Masanori Higuchi

Reduction of non-periodic environmental magnetic noise in MEG measurement by continuously adjusted least squares method

We developed a signal processing method named CALM (Continuously Adjusted Least squares Method) to reduce the non-periodical low-frequency noise during magnetoencephalography (MEG) measurements. With this method, we have successfully measured motor related cortical fields (MRCF) under daytime heavy urban noise.

Ocular dominance affects magnitude of dipole moment: an Meg study

To investigate whether the ocular dominance affects laterality in the activity of the primary visual cortex, we examined the relationship between the ocular dominance and latency or dipole moment measured by checkerboard-pattern and magnetoencephalography in 11 right-handed healthy male participants. Participants with left-eye dominance showed a dipole moment of 21.5±6.1 nAm with left-eye stimulation and 16.1±3.6 nAm with right, whereas those with right-eye dominance showed a dipole moment of 18.0±5.2 and 21.5±2.7 nAm with left-eye and right-eye stimulation of the infero-medial quadrant visual field, respectively. Thus, the dipole moment was higher when the dominant eye was stimulated, which implies that ocular dominance is regulated by the ipsilateral occipital lobe.

Calibration for a Multichannel Magnetic Sensor Array of a Magnetospinography System

A method for calibrating a multichannel superconducting quantum interference device (SQUID) magnetic sensor array of a magnetospinography (MSG) system is proposed. The MSG system detects weak magnetic fields generated by the neural activity of the spinal cord using the array of SQUID sensors. To calibrate 120 SQUID magnetic sensors, we employed an array of precisely machined circular coils to generate fundamental magnetic fields instead of employing Helmholtz coils, which are generally used for the calibration of magnetic sensors. The effective position, orientation, and sensitivity of each SQUID sensor were determined simultaneously by a parametric numerical optimization algorithm. These sensor parameters are essential for improving the accuracy of the magnetic source analysis. Using this method, we could calibrate the magnetic sensors even though they were installed in a non-transparent cryostat. In addition, the shifts from the designed position and orientation of the sensors as well as their variability were evaluated.

Development of SQUID-Based Compact Low-Field MRI System

We have developed a compact low-field magnetic resonance imaging (MRI) system that can be integrated with a magnetoencephalography system to facilitate the simultaneous measurement of magnetoencephalography and MRI for small animals. The superconducting quantum interference device gradiometer that detects the nuclear magnetic resonance signal was operated by a band-pass type flux-locked loop, and the artifact produced by the decay of the polarizing field was improved. Our MRI system includes a desktop-sized planar coil set (350 × 350 × 188 mm3). It has five pairs of coils: a pair of cylindrical coils for the polarizing field and four pairs of shielded planar coils for the measurement field and the 3-D gradient fields. On account of the grace of the coil patterns that were created by a target field method, the homogeneities of the magnetic field were below ±0.2% over 40 mm diameter sphere volume (DSV). The pulse sequence to detect the spin-echo signal was designed for 2-D Fourier imaging. A 41-mm field-of-view with a pixel size of 1.3 mm was achieved. We carried out a 2-D imaging measurement using a phantom created by a 2 × 103 mm3 NiCl2 aqueous solution and obtained a clear image.

Development of a whole-head child MEG system

A whole-head magnetoencephalography (MEG) system was developed to study cognitive processing in young children. The child MEG system has a helmet-shaped sensor array designed to fit child head sizes. The sensor array is composed of 64 LTS-SQUID axial-type gradiometric magnetometers with 50 mm of baseline length, arranged about 100 mm from the center of the child’s head. The sensor array is installed in a helmet of a horizontal dewar with a head circumference of about 530 mm, which was determined on the basis of the preliminary investigation of the standard pre-school children’s head. The liquid helium capacity of the dewar is roughly 100 liters, and the helium consumption rate is less than 6 liters/day. The sensors have been positioned in the dewar using a ship-in-a-bottle approach. To verify the performance of the child MEG system, an auditory evoked field measurement was taken of a healthy 4-year-old child subject. Large simultaneous magnetic field components corresponding to the P100m were successfully observed in the child over both the right and the left hemispheres. The latency of the effect was at around 130 ms, and two equivalent current dipoles were found in the temporal lobes in both hemispheres of the child subject.

