Theoretical physics predicts that currents in biological media result in magnetic fields; however, the predicted fields are so small that they were measurable only recently. One of the first biomagnetic measurements was of fields associated with heart function, measured by Cohen et al. in 1970. Coil magnetometers of the kind they used are usually not sensitive enough for the detection of brain function, which is an order of 10−4 smaller than fields produced by the heart. Consequently it was not until a Josephson junction was incorporated into a superconductive quantum interference device (SQUID) that magnetometers with the required high sensitivity were available for the measurement of brain function. The SQUID is used as ultrasensitive magnetic flux detector. The problem with SQUIDs is that, in order to maintain their superconductivity, the sensor has to be cooled to the temperature of liquid helium (4.2° K). In order to accomplish these low temperatures the SQUID is immersed in liquid helium inside a helium dewar. The rf-SQUID is a superconducting ring with one Josephson junction (weak link) in it. The do-SQUID has two weak links in the ring. Flux transformers transfer flux from a sensing coil to the SQUID. For example, a closed loop of superconducting wire maintains the total magnetic flux inside the loop. If this loop contains two coils, coupled in series, a change of the magnetic flux through one of the coils causes a change in the magnetic flux in the other coil. Thus, magnetic flux is transferred from the sensing coil Ll to the signal coil Ls inside the SQUID (Figure 25.1). In order to increase the signal-to-noise ratio, differential magnetometers, referred to as gradiometers, are utilized in preference to the simple magnetometer (Figure 25.2). The first-order gradiometer has two sensing coils, Ll and L2.
