High-Resolution Magnetic Imaging: Cellular Action Currents and Other Applications

Abstract

The SQUID magnetometers used for biomagnetic studies typically have pickup coils with a 1- to 3-cm diameter that are at a comparable distance from the biological sample. While multiple measurements with such systems can be used to localize dipole-like sources with millimeter accuracy, the ability to resolve two adjacent dipoles, i.e, the imaging resolution, is limited to approximately the source-to-coil separation of 1 to 2 cm. While such imaging resolution may be sufficient for many studies on humans, it is inadequate for studies of action currents at the cellular level, where the characteristic dimensions can be on the order of several hundred microns, and for many other applications, such as nondestructive evaluation (NDE). High resolution SQUID magnetometers, with miniature pickup coils a millimeter from the sample, have been used for a wide variety of in vivo and in vitro biomagnetic measurements, such as recording magnetic fields from nerves, skeletal muscle, cardiac tissue, intestinal smooth muscle, developing embryos, and the brain. Several of these studies and numerical simulations are presented to demonstrate the potential benefits of high-resolution magnetic imaging, and to provide the basis for understanding the factors that govern the spatial resolution of SQUID images of magnetic fields. The use of simple scaling arguments demonstrates that the performance enhancements achieved by minaturization of SQUID microscopes and arrays are governed not only by the sensitivity of the SQUID sensor, but also by the rate of fall-off of the field produced by the elements of a distributed source. Techniques are reviewed for the optimization of high-resolution SQUIDs for both biomagnetism and NDE. Because of the loss in sensitivity as pickup coils or SQUIDs are made smaller, high resolution SQUID imaging will definitely benefit from the development of lower noise SQUID sensors.