Journal of magnetic resonance最新文献

Changing the resonant nucleus by altering the static field, compensation of γ and B0 effects in T2 and T2* measurements of porous media

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-12-01 DOI: 10.1016/j.jmr.2024.107811

Rheya Rajeev , Naser Ansaribaranghar , Andrés Ramírez Aguilera , Florea Marica , Laura Romero de Zerón , Bruce J. Balcom

Multinuclear ¹H, ¹³C, and ²³Na magnetic resonance (MR) has many advantages for studying porous media systems containing hydrocarbons and brine. In recent work, we have explored changing the nucleus measured, keeping the Larmor frequency constant, by changing the static magnetic field B₀. Increasing the static B₀ field distorts the field in the pore space due to susceptibility mismatch between the matrix and pore fluid. Distortion of the magnetic field in the pore space scales with the applied static field. The gradients that result from the spatial variation of the distorted field will also scale with B₀. The equations that describe the inhomogeneous broadening in T₂* show that the MR result depends on

γ

B₀. The diffusion through internal field gradients effect on T₂ depends on the product of

γ

and G, with G depending on B₀.

Increasing the static field to bring a nucleus with lower

γ

into resonance at the same frequency will result in the products

γ

B₀ and

γ

G being constant, and therefore, inhomogeneous broadening and diffusion attenuation effects in porous media are predicted to be constant. We explore the T₂* hypothesis with ²³Na and ¹H measurements of brine in porous reservoir core plugs. We explore the diffusion through internal field gradients effect hypothesis with ¹H and ¹³C measurements of decane saturated glass beads.

The nuclei chosen for study: ¹H, ¹³C, and ²³Na are the three most important nuclei for studies of fluids (brine and hydrocarbons) in reservoir core plugs. These three nuclei have a common resonance frequency of 33.7 MHz at static fields of 0.79 T, 3.19 T, and 2.99 T, respectively. All three fields are readily achieved with our variable field superconducting magnet.

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引用次数: 0

A compact and mobile stray-field NMR sensor 结构紧凑的移动式杂散场核磁共振传感器

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-22 DOI: 10.1016/j.jmr.2024.107798

Belal M.K. Alnajjar , Jürgen Frick , Bernhard Blümich , Jens Anders

In this paper, we introduce a compact, single-sided stray field sensor for NMR relaxometry applications. The sensor consists of four main components: the magnet, the RF coil, the spectrometer, and the translation stage. Our proposed magnet, an improved design of the Profile NMR

-

MOUSE, is designed for low weight, compactness, and magnetic field homogeneity, achieved through various shim strategies using a mixed genetic algorithm. The magnet comprises eight NdFeB blocks, generating a magnetic field of 0.424 T within the sensitive region, positioned 12 mm above the magnet surface. For high spatial resolution measurements, we optimized the sensor performance by using a custom-designed rf coil, providing maximum sensitivity, lateral selectivity, and a dead time of less than 20 µs. Moreover, we utilized 3D-printed structures to precisely align the sensitive slice within the object, using an experimental approach based on CPMG measurements. The presented setup achieved a spatial resolution of 50 µm, with resolution changes proportional to acquisition time. We demonstrate the sensor’s versatility and high resolution with measurements on materials such as cosmetics, elastomers, glue, and wood, verifying the good performance of our design, our alignment strategy, and the measuring scheme.

本文介绍了一种用于核磁共振弛豫测量应用的紧凑型单面杂散场传感器。该传感器由四个主要部件组成：磁铁、射频线圈、光谱仪和平移台。我们提出的磁体是对 Profile NMR-MOUSE 的改进设计，重量轻、结构紧凑、磁场均匀，通过使用混合遗传算法的各种垫片策略实现。磁体由八个钕铁硼块组成，在敏感区域内产生 0.424 T 的磁场，位于磁体表面上方 12 毫米处。为了进行高空间分辨率测量，我们使用定制设计的射频线圈优化了传感器性能，使其具有最高灵敏度和横向选择性，死区时间小于 20 µs。此外，我们还利用 3D 打印结构，采用基于 CPMG 测量的实验方法，在物体内部精确对准敏感切片。所介绍的装置实现了 50 微米的空间分辨率，分辨率的变化与采集时间成正比。我们通过对化妆品、弹性体、胶水和木材等材料的测量，证明了传感器的多功能性和高分辨率，验证了我们的设计、对准策略和测量方案的良好性能。

