The electrical activity of the human heart and brain are noninvasively visualized and analyzed using a multi-channel superconducting quantum interference device (SQUID) system. Multi-channel SQUID systems for magneto-cardiogram (MCG) and magneto-encephalogram (MEG) procedures have been developed by improving SQUID and cryogenic technology. Recently, we have developed two types of biomagnetic systems. The SQUID system, which utilizes a low-temperature superconductor (LTS) SQUID, is capable of detecting very weak signals, because the magnetic sensor is sensitive to signals above a few fT/ (Hz)1/2; this enables even fetal heart disease to be detected within the system's scope. A specially developed display method eases the estimation of distributed current sources. Abnormalities in the current distribution for an ischemic heart and propagation pathway for an arrhythmia are thus easily detected. Moreover, cortical functional abnormalities in patients with chronic dizziness can be visualized. A high-temperature superconductor (HTS) SQUID device system is under development as a next-generation product. The device system is designed to save space and provide mobility for the overall system, including the magnetically shielded cylinder. Due to an improvement in HTS-SQUID sensitivity and the introduction of a noise reduction technique, the real-time measurement of cardio-magnetic fields has become feasible.
A SQUID system for application to the biological immunoassay process is shown. In this system, the biological binding-reaction between an antigen and its antibody is detected using a magnetic marker and a SQUID magnetometer; that is, the binding reaction is detected by measuring the magnetic field from the marker. A so-called SQUID microscope was used in order to achieve a close distance between the cooled SQUID and the room-temperature sample. Three methods have so far been developed for measurement: susceptibility, relaxation and remanence. The measurement method is chosen by the properties of the magnetic marker. It is pointed out that a marker that is optimized for the immunoassay should be developed. For this purpose, we have developed a new marker made of an Fe3O4 particle having a diameter of 25 nm. Since the new marker can keep a remanence after a field of 0.1 T is applied, we use the remanent field of the marker to detect the binding reaction. We conducted an experiment to detect an antigen called Interleukin 8 (IL8). It was shown that the present system can detect IL8 at a weight of 0.1 pg.