SQUID magnetometers for biomagnetism and nondestructive testing: important questions and initial answers

Abstract
For two decades, academic and industrial researchers worldwide have used SQUID magnetometers to measure magnetic signals from the heart, brain, lungs, liver, nerves, skeletal muscle, stomach, intestines, eyes, and other organs, and have invested heavily in developing and promoting this technology. While there are as yet few accepted clinical applications of SQUIDs, various trends are encouraging. The introduction of SQUIDs to the nondestructive testing (NDT) of aircraft and other structural systems and materials is following a similar course: most of the effort is directed towards instrumentation development and demonstrations in simple systems, and instruments suitable for specific commercial applications are just now being prototyped. To assess the potential of either technology, it is useful to ask critical questions: why are we doing this, what have we learned so far, how easy is it, what does it cost, how might we best utilize advances in digital SQUIDs and high-temperature superconductivity, and what can competing technologies provide? Answers to such questions can help identify those specific technological niches for which SQUIDs are uniquely suited, and guide the optimization of SQUID systems that are targeted for particular NDT or biomagnetic measurements.