Islam Haisam, Glover Gary H
Lucas Center, Departments of Bioengineering and Radiology, Stanford University, Stanford, California, USA.
Magn Reson Med. 2016 Feb;75(2):817-22. doi: 10.1002/mrm.25650. Epub 2015 Mar 25.
Imaging using reduced FOV excitation allows higher resolution or signal-to-noise ratio (SNR) per scan time but often requires long radiofrequency pulses. The goal of this study was to improve a recent reduced field of view (FOV) method that uses a second-order shim gradient to decrease pulse length and evaluate its use in functional MRI (fMRI) applications.
The method, which was initially limited to excite thin disc-shaped regions at the isocenter, was extended to excite thicker regions off the isocenter and produced accurate excitation profiles on a grid phantom. Visual stimulation fMRI scans were performed with full and reduced FOV. The resolution of the time series images and functional activation maps were assessed using the full-width half-maxima of the autocorrelation functions (FACFs) of the noise images and the activation map values, respectively.
The resolution was higher in the reduced FOV time series images (4.1% ± 3.7% FACF reduction, P < 0.02) and functional activation maps (3.1% ± 3.4% FACF reduction, P < 0.01), but the SNR was lower (by 26.5% ± 16.9%). However, for a few subjects, the targeted region could not be localized to the reduced FOV due to the low Z2 gradient strength.
The results of this study suggest that the proposed method is feasible, though it would benefit from a stronger gradient coil.
使用缩小视野激发进行成像可在每次扫描时间内实现更高的分辨率或信噪比（SNR），但通常需要较长的射频脉冲。本研究的目的是改进一种最近的缩小视野（FOV）方法，该方法使用二阶匀场梯度来缩短脉冲长度，并评估其在功能磁共振成像（fMRI）应用中的用途。
该方法最初仅限于激发等中心处的薄盘形区域，现已扩展到激发等中心以外的较厚区域，并在网格体模上产生了准确的激发轮廓。使用全视野和缩小视野进行视觉刺激fMRI扫描。分别使用噪声图像自相关函数（FACF）的半高全宽和激活图值评估时间序列图像和功能激活图的分辨率。
缩小视野时间序列图像（FACF降低4.1%±3.7%，P<0.02）和功能激活图（FACF降低3.1%±3.4%，P<0.01）的分辨率更高，但SNR更低（降低26.5%±16.9%）。然而，对于少数受试者，由于Z2梯度强度较低，目标区域无法定位到缩小视野。
本研究结果表明，所提出的方法是可行的，尽管使用更强的梯度线圈会更有利。
