Kikuo Yamaguchi

Asymmetries of prefrontal cortex in human episodic memory: effects of transcranial magnetic stimulation on learning abstract patterns.

Functional neuroimaging suggests asymmetries of memory encoding and retrieval in the prefrontal lobes, but different hypotheses have been presented concerning the nature of prefrontal hemispheric specialization. We studied an associative memory task involving pairs of Kanji (Chinese) pictographs and unfamiliar abstract patterns. Subjects were ten Japanese adults fluent in Kanji, so only the abstract patterns represented novel material. During encoding, transcranial magnetic stimulation (TMS) was applied over the left and right dorsolateral prefrontal cortex (DLPFC). A significant (P<0.05) reduction in subsequent recall of new associations was seen only with TMS over the right DLPFC. This result suggests that the right DLPFC contributes to encoding of visual-object associations, and is consistent with a material-specific rather than a process-specific model of mnemonic function in DLPFC.

Conductivity tensor imaging of the brain using diffusion-weighted magnetic resonance imaging

Conductivity tensor images of the rat brain were obtained by a method based on diffusion-weighted magnetic resonance imaging (MRI). Diffusion-weighted images were acquired by a 4.7 T MRI system with motion probing gradients (MPGs) applied in three directions. Conductivities in each MPG direction were calculated from the fast component of the apparent diffusion coefficient and the fraction of the fast component, and two-dimensional conductivity tensor was estimated. Regions of interest (ROIs) were selected in the cortex and the corpus callosum. The mean conductivities in each ROI were 0.014 S/m and 0.018 S/m, respectively. The corpus callosum exhibited higher conductivity anisotropy resulting from anisotropic tissue structures such as axons and dendrites.

A method for NMR imaging of a magnetic field generated by electric current

This paper proposes a method to detect electric currents using nuclear magnetic resonance (NMR) imaging. The motion of magnetization during applications of electric current and radio-frequency (RF) pulses was formulated based on the rotating-frame Bloch equation. The relationship between a magnetic field generated by electric current and the signal intensity of an NMR image was calculated for a Gaussian RF pulse with a duration of 10 ms. Images of a phantom were obtained with and without application of electric current to a straight wire. The magnetic field generated by the electric current caused a decrease in the signal intensity around the wire. The magnetic field measured from the images was in good agreement with the theoretically calculated magnetic field. In the case of an RF pulse duration of 10 ms, a maximum sensitivity of 3.0/spl times/10/sup 5/ T/sup -1/ was obtained at a magnetic field intensity of 2.7/spl times/10/sup -6/ T.

Current distribution image of the rat brain using diffusion weighted magnetic resonance imaging

In this article, we report that the apparent diffusion coefficient (ADC) of rat brain tissue increased by applied external electrical currents. A subtraction magnetic resonance imaging (MRI) map of ADCs with and without applied currents was obtained to quantitatively evaluate the effect of the electrical currents on ADC. The ADC in tissue was directly estimated by the mean diffusivity (MD), or the sum of the diagonal elements of the diffusion tensor Trace(D)/2. Anisotropic diffusion was estimated by fractional anisotropy FA(D). The experimental results show that the MD with applied currents of 4, 6, 8, and 10 mA increased by 6%, 8%, 16%, and 40%, respectively. The mechanism is partially explained by the movement of ions through extracellular space in tissue.

Magnetic resonance imaging of convection in an electrolyte solution and extracellular fluid associated with stationary electric currents

Convection in an electrolyte solution associated with stationary electric currents has been investigated as one of the Rayleigh–Benard type instability problems in fluid mechanics. However, details of convective flow associated with current have not yet been clarified because visualization of three-dimensional flow is difficult. In the present study, convections in a 0.9% NaCl solution and extracellular fluid were visualized using magnetic resonance imaging. Flow velocities were measured from the increase of the signal intensity based on in-flow effects of unsaturated spins. The maximum velocity with a stationary current of 1.0 mA/cm2 was 0.85 cm/s in the NaCl solution. The onset of the convection was within a few seconds after turning the current on, which depended on the distance from the electrode. The convection was observed with a current frequency lower than 2.0 Hz. In the muscle tissue of a rat, convection in extracellular fluid was observed with an injection of a 10 mA stationary current from a pa...

Multicomponent proton spin-spin relaxation of fibrin gels with magnetically oriented and randomly oriented fibrin fiber structures

We investigated the effect of structural differences in fibrin fibers on the T2 relaxation time. Fibrin fibers have the characteristic of orienting parallel to high magnetic fields during polymerization. Two fibrin gels were polymerized from a fibrinogen solution with and without a 7.05 T magnetic field. Water molecules in the fibrin gel that were polymerized in the high magnetic field exhibited only one relaxation time T2=0.35 s, whereas, water molecules in the fibrin gel that were not exposed to a magnetic field during polymerization had at least two exponential components in the T2 relaxation. The long component, T2=0.35 s, was the same order as the T2 of the fibrinogen solution (=0.41 s) and the fibrin gel polymerized in the high magnetic field. The short component was T2=0.07 s. This difference is attributed to a change in the magnetic dipole–dipole interactions between water molecules and fibrin fibers.

High-resolution magnetic resonance angiography of the rat brain: detection of Cerebral vasospasms in subarachnoid hemorrhage models

In this study, we detected cerebral vasospasms following subarachnoid hemorrhages (SAH) in rats using high-resolution magnetic resonance (MR) angiography. Measurements were performed using a 4.7 T MR imaging system with a 26-mm radiofrequency coil. Cerebral arteries were visualized based on the time-of-flight method. Optimization studies for the repetition time (TR) and the flip angle were performed to yield the best detection of the circle of Willis and the main cerebral arteries. The optimum image contrast was obtained with a TR of 60 ms and a flip angle of 60/spl deg/. The SAH was induced by intracisternal injection of autologous blood, and MR angiograms were obtained 7 days after the injection. The cerebral vasospasms following the SAH caused a decrease in the diameter of arteries. A decrease in the signal intensity of MR angiogram was observed in the main cerebral arteries due to the decrease in the diameter.

Magnetic resonance imaging of fluid motion associated with electrodeposition processes

The visualization of spatial distribution of flow is important for the study of heat and mass transfer and other industrial applications. In the present study, magnetic resonance imaging of fluid motion associated with electrodeposition processes was proposed.

Carbon Monoxide Hydrogenation Catalyzed by Zeolite-Supported Ruthenium Carbonyl Clusters in the Presence of Methyl Iodide

Ru 3 (CO) 12 supported on NaY zeolite promoted with MeI exhibited selective formation of ethane from CO and H 2 . Formaldehyde is considered to be viable as a primary reaction intermediate to hydrocarbons, in particular, ethane. The formation of ruthenium carbonyl iodide was inferred on the basis of IR spectra. Supporting the carbonyl on Al 2 O 3 or active carbon could not give a catalyst selective for the C 2 production.