ＳＱＵＩＤ磁束計による生体磁場計測 ２． その臨床応用について

Abstract

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.