Hybrid B1+-shimming and gradient adaptions for improved pseudo-continuous arterial spin labeling at 7 Tesla
Hybrid B1+-shimming and gradient adaptions for improved pseudo-continuous arterial spin labeling at 7 Tesla
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DOI：
10.1002/mrm.28982
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发表时间：
2021-08-19
影响因子：
3.3
通讯作者：
Nagel, Armin M.
中科院分区：
文献类型：
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作者：
Meixner, Christian R.;Eisen, Christian K.;Nagel, Armin M.
目的通过利用具有适应的梯度强度和背景抑制的混合同质性和效率优化的B1+-垫片来改进7T的伪连续动脉自旋标记(PCASL)。方法在7T下进行了三个实验，每个实验有5名志愿者：(1)引入一种混合(即，同质性效率优化)B1+-SHIM，并对其进行变速率选择性激发pcASL标记。因此，获取V3节段的B1+图和飞行时间图像来识别供血动脉。为了验证，在圆极化(CP)模式和混合B1+-垫片模式下应用了梯度回波序列。此外，还评估了PCASL灌注图像的灰质(时间)信噪比(TSNR)。(2)对pcASL标记进行了Bloch模拟，并进行了实验验证，重点是切片选择性梯度。(3)将背景抑制添加到B1+-垫片、梯度适应的7T序列，然后将其与3T时的匹配序列进行比较。结果与CP模式相比，B1+-ShIM改善了标记面内的信号(23.6%)，SNR/tSNR增加(+11%/+11%)，但差异无统计学意义。与仿真结果一致，自适应梯度显著提高了tSNR(35%)和SNR(45%)。背景抑制进一步改善了7T时的灌注图像，该方案在3T时的表现与分辨率匹配的方案一样好。结论所提出的混合B1+-位相-垫片与适应的切片选择梯度和背景抑制相结合，在7T的次优B1+条件下显示出极大的改进pcASL标记的潜力。
Purpose To improve pseudo-continuous arterial spin labeling (pcASL) at 7T by exploiting a hybrid homogeneity- and efficiency-optimized B1+-shim with adapted gradient strength as well as background suppression. Methods The following three experiments were performed at 7T, each employing five volunteers: (1) A hybrid (ie, homogeneity-efficiency optimized) B1+-shim was introduced and evaluated for variable-rate selective excitation pcASL labeling. Therefore, B1+-maps in the V3 segment and time-of-flight images were acquired to identify the feeding arteries. For validation, a gradient-echo sequence was applied in circular polarized (CP) mode and with the hybrid B1+-shim. Additionally, the gray matter (temporal) signal-to-noise ratio (tSNR) in pcASL perfusion images was evaluated. (2) Bloch simulations for the pcASL labeling were conducted and validated experimentally, with a focus on the slice-selective gradients. (3) Background suppression was added to the B1+-shimmed, gradient-adapted 7T sequence and this was then compared to a matched sequence at 3T. Results The B1+-shim improved the signal within the labeling plane (23.6%) and the SNR/tSNR increased (+11%/+11%) compared to its value in CP mode; however, the increase was not significant. In accordance with the simulations, the adapted gradients increased the tSNR (35%) and SNR (45%) significantly. Background suppression further improved the perfusion images at 7T, and this protocol performed as well as a resolution-matched protocol at 3T. Conclusion The combination of the proposed hybrid B1+-phase-shim with the adapted slice-selective gradients and background suppression shows great potential for improved pcASL labeling under suboptimal B1+ conditions at 7T.