can be obtained by measuring at two or more different frequencies. The frequencies used depend on the application, and their selection depends on the frequency behaviour of the impedance for the target tissue. An analysis using available data and theoretical models for tissue impedance yields the expected impedance and boundary voltage changes, therefore setting the measurement instrument specifications. The instrument errors produced by different sources are analysed, and, from this analysis it is possible to determine the feasibility of building the instrument, the limit values for some parameters (or components) and indications on the most suitable design of critical parts. This analysis also shows what kinds of error can be expected in the reconstructed images. It is concluded that it is possible to build an instrument with limited errors, allowing static images to be obtained. An instrument has been built that meets some of the design requirements and fails in others because of technological problems. In vivo images obtained with this instrument will be presented in Part 2 of this work.

Common-mode feedback in electrical impedance tomography

When a current is injected into a body, in addition to the voltage profile developed on the surface, a common-mode voltage (CMV) which produces errors in the measurement also appears. The great accuracy needed to reconstruct images in electrical impedance tomography (EIT) requires the use of differential amplifiers with a high common-mode rejection ratio (CMRR) to avoid this error. Nevertheless, the effective CMRR is lower than the differential amplifier ratio due to mismatches in the electrode impedances and other circuits in the measurement channel. The use of common-mode feedback (CMFB) is an alternative to reducing the error produced by the CMV. The stability of the feedback loop is analysed for a broadband system. Simulation and experimental results show that it is possible to obtain an improvement of 40 dB in the measurements at frequencies of up to 10 kHz.