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引用次数: 0

Optimizing EPR pulses for broadband excitation and refocusing 优化宽带激发和再聚焦的 EPR 脉冲

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-19 DOI: 10.1016/j.jmr.2024.107807

Eric R. Lowe , Stefan Stoll , J.P. Kestner

In this paper, we numerically optimize broadband pulse shapes that maximize Hahn echo amplitudes. Pulses are parameterized as neural networks (NN), nonlinear amplitude limited Fourier series (FS), and discrete time series (DT). These are compared to an optimized choice of the conventional hyperbolic secant (HS) pulse shape. A power constraint is included, as are realistic shape distortions due to power amplifier nonlinearity and the transfer function of the microwave resonator. We find that the NN, FS, and DT parameterizations perform equivalently, offer improvements over the best HS pulses, and contain a large number of equivalent optimal maxima, implying the flexibility to include further constraints or optimization goals in future designs.

在本文中，我们通过数值方法优化了能使哈恩回波幅度最大化的宽带脉冲形状。脉冲参数为神经网络（NN）、非线性幅度限制傅里叶级数（FS）和离散时间级数（DT）。这些参数与传统双曲正割（HS）脉冲形状的优化选择进行了比较。由于功率放大器的非线性和微波谐振器的传递函数，还包括功率约束和实际的形状失真。我们发现，NN、FS 和 DT 参数化的性能相当，比最佳 HS 脉冲有所改进，并包含大量等效的最优最大值，这意味着在未来的设计中可以灵活地加入更多的约束条件或优化目标。

引用次数: 0

Eliminating electromagnetic interference for RF shielding-free MRI via k-space convolution: Insights from MR parallel imaging advances 通过 k 空间卷积消除无射频屏蔽磁共振成像的电磁干扰：磁共振并行成像技术的发展启示。

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-19 DOI: 10.1016/j.jmr.2024.107808

Yilong Liu , Linfang Xiao , Mengye Lyu , Ruixing Zhu

Recent advances in ultra-low field MRI have attracted attention from both academic and industrial MR communities for its potential in democratizing MRI applications. One of the most striking features on those advances is shielding-free imaging by actively sensing and eliminating the electromagnetic interference (EMI). In this study, we review the analytical approaches for EMI estimation/elimination, and investigate their theoretical basis and relations with parallel imaging reconstruction. We provide further understanding of the existing approaches, formulating EMI estimation as convolution in k-space or multiplication in spectrum-space. We further propose to use tailored convolutional kernel to adaptively fit the varying EMI coupling across the acquisition window. These methods were evaluated with both simulation study and human brain imaging. The results show that using tailored convolutional kernel can achieve more robust performance against system and acquisition imperfections.

超低磁场磁共振成像技术的最新进展吸引了学术界和工业界的关注，因为它具有使磁共振成像应用平民化的潜力。这些进展的最显著特点之一是通过主动感应和消除电磁干扰（EMI）实现无屏蔽成像。在本研究中，我们回顾了电磁干扰估计/消除的分析方法，并研究了它们的理论基础以及与并行成像重建的关系。我们进一步理解了现有的方法，将 EMI 估算表述为 k 空间中的卷积或频谱空间中的乘法。我们还建议使用定制的卷积核来适应整个采集窗口中不断变化的 EMI 耦合。我们通过模拟研究和人脑成像对这些方法进行了评估。结果表明，使用定制的卷积核可以在系统和采集不完善的情况下实现更稳健的性能。

引用次数: 0

Proton hyperfine couplings and Overhauser DNP 质子高频耦合和奥弗豪斯 DNP。

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-17 DOI: 10.1016/j.jmr.2024.107797

Michael Mardini , Christy George , Ravi Shankar Palani , Xizi Du , Kong Ooi Tan , Ivan Sergeyev , Yangping Liu , Robert G. Griffin

We have prepared trityl radicals with protons at the positions of the -COOH group in the phenyl rings and examined their EPR spectra, which show large

