E. Costa Monteiro

High sensitivity giant magnetoimpedance (GMI) magnetic transducer: magnitude versus phase sensing

This paper presents the results of investigations recently done by our research group that lead to a new and much better approach to the design of GMI (giant magnetoimpedance) magnetic transducers, which considers the changes in the impedance phase characteristics of GMI sensors due to varying low-intensity magnetic fields, instead of the usual impedance magnitude characteristics considered in the GMI literature. The development process of this new class of magnetic field transducers is discussed, beginning with the definition of the ideal conditioning of the GMI sensor elements (the dc level and frequency of the excitation current and the sample length), proceeding to compare the differences observed between the impedance magnitude and phase of GMI sensors and closing with the electronic circuits that condition ribbon-shaped GMI sensors and read their phase or magnitude variation as a function of the longitudinal magnetic field. Simulation studies, including the full electronic circuit and based on the experimental data obtained from measured GMI curves, have shown that an improvement in the sensitivity of GMI magnetometers larger than ten times can be expected when phase-based transducers are used instead of magnitude-based transducers. Finally, it is also shown that phase-based transducers are highly adequate for miniaturization purposes.

Detection of reentry currents in atrial flutter by magnetocardiography

The magnetic signal produced by atrial flutter induced in isolated rabbit hearts has been detected. A simple model describing a circus reentry path is discussed and fitted to the experimental data. Agreement between the simulated magnetic field obtained using the model and animal experimental results suggests that, at least in the preparation used, the presence of a reentry current can be checked and described in a zero-order approximation by the circular motion of a constant-intensity current dipole. It appears that magnetocardiography can be used as a noninvasive technique to locate and to provide information about the radius of the circuit of such reentry currents.<<ETX>>

Ring shaped magnetic field transducer based on the GMI effect

In this paper the design of a magnetic-field-to-voltage transducer based on the giant magnetoimpedance phenomenon (GMI) is proposed, characterized by an innovative geometric configuration. In order to attain the best near-field sensibility and far-field immunity, the transducers sensitive element and electronic circuit were planned and implemented. By thoroughly characterizing them it was possible to obtain an estimate of the transducers sensibility, which is approximately 12 V Oe−1. This value is comparable to those observed in two of the most important existing magnetic sensors: the fluxgate and the Hall effect sensor. The main application of the developed transducer is the localization of magnetic foreign bodies in humans, based on a previously developed and tested SQUID sensor technique. In order to provide a better interpretation of the experimental results, a theoretical model of the magnetic field associated with a needle and of the signal it generates in the transducer was created. Measurements with a needle were performed to analyze the behavior of the prototype, which has a high sensitivity, as expected, but presents strong hysteresis, lack of linearity and low immunity to uniform fields. However, despite the improvements that can still be done and have already been identified, the developed transducer has many promising applications, and has the advantage of reduced fabrication and operation costs.

Magnetic field transducers based on the phase characteristics of GMI sensors and aimed at biomedical applications

For the last four years the Laboratory of Biometrology of PUC-Rio has been working in the development of magnetic field transducers to be used in biomedical applications — especially in the three-dimensional localization of needles inserted in the human body and in the measurment of arterial pulse waves. While previous investigations were based on the behavior of the magnitude of the impedance of Giant Magnetoimpedance (GMI) ribbon-shaped sensors, this manuscript presents the preliminary results of a new research that considers the changes in the phase characteristics of GMI sensors due to varying low-intensity magnetic fields. In spite of being less explored in the literature, the work carried out so far indicates that the sensitivity of the phase can lead to more promising results than the ones already obtained with transducers based on the variation of the impedance magnitude. By means of examples showing that the sensitivity of the phase is affected by parameters (amplitude, frequency and DC level) of the AC biasing current that flows throught the sensor, this manuscript discusses how an ideal stimulation condition was derived in order to obtain more sensitive transducers. It is also examined the influence of the ribbon length in the sensitivity. A new conditioning electronic circuit — responsible for the excitation and measurement of the GMI sensor, and designed to work in the 100kHz to 5Mhz range — has been developed and is presented in the manuscript. Simulation studies of the complete transducer, including the conditioning circuit and based on data obtained from measured curves, have shown that an improvement of 10 to 100 times can be expected when compared to the sensitivity of previous magnitude-based transducers.

