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Recently-developed, high-resolution Superconducting QUantum Interference Device (SQUID) magnetometers can provide magnetic images of small objects with a signal-to-noise ratio and spatial resolution that are an order of magnitude better than is achievable with conventional SQUID magnetometers. The MicroSQUIDtm magnetometer has 4 channels, each of which is a differential magnetometer with a 3 mm diameter pick-up coil located less than 1.5 mm from the room-temperature region outside the Dewar. The system noise is approximately 100 fT/Hz1/2. We present magnetic field data recorded from several two-dimensional current distributions, and from the remanent magnetization in rocks, photocopier images, and magnetic contamination in metallic tubes. The system is capable of locating to better than 10 µm a straight wire carrying a 20 µA peak-to-peak sine wave current. The system can detect growth currents in a fertile chicken egg, and signals from a voluntarily-activated single motor unit in the human thumb. Magnetic fields produced by two-dimensional current distributions can be converted into current density images using Fourier-transform spatial filtering algorithms. This approach allows us to examine the difference between the imaging and localizing resolution of a magnetometer, and allows us to devise apodized pick-up coils with increased spatial resolution.
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