P. Costa Ribeiro

Discrete spatial filtering with SQUID gradiometers in biomagnetism

First‐, second‐, and third‐order gradiometers used in detecting biomagnetic signals are analyzed as spatial filters. Their transfer functions independent of the source to be measured are presented and both the magnitude and phase characteristics of the transfer functions are analyzed. The distortion introduced by the gradiometer can be estimated from these characteristics. In order to treat the signal in that approach, the spatial Fourier transform of a magnetic signal produced by a current dipole at a given distance is discussed.

Digital filter design approach for SQUID gradiometers

A review of the traditional method for designing gradiometers is made. A nonrecursive digital filter model for the gradiometer is presented, giving a new set of parameters for the gradiometer identification. Some designs are analyzed using the proposed set. As an example, a true differentiator is designed to be used as the SQUID input coil. It is shown that the differentiator has the same noise rejection as the conventional gradiometer but provides more signal sensitivity.

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.

A survey of calibration procedures for SQUID gradiometers

The accuracy of three distinct experimental procedures for calibrating axial SQUID gradiometers has been compared, for the same gradiometer design and experimental setup. Each procedure employs a different field source, and a nonlinear least-squares optimization is used to fit the measured voltage to the theoretical field, thus determining Tesla/Volt calibration factors. We also studied the effect of noise and gradiometer imbalance on the accuracy of each procedure.

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.

Magnetic memory of oil paintings

The color in oil paints is obtained sometimes from magnetic minerals. This fact implies that oil paintings could present an intrinsic magnetization. Magnetic imaging of an oil painting consists in the measurement of magnetic flux related to the magnetization of the painting. In this report results of magnetic measurements show that oil paints from different manufacturers (Maimeri, Pebeo, and Grumbacher) are magnetic. The magnetic imagings of four similar oil paintings done with the same visual color, obtained using different mixtures of oil paints for each painting, were different. This shows that the magnetic imaging can be used as a fingerprint for the oil paintings. Magnetic imaging was obtained for oil paintings done by Oswald, Bianco, and Timoteo, all of them are Brazilian painters. The imaging obtained in measurements at different times was always the same, showing that the magnetic images are stable. This leads to the conclusion that magnetic imaging would be a common authentication technique of pa...

Magnetic Susceptibility Tomography with Nonuniform Field

Imaging systems that generate slice views of the body are called tomographic scanners. Several modalities of scanners that generate such cross sectional images have been developed for biomedical applications, such as X-ray tomography, PET and MRI. Magnetic susceptibility tomography (MST) may be a new tool for visualizing the interior of the body. Previous works have shown that it is possible to reconstruct a 4×4×4 susceptibility distribution from the perturbation produced under a uniform field. In this paper the solution to the inverse problem is addressed in the framework of reconstruction from projections. A nonuniform field is considered together with uniform field.

Detection of Magnetic Atrial Flutter Waves in Isolated Rabbit Hearts

The non-invasive character of Magnetocardiography and it’s ability to monitor electrical phenomena place the MCG technique in a very good position for studying arrhythmias like flutter and fibrillation. Although these arrhythmias have long been studied, there are still questions about depolarization current patterns (Boineau et al., 1984). The periodicity of flutter, contrasting with the more chaotic behavior of fibrillation, places the former as more suitable for analysis using a one channel system. Models of reentry currents models are more commonly used to explain flutter, but the existence of ectopic foci is not completely ruled-out in some forms of such arrhythmia.

The magnetic imaging of oil paintings

We propose a new technique for authentication of oil paintings, using a scanning SQUID technique to measure its magnetic field. The paintings are pre-magnetized in an homogeneous field of 100 G. It was observed that the response depends on the ferromagnetic properties of each paint independent of its colour. This shows that a magnetic image could be used as a magnetic signature for authentication purposes.