Sadamu Tomita

A new method for magnetoencephalography: a three-dimensional magnetometer-spatial filter system

A novel three-dimensional magnetometer-spatial filter system was developed to study human brain activity with high spatiotemporal resolution. The system combines the high temporal resolution of magnetoencephalography with the high spatial resolution achieved by using three-dimensional magnetometers and spatial filters to measure the direction and intensity of magnetic fields generated during brain activity. Simulation and phantom studies indicate that the system is capable of mapping current sources of magnetic fields with a spatial resolution comparable to that of any other brain functional imaging technique while maintaining millisecond temporal resolution. Application of this system to the human brain resolved magnetoencephalographic responses evoked by motion stimuli on a millisecond scale into responses occurring in visual cortical areas V1, V2/3 and V5. It also revealed signals related to contextual modulation in V1 and V2/3. This system provides a new way of studying the dynamics of human brain function.

Influence of head model in biomagnetic source localization

SummaryWe evaluated the influence of the head model on biomagnetic source localization by utilizing a computer simulation. We localized the source of a magnetic field that was calculated using a realistic head model, and then evaluated the localization errors. It was seen that the sphere model adequately localized the dipole in cases near the sensor, but not in cases where the dipole was deeply situated.

A 129-channel Vector Neuromagnetic Imaging System

We developed a 129-channel vector neuromagnetic imaging system detecting magnetic fields as a vector value. It is expected that further information can be obtained from vector neuromagnetic field than a conventional system. An axial vector first-order gradiometer was developed as a detection coil, which consists of three first-order differential coils wound on the surface of the same bobbin. These three coils are perpendicular to each other. In addition to a radial component of magnetic fields, two tangential components can be calculated from the measured field at each measuring point. The measuring area with a diameter of nearly 20 cm has 43 measuring points. The DC-SQUID which we developed, adopted a double-washer superconducting ring and employed week link Josephson junctions instead of the conventional tunnel junction. Therefore magnetic flux penetrating the SQUID ring is canceled. In addition, the feedback coil is directly connected with the input coil magnetically so that the current in the detection coil can be canceled by the feedback flux. Therefore a crosstalk between adjacent detection coils is negligible. The read-out electronics is based on the Direct Offset Integration Technique. The system noise in a magnetically shielded room is less than 10fT/Hz1/2 above 10Hz. We confirmed that this system could measure various evoked responses. In this report the features of this system and an application will be shown.

Moving mesh method for reconstructing some spread sources in the brain.

The purpose of this paper is to propose a new algorithm for the analysis of biomagnetic field data obtained from magnetoencephalography (MEG) measurements. This new method overcomes two major problems faced by the current method of data analysis. The first problem is the need to determine the number of sites of brain activity before calculations can be performed. The second problem is inability of the analysis to provide any information regarding the volume of the brain activity. The new data analysis method, called the Moving Mesh Method (MMM), is capable of analyzing MEG data without the need to determine the number of sources beforehand. In addition, the MMM determines the location of brain activity as a three dimensional volume, instead of as a point source of activity. The MMM uses an iterative method of calculating the position of the sources to achieve greater accuracy, and a regularized g-inverse matrix to stabilize its solution. The feasibility of the MMM was examined by two methods. First, a computer simulation was used to confirm the MMMs capability to analyzing MEG data. In the second experiment, the MMM was applied to analyze somatosensory evoked fields obtained using a new imaging system (Shimadzu Biomagnetic Imaging System, Model-100). From the interpretation of the results, we have concluded that the MMM is a feasible method of biomagnetic data analysis.

Visualization of Tonotopy of Auditory Cortex Using a Newly Developed 129 Channel Vector Magnetoencephalography

The existence of the human tonotopic organization has been revealed using the neuromagnetic measurement. These early studies on the tonotopic organization in the human were performed by measuring sequentially at many sites using the magnetoencephalography (MEG) with a few channels 1, 2, 3, so these studies needed a vast effort and time to fulfill a study. This meant that we could not use mangentoencephalography (MEG) to investigate the auditory function of patients in clinical setting. Now, many MEG systems, which are commercially available, consist of multi-channel first-order axial gradometers or planar gradiometers. These have some advantages; specially, axial gradiometer is sensitive to the radial component of magnetic fields, and planar gradiometer is favorable to obtain their tangential component. Even using these two systems, however, we could not obtain three dimensional components about magnetic fields. Consequently, we have developed a new 129 channel vector magnetometer system5).