Spin-Lock Imaging for Direct Detection of Oscillating Magnetic Fields with MRI: Simulations and Phantom Studies

Abstract

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.

Neural magnetic field dependent fMRI toward direct functional connectivity measurements: A phantom study

“…Furthermore, the spinlock sequence intrinsically has frequency selectivity. The combination of the signal changes with α = 90° and α ≠ 90°, therefore, could allow the detection of functional connectivity 29 . The partial spinlock sequence with α ≠ 90° is sensitive to both the amplitude and the phase of the measured magnetic fields.…”

“…Sequence and behaviour of magnetisation. Figure 1(a) shows the partial spinlock sequence based on the conventional spin-echo sequence 29 . This partial spinlock sequence consists of a spinlock module followed by a spin-echo sequence.…”

Towards robust in vivo quantification of oscillating biomagnetic fields using Rotary Excitation based MRI

“…Thus, low susceptibility to inhomogeneities combined with high sensitivity to dynamic magnetic field components is required. Most SIRS [7,11,13,14,15,16,17] and REX [4,16] studies were performed without elaborated compensation techniques. For SIRS a ramped SL technique compensating for B 1 + imperfections has been introduced [18], while for REX measurements the widely used composite SL (C-SL) technique [19] has already been applied.…”

“…have not been considered. In particular, there is evidence of a strong dependence of REX signals on the SL parameters used (e.g., choice of spin-lock pulse duration tSL) [13,23], which has not yet been considered for compensated REX techniques.…”

Towards robust in vivo quantification of oscillating biomagnetic fields using Rotary Excitation based MRI

“…Thus, low susceptibility to inhomogeneities combined with high sensitivity to dynamic magnetic field components is required. Most SIRS 7 , 11 , 13 – 17 and REX 4 , 16 studies were performed without elaborated compensation techniques. For SIRS, a ramped SL technique compensating for imperfections has been introduced 18 , while for REX measurements the widely used composite SL (C-SL) technique 19 has already been applied.…”