High resolution imaging of biomagnetic fields generated by action currents in cardiac tissue using a LTS-SQUID microscope

We have developed a scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic fields of room-temperature samples with sub-millimeter resolution. In our design, we use hand-wound niobium pickup coils coupled to commercially available low temperature SQUID sensors. The SQUID sensor and the pickup coil are located in the vacuum space of the cryostat and are separated by a 25 mum sapphire window from the room-temperature sample. The spacing between the tip of the pickup coil and the sample is typically < 100 mum. We have used pickup coil diameters of 250 and 500 mum with various numbers of turns depending on the desired spatial resolution. For a 500 mum diameter pickup coil we achieved a field sensitivity of 330 fT/Hz(-12) for frequencies above 1 Hz. This allows us to record magnetocardiograms (MCGs) on the surface of an isolated Langendorff perfused rabbit heart. We recorded MCGs at 1600 locations on a 16 x 16 mm(2) grid on the surface of an isolated rabbit heart stimulated with a point electrode in the center of the grid. The MCGs were combined to produce a time series of two-dimensional magnetic field maps. which show a clear. octopolar pattern during the current injection, a similar pattern With a reversal in currents immediately after current injection, and the generation and propagation of an elliptical action-current wave front. (C) 2001 Elsevier Science B.V. All rights reserved.