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Recently, the main issue in neuroscience has been the imaging of the functional connectivity in the brain. No modality that can measure functional connectivity directly, however, has been developed yet. Here, we show the novel MRI sequence, called the partial spinlock sequence toward direct measurements of functional connectivity. This study investigates a probable measurement of phase differences directly associated with functional connectivity. By employing partial spinlock imaging, the neural magnetic field might influence the magnetic resonance signals. Using simulation and phantom studies to model the neural magnetic fields, we showed that magnetic resonance signals vary depending on the phase of an externally applied oscillating magnetic field with non-right flip angles. These results suggest that the partial spinlock sequence is a promising modality for functional connectivity measurements. [ABSTRACT FROM AUTHOR]

Recently, optically pumped atomic magnetometers (OPAMs) replacing superconductive quantum interference device-based magnetometers have attracted a considerable amount of attention. To apply the OPAMs to biomagnetic field measurements, simultaneous multilocation measurements are required. To fulfill the specific requirement, we have developed a hybrid cell, which contains potassium and rubidium atoms. The OPAM with a hybrid cell can enhance the spatial homogeneity due to the suppression of pump-beam absorption. Thus, we can have multiple measurement locations inside a hybrid cell with multiple pump and probe lasers. In this paper, we proposed a novel method to accomplish simultaneous multilocation measurements based on pump-beam modulation. Using two optical choppers, we operated a two-channel OPAM with two modulated pump beams. We investigated the signal strength of the OPAM as a function of modulation frequency and found that the signal strength decreased as the modulation frequency increased. Subsequently, we measured the magnetic field distributions generated from a loop coil to validate the proposed method and could confirm that they showed good agreements with the theoretical values. These results demonstrate the feasibility of the proposed method based on pump-beam modulation. [ABSTRACT FROM AUTHOR]

We measured the spatial homogeneity of spin polarization, which is directly related to the sensitivity and magnetic linewidth of an optically pumped atomic magnetometer with a hybrid cell of K and Rb atoms. This was done by changing the position of the probe beam. Furthermore, for comparison, we also measured the sensitivity and magnetic linewidth of atomic magnetometers with single K and single Rb cells, and found that optically thick atoms were spin polarized homogeneously with the hybrid cell. Optically thin alkali metal vapor can be spin polarized homogeneously, and the spin polarization transfers to an optically thick alkali metal vapor. An atomic magnetometer with the hybrid cell was found to be more effective in realizing simultaneous signal measurements at different locations with the uniform sensitivity and magnetic linewidth. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we describe the detection of rotating magnetic fields using optically pumped atomic magnetometers (OPAMs) for measuring magnetic resonance (MR) signals. From the results of rotating- and alternating-magnetic-field measurements, we found that to detect a rotating magnetic field with high sensitivity, the rotation direction of the magnetic field to be measured must select the bias-magnetic-field direction of OPAM. In addition, the OPAM sensitivity for rotating magnetic fields should be twice that for alternating magnetic fields. These results indicate that for measuring MR signals, magnetic fields caused by rotating magnetizations can be detected with the sensitivity of order at 1kHz using OPAMs. [Copyright &y& Elsevier]

• MR imaging simulation with spin-lock preparations. • Comparison of the simulated images with experimental ones. • Analytical solution of the Bloch equation during RF pulse and gradient field. • The simulated images with the sufficient sub-voxels agreed with the experimental ones. • GPU parallel computing is effective to avoid the impractical calculation time. Spin-lock preparation was studied to detect tiny oscillatory magnetic fields such as a neural magnetic field without the blood oxygen level-dependent effect. This approach is a direct measurement and independent of static magnetic field strength. Accordingly, it is anticipated as a feasible functional magnetic resonance imaging (fMRI) in low and ultra-low-field MRI. Several reports have been published on spin-lock preparation but reports on imaging simulation are rare. Research in this area can assist in investigating magnetic resonance signal changes and, accordingly, can help to develop new spin-lock methods. In this study, we propose an imaging simulation method with an analytical solution using the Bloch equation. To demonstrate the feasibility of our proposed method, we compared simulated images with experimental results in which the number of sub-voxels and the amplitude and phase of the target oscillatory magnetic fields varied. In addition, we also applied graphics processing unit parallel computing and investigated the feasibility of avoiding an impracticable calculation time by doing so. [ABSTRACT FROM AUTHOR]

