Jesmanowicz A, Bandettini P A, Hyde J S
Biophysics Research Institute, Medical College of Wisconsin, Milwaukee 53226-0509, USA.
Magn Reson Med. 1998 Nov;40(5):754-62. doi: 10.1002/mrm.1910400517.
Half k-space gradient-recalled echo-planar imaging (GR-EPI) is discussed in detail. T2* decay during full k-space GR-EPI gives rise to unequal weighting of the lines of k-space, loss of signal intensity at the center of k-space, and a point-spread function that limits resolution. In addition, the long readout time for high-resolution full k-space acquisition gives rise to severe susceptibility effects. These problems are substantially reduced by acquiring only half of k-space and filling the empty side by Hermitian conjugate formation. Details of the pulse sequence and image reconstruction are presented. The point-spread function is 3(1/2) times narrower for half than full k-space acquisition. Experiments as well as theoretical considerations were carried out in a context of fMRI using a whole-brain local gradient and an RF coil at 3 Tesla. Using a bandwidth of +/-83 kHz, well-resolved single-shot images of the human brain, as well as good quality fMRI data sets were obtained with a matrix of 192 x 192 over 16 x 16 cm2 FOV using half k-space techniques. The combination of high spatial resolution using the methods presented in this article and the high temporal resolution of EPI opens opportunities for research into fMRI contrast mechanisms. Increase of percent signal change as the resolution increases is attributed to reduction of partial volume effects of activated voxels. Histograms of fMRI pixel responses are progressively weighted to higher percent signal changes as the resolution increases. The conclusion has been reached that half k-space GR-EPI is generally superior to full k-space GR-EPI and should be used even for low-resolution (64 x 64) EPI.
详细讨论了半k空间梯度回波平面成像（GR-EPI）。全k空间GR-EPI过程中的T2*衰减导致k空间各行加权不均、k空间中心信号强度损失以及限制分辨率的点扩散函数。此外，高分辨率全k空间采集的长读出时间会产生严重的磁化率效应。通过仅采集一半k空间并通过埃尔米特共轭形成填充空的一侧，这些问题可得到显著减少。文中介绍了脉冲序列和图像重建的细节。半k空间采集的点扩散函数比全k空间采集窄3（1/2）倍。在3特斯拉的功能磁共振成像（fMRI）背景下，使用全脑局部梯度和射频线圈进行了实验以及理论考量。使用±83 kHz的带宽，通过半k空间技术在16×16 cm2视野上以192×192的矩阵获得了人脑清晰分辨的单次激发图像以及高质量的fMRI数据集。本文介绍的方法所具有的高空间分辨率与EPI的高时间分辨率相结合，为研究fMRI对比机制提供了机会。随着分辨率提高，信号变化百分比增加归因于激活体素部分容积效应的减少。随着分辨率提高，fMRI像素响应直方图逐渐向更高的信号变化百分比加权。得出的结论是，半k空间GR-EPI通常优于全k空间GR-EPI，甚至对于低分辨率（64×64）的EPI也应使用。