Magnetic Marker Localization System Using a Super-Low-Frequency Signal

In this paper, a magnetic marker localization system using a super-low-frequency signal-around 100 Hz-is proposed for a surgical navigation system. This method avoids the effect of eddy currents flowing inside metallic surgical tools. The frequency dependence of the magnetic field distortion and localization error due to eddy currents were simulated to determine the optimum signal frequency. From the results of the simulation, the influence of eddy currents on the magnetic marker localization can be insignificant below 1 kHz. The prototype system used marker signals at 94 and 143 Hz and comprised 16-channel fluxgate magnetometers. Markers were successfully localized and the developed system was operational even though metal tools were present in the observation area.

A 9-channel relaxation oscillation SQUID magnetometer system integrated in a 16 mm ? 16 mm area

We have developed a nine-channel superconducting quantum interference (SQUID) magnetometer system integrated in a 16 mm × 16 mm area. Multi-loop relaxation oscillation SQUIDs (ROSs) are used as magnetometers. Typical field resolution of the ROS is 10 fT Hz−0.5 at the white region. The distance between neighbouring two magnetometers is 4 mm and the magnetometers are as close as 3 mm to subjects. Therefore, it is possible to measure magnetic fields with high spatial resolution and high S/N ratio. Using this system, we measured the magnetic signals due to the action potential propagating along the peripheral nerve of a human.

A hybrid SQUID gradiometer with 3-dimensional thin-film pick-up coils fabricated using an excimer laser

A compact hybrid superconducting quantum interference device (SQUID) gradiometer consisting of a coaxial pick-up coil with a superconducting Nb thin-film and a square double-washer dc-SQUID is developed. To make the pick-up coils, a 3-dimensional thin-film patterning technique is developed using an excimer laser. The gradiometer operates with flux locked loop electronics, a low-noise amplifier, and a high-speed (1 MHz) phase-sensitive detector. This gradiometer is sensitive to levels as low as 9 fT//spl radic/Hz.<<ETX>>

Measurement of Magnetic Resonance Signal from a Rat Head in Ultra-Low Magnetic Field

Magnetic resonance imaging (MRI) in ultra-low magnetic fields is an attractive measurement method for simultaneous detection of the anatomical images and biomagnetic signals in applications such as agnetoencephalography (MEG). To combine the ultra-low field MRI with small-animal MEG systems, we developed a compact ultra-low field MRI system based on the superconducting quantum interference device (SQUID) techniques. In this study, we demonstrate ultra-low field MRI measurement of a rat head. By comparing the obtained result with the MR image acquired by a conventional MRI system, the localized source of the MR signal measured by using our ultra-low field MRI system was determined to be in agreement with the rat brain region.

Electric Current Imaging by Ultrasonography and SQUID Magnetometry

In this paper, we propose a new method for imaging the distribution of electric currents; this method involves a combination of ultrasonography and SQUID magnetometry. To evaluate this new method, we used a block of gelatin pierced through by a lead wire as a model of a biological object. An electric current was applied to the wire and the magnetic signal generated around the wire was measured by a superconducting quantum interference device (SQUID). At the same time, an ultrasound image of the wire was taken by means of an ultrasound imaging machine. Subsequently, the magnetic field image was aligned with the ultrasound image with respect to the position of marker coils relative to the ultrasound probe. The source of the magnetic signal was localized by solving the inverse problem and visualized on the ultrasound image of the wire. The position error was 2.9 mm. The results demonstrated the applicability of the new technique combining SQUID magnetometry with ultrasound imaging.