Dynamic B(0) shimming for multiband imaging using high order spherical harmonic shims.
Dynamic B(0) shimming for multiband imaging using high order spherical harmonic shims.
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DOI：
10.1002/mrm.28438
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发表时间：
2021-01
影响因子：
3.3
通讯作者：
Pan JW
中科院分区：
文献类型：
--
作者：
Hetherington HP;Moon CH;Schwerter M;Shah NJ;Pan JW
描述并实现了一种在7T时利用人脑中的球谐填隙实现动态逐层和多频带B0填隙的策略。对于薄轴切片，球谐垫片可以被分成一对空间退化的垫片(z-退化和非z-退化)，从而只需要1/2的垫片(非z-退化)来进行单切片优化。然而，当组合时，垫片对可用于同时产生相同的面内对称性，但具有作为其z位置的函数的不同幅度。这实现了等同于通过单片逐片优化实现的多频段填隙。所有数据都是在7T时使用球面谐和垫片获得的，该垫片能够通过两个额外的5阶垫片(1-4+)向上垫片至4阶。动态垫片更新使用具有2ms斜坡和约束优化的10A垫片电源来实现，以最大限度地减少涡流。在8个受试者的组中，我们证明：1)动态更新1-4+阶垫片将全脑B0场的标准差从32.4±2.6和24.9±2赫兹降低到15.1±1.7赫兹；2)当只动态更新非z-简并的垫片(14.3±1.5赫兹)时，或者当使用多频带垫片因子2、MBS=2和所有垫片(14.4±2.0赫兹)时，获得了接近相同的性能。高次球面谐波在静态全局匀场的基础上有了很大的改进，并实现了动态多频段匀场，其性能与动态逐片匀场的性能几乎相同。这减少了回声平面成像中的失真。
To describe and implement a strategy for dynamic slice-by-slice and multi-band B0 shimming using spherical harmonic shims in the human brain at 7T. For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) which are spatially degenerate, such that only ½ of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location. This enables multi-band shimming equivalent to that achievable by single slice-by-slice optimization. All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st-4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2ms ramps and constrained optimizations to minimize eddy currents. In groups of 8 subjects we demonstrated that: 1) dynamic updating using 1st-4th+ order shims reduced the standard deviation of the B0 field over the whole brain from 32.4±2.6 and 24.9±2Hz with 1st-2nd and 1st-4th+ static global shimming to 15.1±1.7Hz; 2) near equivalent performance was achieved when dynamically updating only the non-z-degenerate shims (14.3±1.5Hz), or when a using multi-band shim factor of 2, MBs=2, and all shims (14.4±2.0Hz). High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multi-band shimming with near equivalent performance to that of dynamic slice-by-slice shimming. This reduces distortion in echo-planar imaging.
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影响因子：
3.3
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