Zou Chao, Cheng Chuanli, Qiao Yangzi, Wan Qian, Tie Changjun, Pan Min, Liang Dong, Zheng Hairong, Liu Xin
Paul C. Lauterbur Research Center for Biomedical Imaging, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China.
Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen 518055, China.
Quant Imaging Med Surg. 2019 Feb;9(2):247-262. doi: 10.21037/qims.2019.02.07.
Multi-echo gradient echo (GRE) sequence with bipolar readout gradients can reduce achievable echo spacing and thus have higher acquisition efficiency compared to unipolar readout gradients for fat fraction (FF) quantification. However, the eddy current induced phase (EC-phase) in a bipolar sequence corrupts the phase consistency between echoes and can lead to inaccurate fat quantification.
A hierarchical iterative linear-fitting algorithm (HILA) was proposed for EC-phase correction. In each iteration, image blocks were divided into sub-blocks. The EC-phase was fitted to a linear model in each sub-block. The estimated linear phase in each sub-block was then used as a starting value for the next iteration. Finally, a weighted average over all levels was calculated to obtain the final EC-phase map. Monte Carlo simulations were adopted to evaluate how the residual EC-phase would affect FF quantification accuracy. The performance of the proposed HILA method was then compared to the well-established unipolar acquisition method in phantom and experiments on 3T.
The simulations showed that certain ΔTE values, such as ΔTE =~0.80/1.50/1.95 ms, allowed for FF estimation that were relatively robust to the residual EC-phase ranging from -2π/15 to 2π/15 for a 6-echo bipolar acquisition on 3T. The phantom study showed that the maximum mean FF error, after EC-phase correction with the proposed HILA method, was smaller than 2%, implying that HILA can approximate the high-order term of the EC-phase through step-wise linear fitting. There was no significant difference between the FFs from bipolar and unipolar acquisitions on the two MR systems in the experiments.
The proposed HILA method provides a simple and efficient EC-phase correction method for bipolar acquisition without acquiring additional data. The appropriate choice of TEs may further reduce the effect of the residual EC-phase on accurate FF quantification with bipolar readout sequence.
具有双极读出梯度的多回波梯度回波（GRE）序列可以减小可实现的回波间隔，因此与用于脂肪分数（FF）定量的单极读出梯度相比具有更高的采集效率。然而，双极序列中由涡流引起的相位（EC相位）破坏了回波之间的相位一致性，并可能导致脂肪定量不准确。
提出了一种用于EC相位校正的分层迭代线性拟合算法（HILA）。在每次迭代中，图像块被划分为子块。在每个子块中将EC相位拟合到线性模型。然后将每个子块中估计的线性相位用作下一次迭代的起始值。最后，计算所有层级的加权平均值以获得最终的EC相位图。采用蒙特卡罗模拟来评估残余EC相位如何影响FF定量准确性。然后将所提出的HILA方法的性能与在体模和3T实验中成熟的单极采集方法进行比较。
模拟表明，对于3T上的6回波双极采集，某些ΔTE值，例如ΔTE =~0.80/1.50/1.95 ms，允许进行FF估计，对于范围从-2π/15到2π/15的残余EC相位相对稳健。体模研究表明，使用所提出的HILA方法进行EC相位校正后，最大平均FF误差小于2%，这意味着HILA可以通过逐步线性拟合近似EC相位的高阶项。在实验中的两个MR系统上，双极和单极采集的FF之间没有显著差异。
所提出的HILA方法为双极采集提供了一种简单有效的EC相位校正方法，无需采集额外的数据。适当选择TEs可以进一步减少残余EC相位对双极读出序列准确FF定量的影响。
