Conductivity Imaging in Plates Using Current Injection Tomography

The task of reconstructing an unknown distribution of electrical conductivity is widely recognized as a central theoretical problem in eddy-current nondestructive evaluation [1]. Rather than using an eddy-current method, we address this problem using DC injection of current into conductive materials. Experimental methods of the magnetic imaging of injected currents using high-resolution SQUID magnetometers have been described elsewhere [2]. In this paper we describe a tomographic method for using magnetically-imaged, injected currents to reconstruct distributions of electrical conductivity. Much of what we describe should also be applicable to data obtained using uniform colinear eddy currents induced by means of planar sheet inducers [4, 5]

Electric and magnetic fields from two-dimensional anisotropic bisyncytia

Cardiac tissue can be considered macroscopically as a bidomain, anisotropic conductor in which simple depolarization wavefronts produce complex current distributions. Since such distributions may be difficult to measure using electrical techniques, we have developed a mathematical model to determine the feasibility of magnetic localization of these currents. By applying the finite element method to an idealized two-dimensional bisyncytium with anisotropic conductivities, we have calculated the intracellular and extracellular potentials, the current distributions, and the magnetic fields for a circular depolarization wavefront. The calculated magnetic field 1 mm from the tissue is well within the sensitivity of a SQUID magnetometer. Our results show that complex bisyncytial current patterns can be studied magnetically, and these studies should provide valuable insight regarding the electrical anisotropy of cardiac tissue

Detection of a Deep Flaw Inside a Conductor Using a Squid Magnetometer

In general, eddy current techniques are suitable for finding surface-breaking flaws in conductors. Subsurface cracks are very difficult to detect due to the skin depth effect. Acoustic techniques are effective at detecting subsurface voids, but cracks immediately beneath the surface are difficult to discriminate from the surface signal. Superconducting QUantum Interference Device (SQUID) magnetometers, very sensitive instruments for measuring DC and low frequency fields, have been used for detection of flaws in conducting objects [1,2,3]. By injecting DC and low frequency AC currents into a brass bar, we have detected a subsurface flaw using a SQUID magnetometer, and shown these data to be consistent with our theoretical calculation

Sensitivity and spatial resolution of square loop SQUID magnetometers

We calculate the flux threading the pick-up coil of a square SQUID magnetometer in the presence of a current dipole source. The result reproduces that of a circle coil magnetometer calculated by Wikswo with only small differences. However it has a simpler form so that it is possible to derive from it closed form expressions for the current dipole sensitivity and the spatial resolution. The results are useful to assess the overall performance of the device and to compare different designs

Magnetic Microscopy Promises a Leap in Sensitivity and Resolution

Twenty years ago, Kirschvink argued that many paleomagnetic studies were limited by the sensitivity of the magnetometer systems then in use [Kirschvink, 1981]. He showed that sedimentary rocks could preserve detrital remanent magnetizations at levels of 10^(-14) to 10^(-15) Am^2, about 100-1000 times below the noise level of today's best superconducting (SQUID) rock magnetometers. If a more sensitive magnetometer could be built, it would dramatically expand the range and variety of rock types amenable to paleomagnetic analysis. Just such an instrument is now on the horizon: the low-temperature superconductivity (LTS) SQUID Microscope

A comparison of SQUID imaging techniques for small defects in nonmagnetic tubes

Although superconducting quantum interference devices (SQUIDs) provide an exquisitively sensitive means for measuring magnetic fields, their usage in the past has been limited chiefly to biomagnetic research. However, over the past few years interest in applying SQUID techniques to the field of nondestructive evaluation (NDE) has blossomed [1]. Many experiments have exploited the sensitivity of SQUIDs for diverse NDE applications, especially those requiring large separation distances between the sensor and the item to be inspected. Our work instead has focused on the potential to detect very small defects with SQUIDs, specifically in thin-walled tubes. In this paper, we discuss three different methods for creating magnetic fields in tubes. The methods comprise (a) directly injecting a current through the tube, (b) using a separate induction coil to create induced currents in the tube, and (c) utilizing a ferromagnetic tracer technique. To illustrate the capabilities of each method, we present two-dimensional maps of the spatial distribution of the magnetic field as measured by a SQUID magnetometer — that is, SQUID images. The images will also be used to compare the sensing methods with respect to such practical considerations as relative sensitivity and signal-to-noise ratio

Non-invasive detection of animal nerve impulses with an atomic magnetometer operating near quantum limited sensitivity

Magnetic fields generated by human and animal organs, such as the heart, brain and nervous system carry information useful for biological and medical purposes. These magnetic fields are most commonly detected using cryogenically-cooled superconducting magnetometers. Here we present the frst detection of action potentials from an animal nerve using an optical atomic magnetometer. Using an optimal design we are able to achieve the sensitivity dominated by the quantum shot noise of light and quantum projection noise of atomic spins. Such sensitivity allows us to measure the nerve impulse with a miniature room-temperature sensor which is a critical advantage for biomedical applications. Positioning the sensor at a distance of a few millimeters from the nerve, corresponding to the distance between the skin and nerves in biological studies, we detect the magnetic field generated by an action potential of a frog sciatic nerve. From the magnetic field measurements we determine the activity of the nerve and the temporal shape of the nerve impulse. This work opens new ways towards implementing optical magnetometers as practical devices for medical diagnostics.Comment: Main text with figures, and methods and supplementary informatio