A functional magnetic resonance imaging (fMRI) method that focuses on neural magnetic fields has great potential to detect neural activities more directly than the conventional method. Because this fMRI method does not depend on blood-oxygenation-level-dependent contrast, improved temporal and spatial resolutions can be expected. Among various approaches of this fMRI method, the one that uses a spin-lock imaging sequence has attracted wide attention because of the possibility to detect small oscillating magnetic fields. To understand the mechanism of this approach, we visualized magnetization behavior during the spin-lock module with externally applied oscillating magnetic fields. A fast-and-simple method with matrix operations was used to solve a time-dependent Bloch equation. In addition, we investigated the influence of the duration of the spin-lock pulse in the spin-lock module, which interacts with the external oscillating magnetic fields, on magnetic resonance signals. Furthermore, to detect minute magnetic fields in the order of sub-nT, we carried out phantom studies on the practical use of this method as an fMRI approach. A single-loop coil generating oscillating magnetic fields was placed inside a saline-filled phantom. Time-dependent performance of magnetization during the spinlock module was thus visually demonstrated to aid understanding of the mechanism of the fMRI method with the spin-lock imaging sequence. In addition to this visualization, we found that the decrease in magnetization depends on the duration of the spin-lock pulse. Longer durations are appropriate for detecting minute sub-nT magnetic fields such as neural magnetic fields. Furthermore, we were able to detect magnetic fields of approximately 200 pT by choosing a spin-lock pulse of long duration and increasing the number of MR image acquisitions. Our results provide useful information for the understanding of the mechanism of direct detection of oscillating neural magnetic fields using MRI with a spin-lock imaging sequence. In addition, we propose an improved selection scheme for the duration of the spin-lock pulse and the feasibility of detecting oscillating magnetic fields of 200 pT considering practical application of fMRI.
DiŠusion tensor imaging (DTI) is a magnetic resonance (MR) imaging technique that has attracted attention in recent years for applications such as nerveˆber tracking, neurography, and tumor detection. In DTI measurements, 2 motion-probing gradient (MPG) pulses are applied to evaluate water diŠusion. In DTI for nerveˆber tracking, acquisition parameters, such as strength, duration, and separation of MPGs, in‰uence the MR signal.In this study, we set acquisition parameters in DTI to emphasize fractional anisotropy to clarify the direction of nerveˆbers. We performed Monte Carlo simulations of restricted diŠusion in a cylinder model and phantom measurements with capillary plates to examine the relationship between the acquisition parameters in DTI and the size of restricted structures, particularly their diameter and length, which we will refer to as``compartment size.' ' We conˆrmed that normalized signal intensities in DTI measurements depend on diŠu-sion time, which, in turn, depends on the separation and duration of the MPG, and they decrease with increase in compartment size. Furthermore, our simulation and phantom results suggest that use of a longer diŠusion time eŠectively emphasizes fractional anisotropy to clarify the direction of nerveˆbers.
A new MRI method using the spin-lock sequence has attracted wide attention because of its potential for detecting small oscillating magnetic fields. However, as the mechanism involved is complicated, we visualized the magnetization performance during the spin-lock sequence in order to better understand interaction of the spin-lock pulse and the externally applied oscillating magnetic fields by means of a fast-and-simple method using matrix operations to solve a time-dependent Bloch equation. To improve spin-lock imaging in the detection of small magnetic fields (in an fMRI experiment that modeled neural magnetic fields), we observed that the phenomenon decreases MR signals, which led us to investigate how spin-lock parameters cause the MR signal to decrease; based on this, we determined that MR signals decrease in oscillating magnetic fields that are resonant with the spin-lock pulse. We also determined that MR signals decrease is directly proportional to spin-lock duration. Our results suggest that MRI can feasibly detect oscillating magnetic fields directly by using of the spin-lock sequence.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.