It passed more than 30 years since the ultra-high vacuum technique was developed. The ultimate pressure available in an ultra-high vacuum system gradually decreased with the advent of sputter ion pumps (SIP), turbomolecular pumps (TMP) and cryopumps (CP), but still remains in the range of 10-9Pa now. Recently there are increasing number of reports for the generation of extremely high vacuum (XHV), which corresponds to the pressure less than 1×10-10Pa. The vacuum pumps used for the generation of XHV are liquid-nitrogen cooled sublimation pumps, liquid-helium and liquid-nitrogen cooled cryopumps, and helium-refrigerator cooled cryopumps. They are closely related with cryogenic engineering. We summarize recent successful works for the generation of XHV. The production technique of XHV is advancing steadily, but total and partial pressure gauges which can measure such low pressure without disturbing vacuum condition have not been developed yet, and they are strongly demanded.
The recent development of SQUID system for biomagnetic study is remarkable. The high sensitive SQUID system makes possible to study the brain function and the research for the application of biomagnetic measurement to the diagnosis is in progress. In this review, the techniques for measuring the weak magnetic fields emanating from active positions in a human body are described and the estimation of source location of the biomagnetic field is described, The SQUID sensor, magnetically shielded room, reduction of magnetic noise and multi-channel SQUID systems are presented.
CAT and MRI dramatically changed modern medicine by its excellent anatomical resolution. For functional analysis of central nervous system, however, good old EEG (electro-encephalo-graphy) is still the main weapon. Although computer topography brought some improvement in this field, poor spatial resolution has been the main shortcoming for EEG. This is because voltage distribution of neuron activity over the skull is easily influenced by the complicated human head structure. It is known that magnetic field is emitted from the brain together with electrical neuron activity. Distribution of magnetic field over the skull is less sensitive to the complicated head structure. Thus it is theoretically possible to calculate the accurate location of neuron activity by recording magnetic field over the skull. The magnetic field, however, is too weak for conventional magnetic sensors to detect. Recently SQUID (superconducting quantum interference device) has been introduced as an extremely sensitive magnetic sensor. With SQUID system, it is possible to record weak magnetic field from the brain. Multi-channel SQUID system is the main concern since accuracy of neuron activity localization and time needed for recording are dramatically improved. Study of tonotopic and amplitopic organization of human auditory cortex is presented as an example of SQUID application. Multi-channel DC-SQUID system, installed at the Center for Neuromagnetism of New York University Medical Center, was used. Auditory stimulus of various frequency and intensity was applied to the right ear in randomized order with randomized interstimulus interval. The magnetic response was recorded from left hemisphere with one probe fixed at maxima and the other probe at minima of the magnetic field. Signals were then averaged and dipole location was calculated using spherical model method. The location of auditory response was in primary auditory cortex (Area 41). The location of neuron activity shifted medially as auditory stimulus frequency increased. On the other hand, location of neuron activity tended to shift anterioly in the main as auditory stimulus intensity increased. Thus, SQUID system made it possible to analyze minute neuron network function and its location without any invasion and anesthesia to human. SQUID system has been mainly used to study auditory, visual and somatosensory response of the human brain. Magnetic signal during motor activity is also analyzed to elucidate the mechanism of motor initiation, The main current interest of SQUID application is the diagnosis of epilepsy. Magnetic signal emitted during epilepsy is large and distinctive. With SQUID system, location of epilepsy focus and its modality of spreading can be detected. Once the focus is found, it may be possible to treat epilepsy with minor surgery such as electrical cauterization without opening the skull. From the technical point of view, however, the current SQUID system is still under development. The number of channel and complicated structure may be renovated by utilizing recent IC production technology.