論文

The aim of this study was to evaluate the effectiveness of optical stimulation in optogenetically modified rats for magnetoencephalographic (MEG) measurements in small animals. Inducing such responses through external stimulation and recording them with MEG is important for elucidating the fundamental brain mechanisms. Various stimulation methods, including electrical stimulation, have been proposed, but obtaining somatosensory responses in small animals with MEG remains difficult due to mechanical noise and electromagnetic interference. In this work, we employed optical stimulation of whiskers in channelrhodopsin-2 (ChR2)-expressing rats. Optical stimulation provides precise control, localized delivery via optical fibers, and nonmagnetic compatibility, making it well suited for MEG measurements. We used optically pumped magnetometers (OPMs), which offer the advantage of operating without a helium-based cryogenic cooling system, unlike conventional superconducting quantum interference device SQUID sensors. A custom jig was developed to construct a gradiometric measurement system with five OPM sensors inside a magnetic shield room. Whiskers of ChR2 rats were stimulated with blue light (473 nm), and brain magnetic fields were recorded at 64 locations with 6.25 mm spacing. Averaged responses revealed somatosensory-evoked magnetic fields of several hundred fT at 50 ms latency, yielding reliable dipolar field maps over a 5 × 5 cm2 area.

T2 relaxation time in magnetic resonance imaging (MRI), which is determined by magnetic dipole interactions, has been employed as a parameter for tumor detection. However, owing to the lack of MRI detection sensitivity, nuclear medicine imaging is currently the basic option for tracking low concentrations of chemical probes. Previous studies utilizing cascade radionuclides have focused on the relaxation due to electric quadrupole interactions. However, magnetic dipole interactions, which are crucial in MRI, remains to be elucidated. In this study, we determined the magnetic relaxation rate by using 111In, a cascade radionuclide used in clinical single-photon emission computed tomography (SPECT) scans. As the angle between the gamma rays from the nuclei is affected by the electrical and magnetic interactions acting on the nuclei, we measured the angular correlation ratio by using eight gadolinium gallium garnet (GAGG) multi-pixel photon counter (MPPC) 8 × 8 array detectors and extracted the magnetic relaxation rate. Consequently, we obtained a rate that increased with the Fe concentration, similar to the T2 relaxation rate, although it was influenced by the inhomogeneity of the external magnetic field. This study utilized low concentrations of the liquid-state radionuclide, which is commonly used in clinical nuclear medicine imaging scans and is expected to provide much higher sensitivity and more selective detection of tumors than conventional MRI.