Department of Medicine, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii 96813-2427, USA.
Magn Reson Med. 2010 Mar;63(3):828-34. doi: 10.1002/mrm.22209.
Three-dimensional spatial-spectral radiofrequency pulses using a stack-of-spirals trajectory can achieve two-dimensional spatial localization and one-dimensional spectral selection simultaneously. These pulses are useful, for example, in reduced field-of-view applications that also require frequency specificity such as lipid imaging. A limitation of the pulse design is that the length of the spiral trajectory is fixed by the frequency separation of lipid and water. This restricts the highest possible excitation resolution of the spatial profile over a given field of excitation. In this work, we examine the use of periodically rotated variable-density spirals to increase the spatial excitation resolution without changing the frequency selectivity. Variable-density spirals are used to undersample the high spatial frequencies such that higher excitation resolutions can be obtained with a small expense in increased aliasing of the slice profile. The periodic rotation of the spiral trajectories reduces the impact of the undersampling by distributing the aliasing in the frequency domain. The technique is demonstrated with simulations, phantom studies, and imaging human leg muscle at 3 T. It was found in the human study that the spatial excitation resolution could be improved from 6 x 6 to 8 x 8 (matrix size over a fixed field of view) while decreasing aliasing by approximately 40-60%.
使用螺旋轨迹的三维空间-谱射频脉冲可以实现二维空间定位和一维谱选择。这些脉冲在需要频率特异性的小视野应用中非常有用，例如脂质成像。脉冲设计的一个限制是，脂质和水的频率分离决定了螺旋轨迹的长度。这限制了在给定激发场中空间轮廓的最大可能激发分辨率。在这项工作中，我们研究了使用周期性旋转的可变密度螺旋来提高空间激发分辨率，而不改变频率选择性。可变密度螺旋用于对高空间频率进行欠采样，以便在增加切片轮廓的混叠的小代价下获得更高的激发分辨率。螺旋轨迹的周期性旋转通过在频域中分布混叠来减少欠采样的影响。该技术通过模拟、体模研究以及在 3T 下对人体腿部肌肉进行成像得到了验证。在人体研究中发现，空间激发分辨率可以从 6x6 提高到 8x8（在固定视野上的矩阵大小），同时混叠减少约 40-60%。
