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  • 1
    Electronic Resource
    Electronic Resource
    Apodized pickup coils for improved spatial resolution of SQUID magnetometers (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 2439-2448 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The spatial resolution of a SQUID magnetometer is known to be determined by the size of the magnetometer pickup coil and the distance of this coil from the source of the magnetic field. With the advent of spatial-filtering techniques for converting magnetic field maps to current density images, it is clear that the quality and spatial resolution of the current image is also affected by the detailed characteristics of the transfer function between the current sources and the pickup coil. We show that the quality of a current image can be improved over that obtained with conventional pickup coils by using a multiturn pickup coil whose interturn spacing is adjusted to provide a source-coil transfer function with specific characteristics. This process is functionally equivalent to apodization techniques applied in optical image processing, in which the aperture function of the telescope is adjusted to improve spatial resolution. In coil design, the process of apodization involves designing a magnetometer coil whose transfer function has its first zero at a relatively large spatial frequency. Using apodization, we can decrease coil size and increase the number of turns without a large increase in coil inductance. Our model calculations demonstrate that apodization allows us to optimize the ability of a coil design to image a current distribution, subject to the constraints of signal-to-noise ratio, spatial resolution, and coil inductance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141336
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  • 2
    Electronic Resource
    Electronic Resource
    High-resolution high-speed synchronous epifluorescence imaging of cardiac activation (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 213-217 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An optical imaging technique with high spatial and temporal resolution was developed to record fractional changes in laser-induced epifluorescence associated with the cardiac transmembrane potential during and after the application of monophasic point stimuli. The technique takes advantage of the repeatability of the recorded events, and uses a synchronized laser strobing mechanism to overcome the speed limitation inherent to slow-scan charge-coupled device cameras, and achieves an effective frame rate of 500 frames/s at a spatial resolution of 100×100 pixels in a single frame with a pixel resolution of 75 μm. The signal-to-noise ratio can be improved with boxcar averaging. Patterns of virtual cathode and anode with distinctive regions of simultaneous depolarization and hyperpolarization during stimulation are demonstrated with stimuli applied to the resting myocardium of an isolated rabbit heart. The technique described in this article provides a powerful tool for investigating repeatable dynamics in the function of electrically active tissue. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148052
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  • 3
    Electronic Resource
    Electronic Resource
    A numerical study of the use of magnetometers to detect hidden flaws in conducting objects (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 2122-2135 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Superconducting quantum interference device magnetometers can be used to detect flaws in conducting objects by measuring the magnetic fields produced by flaw-induced perturbations in an otherwise uniform applied electric current. We examine the electric-potential and magnetic-field distributions from prolate spheroidal flaws in a homogeneous conducting plate. Beginning with Laplace's equation in prolate spherical coordinates, we derive analytical expressions for the potential distribution outside of prolate spheroidal flaws exposed to either a uniform axial or transverse applied current. The influence of the flaw over the uniform field is practically negligible for radial distances three times that of the boundary surface of the flaw. Because the magnetic field due to the perturbation of the uniform currents by the flaw arises from the discontinuity of the component of the current tangential to the surface of the flaw, the magnetic field can be evaluated numerically using the analytical expressions for the current at the flaw surface. In the far field, B falls off as the inverse square of the distance from the flaw, consistent with the flaw acting as a current dipole. The dipole strength is determined by flaw volume and orientation, and the dependence of signal strength upon the direction of the current can be used to infer the shape of the flaw. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361070
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  • 4
    Electronic Resource
    Electronic Resource
    Using a magnetometer to image a two-dimensional current distribution (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 361-372 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We describe a mathematical algorithm to obtain an image of a two-dimensional current distribution from measurements of its magnetic field. The spatial resolution of this image is determined by the signal-to-noise ratio of the magnetometer data and the distance between the magnetometer and the plane of the current distribution. In many cases, the quality of the image can be improved more by decreasing the current-to-magnetometer distance than by decreasing the noise in the magnetometer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.342549
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic shield for wide-bandwidth magnetic measurements for nondestructive testing and biomagnetism (1991)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 2654-2661 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A magnetically shielded chamber has been designed and constructed for nondestructive testing and biomagnetic measurements with a high spatial resolution SQUID magnetometer. The shield consists of two layers of 1.6-mm-thick, high permeability alloy and two layers of 1.3-cm-thick aluminum, and has interior dimensions of 0.6×1.2×1.7 m. Except at 60 Hz, the noise level above 10 Hz is 12 fT/(square root of)Hz. The major noise, due to the 60 Hz power line, has been reduced to 8–36 fT/(square root of)Hz. The thermal noise generated by the shield does not affect the measurement inside the shield. At the center of the shield, the shielding factor against the earth's static magnetic field is 500, and the ac shielding factor at 60 Hz is larger than 3×105. The results have been discussed and compared with the calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142195
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  • 6
    Electronic Resource
    Electronic Resource
    SCANNING SQUID MICROSCOPY (1999)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Materials Research 29 (1999), S. 117-148 
    Details
    ISSN: 0084-6600