• Comparison of two spin-lock methods for low-field functional MRI. • The analytical solution of the Bloch equation under these sequences. • The difference of the methods was clarified experimentally and theoretically. • With 0.3-T MRI scanner, we investigated the detectable strength of the two methods. We demonstrated the feasibility of the spin-lock preparation sequence using low-field magnetic resonance (MR) imaging that prevents interference from blood-oxygenation-level-dependent effects. We focused on two spin-lock preparations: spin-lock Mz (SL-Mz) and stimulus-induced rotary saturation (SIRS) and analyzed the magnetization dynamics during the sequences using the Bloch equation. Next, we performed phantom experiments using a loop coil to investigate the MR signal change as a function of the target signal strength and phase. Furthermore, we performed curve fittings to consider the radio frequency, which agreed with the experimental results. Then, we investigated the detectable strength of the magnetic field, and the SL-Mz detected a signal strength of 2.34 nT. In conclusion, our experimental results showed good agreement with the results obtained using the Bloch equation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Analytical solution of magnetization dynamics under SIRS. • Two modes in the analytical solution. • Modes depending on target oscillating field. • The solution agreed with measurements. Abstract We studied stimulus-induced rotary-saturation preparation (which enables measurement of oscillating magnetic fields using MRI) and derived an analytical solution of the Bloch equation to understand magnetization dynamics mathematically and comprehensively and to conduct simulations without sequential-calculation techniques such as the Runge-Kutta method. We formulated the dynamics using the Bloch equation, introducing an additional rotating frame and some approximations to make it into a homogeneous differential equation. Moreover, we found that there are two modes depending on the target oscillating magnetic field. To confirm the validity of the solution, we experimentally investigated its characteristics and performed curve fitting using the analytical model. Considering the constraints on the frame, the analytical solution was found to agree with experimental data. The experimental data indicate that it is necessary to design robust sequences compensating B 0 or B 1 lock spatial inhomogeneity to improve measurements. Therefore, experimenters should consider the dynamics of magnetization with RF pulses to rewind the spin phase for accurate measurements. [ABSTRACT FROM AUTHOR]

• Dependence of spin-lock-based fMRI sensitivity on target magnetic fields direction. • Spin-lock-based fMRI is insensitive to the magnetic fields perpendicular to B 0. • Same tendency was expected to appear in similar spin-lock sequences for fMRI. • Low-field MRI with pancake-type MRI scanners may overcome the issue by head rotation. Several noninvasive techniques for the direct measurement of the neuronal activity using magnetic resonance imaging (MRI) have recently been reported. As a promising candidate, we focus on a spin-lock MRI sequence (i.e., stimulus-induced rotary saturation (SIRS)) directly measuring a tiny oscillating magnetic field. Previous phantom studies on SIRS have applied the target oscillating magnetic field parallel to the direction of the static magnetic field B 0 . However, in practice, the neuromagnetic fields are not always aligned in the same direction as in such a condition. This study investigates the MR signal changes during SIRS when the target magnetic field direction is not the same as that of the B 0 field through both phantom experiments and Bloch simulations. The experimental results indicate that only the target magnetic field component along the B 0 field affects the signal change, indicating that SIRS has partial sensitivity, even if the target magnetic fields are tilted from the B 0 field. Furthermore, the simulation results show good agreements with the experimental results. These results clarify the sensitivity direction of SIRS-based fMRI and lead to the possibility that the direction of the generated neuromagnetic fields can be estimated, such that we can separate directional information from the other information contained in neuromagnetic fields (e.g., phase information). [ABSTRACT FROM AUTHOR]