应用集成射频/匀场线圈技术恢复 fMRI 中的信号。
Application of an integrated radio-frequency/shim coil technology for signal recovery in fMRI.
机构信息
Brain Imaging and Analysis Center, Duke University, Durham, North Carolina, USA.
Medical Physics Graduate Program, Duke University, Durham, North Carolina, USA.
出版信息
Magn Reson Med. 2021 Dec;86(6):3067-3081. doi: 10.1002/mrm.28925. Epub 2021 Jul 20.
PURPOSE
Gradient-echo echo-planar imaging (EPI), which is typically used for blood oxygenation level-dependent (BOLD) functional MRI (fMRI), suffers from distortions and signal loss caused by localized B inhomogeneities. Such artifacts cannot be effectively corrected for with the low-order spherical harmonic (SH) shim coils available on most scanners. The integrated parallel reception, excitation, and shimming (iPRES) coil technology allows radiofrequency (RF) and direct currents to flow on each coil element, enabling imaging and localized B shimming with one coil array. iPRES was previously used to correct for distortions in spin-echo EPI and is further developed here to also recover signal loss in gradient-echo EPI.
METHODS
The cost function in the shim optimization, which typically uses a single term representing the B inhomogeneity, was modified to include a second term representing the signal loss, with an adjustable weight to optimize the trade-off between distortion correction and signal recovery. Simulations and experiments were performed to investigate the shimming performance.
RESULTS
Slice-optimized shimming with iPRES and the proposed cost function substantially reduced the signal loss in the inferior frontal and temporal brain regions compared to shimming with iPRES and the original cost function or 2 -order SH shimming with either cost function. In breath-holding fMRI experiments, the ΔB and signal loss root-mean-square errors decreased by -34.3% and -56.2%, whereas the EPI signal intensity and number of activated voxels increased by 60.3% and 174.0% in the inferior frontal brain region.
CONCLUSION
iPRES can recover signal loss in gradient-echo EPI, which is expected to improve BOLD fMRI studies in brain regions suffering from signal loss.
目的
梯度回波回波平面成像（EPI）通常用于血氧水平依赖（BOLD）功能磁共振成像（fMRI），但会受到局部 B 不均匀性引起的失真和信号损失的影响。此类伪影无法通过大多数扫描仪上可用的低阶球谐（SH）匀场线圈有效校正。集成的并行接收、激发和匀场（iPRES）线圈技术允许每个线圈元件中流动射频（RF）和直流电，从而可以使用一个线圈阵列进行成像和局部 B 匀场。iPRES 先前曾用于校正自旋回波 EPI 的失真，在此基础上进一步开发，以恢复梯度回波 EPI 中的信号损失。
方法
在匀场优化中的成本函数中，通常使用代表 B 不均匀性的单个项，修改为包含代表信号损失的第二个项，可调权重以优化失真校正和信号恢复之间的权衡。进行了模拟和实验研究以研究匀场性能。
结果
与使用 iPRES 和原始成本函数的匀场相比，使用 iPRES 和提出的成本函数的切片优化匀场可显著降低下额和颞叶脑区的信号损失。与使用任何成本函数的 2 阶 SH 匀场相比，iPRES 和提出的成本函数的切片优化匀场可显著降低下额和颞叶脑区的信号损失。在屏气 fMRI 实验中，ΔB 和信号损失均方根误差分别降低了-34.3%和-56.2%，而在下额脑区 EPI 信号强度和激活体素数量分别增加了 60.3%和 174.0%。
结论
iPRES 可以恢复梯度回波 EPI 中的信号损失，这有望改善受信号损失影响的脑区的 BOLD fMRI 研究。
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