Multiple MLP Neural Networks Applied on the Determination of Segment Limits in ECG Signals

The electrocardiogram (ECG) has a characteristic morphology composed by various waves, corresponding to the activities in different regions of the human heart. These waves have expected ranges of duration and amplitude, and large deviations from such values indicate a series of heart diseases. This work proposes an algorithm, based on multiple multi-layer perceptron (MLP) neural networks, to automatically determine the onset and offset of each component wave, as a first step for implementing a fully automated diagnosis system. Data obtained from the MFT-BIH database have been used, comprising a series of long term measurements in patients and also manual definition of the limit points performed by clinical physicians. The results clearly show the applicability of the MLP model in this biomedical task. Also, the combination of the results provided by all trained neural networks, instead of only the best one, has proven to improve the overall performance of the system.

Medição não-invasiva de ondas de pulso arterial utilizando transdutor de pressão MIG

This manuscript presents a high sensitivity pressure transducer, developed at the Biometrology Laboratory of PUC-Rio for biomedical applications and examined in this article for the registration of arterial pulse waves. Such transducer is based on the Giant Magnetoimpedance (GMI) phenomenon and is an evolution of an unit previously developed at the same Laboratory. Knowing the main characteristics of the the GMI strips used as sensors, the configuration which should yield the highest possible sensitivity has been implemented and tested. The upgrade introduced to the original project has increased its sensitivity and enabled us to record not just the carotid arterial pulse wave (as the previous transducer configuration did), but also the radial and brachial arterial pulse.

Magnetic Measurement Techniques for Locating Foreign Bodies in Humans

There is a high incidence of needles accidentally introduced in the human body. The commonly used radiological techniques present several limitations. They usually do not give the appropriate informations for a surgical extraction procedure, like the depth relative to the skin and only the neddle projections are available. The development of a more accurate, non-invasive and innocuous method for a magnetic foreign body localization is of meaningful clinical importance. In this way, non-invasive magnetic field measurements, using a SQUID magnetometer, were performed to locate, for surgical purposes, needles lost in the bodies of 5 children. Also, another approach for detecting magnetic fields generated by foreign bodies, using a fluxgate magnetometer, is proposed.

A cellular automaton computer model for the study of magnetic detection of cardiac tissue activation during atrial flutter

A discrete cellular automaton computer model was applied to simulate the excitation wave propagation characteristic of the fundamental mechanisms of atrial flutter arrhythmia. The magnetic field produced by the different possibilities of simulated tissue activation associated to the arrhythmia has been calculated. After recording the signal as a function of time, isoamplitude maps were configured. The different computational images obtained could distinguish the various specific configurations of tissue activation.

Magnetic localisation of a current dipole implanted in dogs

In order to evaluate the difficulty in localising a current dipole due to volume conductor current contributions to the magnetocardiogram, accuracy of depth localisation of a commercial coaxial pacemaker cable, used as a current dipole, was studied in two experimental situations: immersed in a prismatic container with NaCl solution and introduced into the lower oesophagus of dogs. Isofield contour maps were obtained by interpolation of the magnetic field measured over a plane and perpendicular to it with a third-order gradiometer coupled to a SQUID. The dipole can be accurately localised in the prismatic container. The observation of an isofield map that is symmetric about the maximum-minimum axis when the dogs are in the dorsal decubitus position with the dipole in the cephalocaudal direction implies that internal inhomogeneities in the dogs volume conductor produce no appreciable effect on the magnetic field. Nevertheless, a large distortion of the magnetic field lines is observed and can be explained by calculations using models that take into account the external boundary of the volume conductor.

Point matching: A new electronic method for homogenizing the phase characteristics of giant magnetoimpedance sensors

Recently, our research group at PUC-Rio discovered that magnetic transducers based on the impedance phase characteristics of GMI sensors have the potential to multiply by one hundred the sensitivity values when compared to magnitude-based GMI transducers. Those GMI sensors can be employed in the measurement of ultra-weak magnetic fields, which intensities are even lower than the environmental magnetic noise. A traditional solution for cancelling the electromagnetic noise and interference makes use of gradiometric configurations, but the performance is strongly tied to the homogeneity of the sensing elements. This paper presents a new method that uses electronic circuits to modify the equivalent impedance of the GMI samples, aiming at homogenizing their phase characteristics and, consequently, improving the performance of gradiometric configurations based on GMI samples. It is also shown a performance comparison between this new method and another homogenization method previously developed.