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The scanning SQUID microscope (SSM) is a powerful tool for imaging magnetic fields above sample surfaces. It has the advantage of high sensitivity and bandwidth and the disadvantages of relatively modest spatial resolution and the requirement of a cooled SQUID sensor. We describe the various implementations of this type of instrument and discuss a number of applications, including magnetic imaging of short circuits in integrated circuits, corrosion currents in aluminum, and trapped flux in superconductors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.matsci.29.1.117
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  • 7
    Electronic Resource
    Electronic Resource
    Scalar multipole expansions and their dipole equivalents (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 4301-4308 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: If the source of a field that satisfies Poisson's equation can be written as the divergence of a vector s, then a scalar multipole expansion of the source can be evaluated in terms of s, which is a dipole density. A multipole expansion in terms of the dipole density can be computed about different origins. This allows us to evaluate the expansion of a dipole displaced from the origin and find a method of approximating some multipole expansions by displaced dipoles. In many physical applications it is known that the source is a displaced dipole, and we can find its location from a multipole expansion at some convenient location. It is possible to derive pictures in terms of dipole densities that in the proper limit become the individual multipoles. There are, however, ambiguities in that for some multipoles more than one picture gives the proper field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.334589
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  • 8
    Electronic Resource
    Electronic Resource
    A mathematical analysis of the magnetic field produced by flaws in two-dimensional current-carrying conductors (1992)
    Springer
    Journal of nondestructive evaluation 11 (1992), S. 89-101 
    Details
    ISSN: 1573-4862
    Keywords: Magnet field perturbation ; circular and elliptical flaws ; conjugate function approach
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics
    Notes: Abstract We examine the magnetic field produced by small flaws in a two-dimensional, conducting plate carrying an otherwise-uniform current. We use a conjugate function approach to calculate the current and voltage distributions about the circular and elliptical flaws in the conducting plate, and examine the dependence of the normal component of the magnetic field upon distance, hole size, elliptical eccentricity, and elliptical orientation. We show that when the field is calculated, far from the hole, the field falls off as 1/z 3, wherez is the distance above the plate, and as 1/r 2, wherer is the distance from the center of the hole to the observation point. We also show that for circular and elliptical flaws, the normal component of the magnetic field in the far-field region is linearly related to the area of the flaw.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00568292
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  • 9
    Electronic Resource
    Electronic Resource
    An improved method for magnetic identification and localization of cracks in conductors (1993)
    Springer
    Journal of nondestructive evaluation 12 (1993), S. 109-119 
    Details
    ISSN: 1573-4862
    Keywords: Magnetic field perturbation ; circular and elliptical flaws ; SQUID magnetometers ; background cancellation ; plates and tubes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Electrical Engineering, Measurement and Control Technology , Mathematics
    Notes: Abstract A SQUID magnetometer can be used to measure the magnetic field produced by flaws in a two-dimensional, conducting plate carrying a current. Identification of the flaw-induced magnetic field is difficult because of the large magnetic field associated with the edges of the plate and the current in the leads that connect the plate to the power supply. We have developed a technique by which the wire and edge fields can be cancelled prior to mapping the magnetic field. In this technique, a similar unflawed conducting sheet is placed adjacent to the flawed plate, with a connection between the sheet and the plate at one edge, and with the opposite edges of the sheet and of the plate connected to the two conductors of a coaxial cable. Thus, an applied current will flow along one conductor of the cable, across the cancelling sheet, cross into the flawed plate, return along the plate, and then return to the power supply along the other conductor of the coaxial cable. As a result of this geometry, there is no magnetic field from the lead-in wires because they are coaxial, and the magnetic field due to the edges of the plate is cancelled by the opposing magnetic field of the edges in the adjacent sheet. The extent of cancellation is determined primarily by the separation between the plate and the cancelling sheet, by the thickness of the plate, and by macroscopic inhomogeneities in their electrical conductivities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00567567
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  • 10
    Electronic Resource
    Electronic Resource
    Delayed Activation and Retrograde Propagation in Cardiac Muscle: Implication of Virtual Electrode Effects (2000)
    Springer
    Annals of biomedical engineering 28 (2000), S. 1318-1325 
    Details
    ISSN: 1573-9686
    Keywords: Heart ; Electrophysiology ; Bidomain ; Reentry
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Point cathodal stimulation of cardiac tissue was shown previously to produce both a dog-bone shaped virtual cathode transverse to the muscle fibers and two longitudinal virtual anodes. We hypothesize that virtual anodes can cause a region of delayed activation, separating two regions of early activation caused by the virtual cathode. Using a high-density electrode array in 42 superfused epicardial slices from 14 canine left ventricles, we observed regions of early and delayed activation and different pathways of retrograde propagation corresponding to the earlier patterns. Retrograde propagation was seen from the transversely located early activation area through areas of delayed activation toward the cathode, and from the early activation area toward the cathode directly. These pathways caused a wide dispersion in the direction of retrograde propagation (2° ± 31°, n = 179, relative to the fast axis of threshold activation; radial velocity: 0.5 ± 0.2m/s, n = 95, in 12 slices from 8 hearts with stimuli of 330 μs, 0.8–30 mA). Delayed activations were observed 0° ± 6° (n = 32) from the axis in 23 maps (at differing stimulation strengths) recorded in 13 slices from 10 hearts. We conclude that point cathodal stimulation induce delayed activation along the fiber axis and retrograde propagation both along and transverse to the axis. © 2000 Biomedical Engineering Society. PAC00: 8719Ff, 8719Hh, 8716Uv, 8754Dt, 8719Nn
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1114/1.1326029
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