-

hyperfine couplings, and their dynamic nuclear polarization (DNP) Zeeman field profiles . By assessing these polarizing agents for high-field and Overhauser effect DNP, we gain insight into the roles that these hyperfine couplings and other molecular properties play in the DNP performance of these radicals. Interestingly, we do not observe OE DNP in any of the three molecules we examined. This suggests that hyperfine couplings by themselves are not sufficient to support OE DNP. In this case the electron spin density is

\sim

75 % localized on the central carbon atom rather than being distributed uniformly over the aromatic rings. This is in contrast to BDPA where the distribution is delocalized. Our findings do not suggest that any of these radicals are particularly well-suited to high-field DNP. Furthermore, we emphasize that polarizing agents can be extremely sensitive to their solvent environment, even obscuring the intrinsic magnetic properties of the radical.

我们制备了质子位于苯基环中-COOH 基团位置的三苯甲基自由基，并研究了它们的 EPR 光谱（显示出较大的超线性耦合）及其动态核极化（DNP）泽曼场剖面。通过评估这些极化剂的高场和奥弗霍瑟效应 DNP，我们深入了解了这些超线性耦合和其他分子特性在这些自由基的 DNP 性能中所起的作用。有趣的是，在我们研究的三种分子中，我们没有观察到 OE DNP。这表明超精细耦合本身不足以支持 OE DNP。在这种情况下，电子自旋密度的 75% 都集中在中心碳原子上，而不是均匀地分布在芳香环上。这与 BDPA 形成鲜明对比，在 BDPA 中，电子自旋分布是分散的。我们的研究结果并不表明这些自由基中的任何一种特别适合高场 DNP。此外，我们还强调，极化剂对其溶剂环境极为敏感，甚至会掩盖自由基的内在磁性。

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引用次数: 0

16-channel sleeve antenna array based on passive decoupling method at 14 T 基于无源去耦方法的 16 通道套筒天线阵列，14 T。

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-16 DOI: 10.1016/j.jmr.2024.107796

Youheng Sun , Miutian Wang , Jianjun Du , Wentao Wang , Gang Yang , Weimin Wang , Qiushi Ren

At ultra-high fields, especially at 14 T, head coil arrays face significant challenges with coupling between elements. Although passive decoupling methods can reduce this coupling, the decoupling elements can cause destructive interference to the RF field of the head array, thus reducing the

B_{1}^{+}

efficiency. The

B_{1}^{+}

loss due to this effect can be even higher than that due to inter-element coupling. In this study, we develop a novel passive decoupling method to improve the performance of head coil arrays at 14 T. Specifically, passive dipole antennas were utilized to decouple the 16-channel sleeve antenna array, with their positioning optimized to minimize destructive interference with the array’s RF field by increasing their distance from the active antennas. We used electromagnetic simulations to optimize the position of the passive dipoles to obtain the best performance of the array. In addition, we introduced a 16-channel dipole antenna array to compare the array performance when evaluating the sleeve antenna array performance using a human body model. We also constructed the optimized sleeve antenna array and measured its S-parameters to verify the effectiveness of the decoupling strategy. Our results show that the improved passive decoupling method can well reduce the destructive interference of the decoupling elements to the RF field. The sleeve antenna array developed under this method exhibits higher

B_{1}^{+}

efficiency and better transmission performance.

在超高场，尤其是 14 T 时，头部线圈阵列面临着元件间耦合的巨大挑战。虽然无源去耦方法可以减少这种耦合，但去耦元件会对磁头阵列的射频场造成破坏性干扰，从而降低 B1+ 效率。这种效应造成的 B1+ 损耗可能比元件间耦合造成的损耗还要高。具体来说，我们利用无源偶极子天线对 16 通道套筒天线阵列进行去耦，并对其位置进行了优化，通过增大其与有源天线的距离，最大限度地减少对阵列射频场的破坏性干扰。我们利用电磁模拟来优化无源偶极子的位置，以获得阵列的最佳性能。此外，我们还引入了一个 16 通道偶极子天线阵列，以便在使用人体模型评估套筒天线阵列性能时对阵列性能进行比较。我们还构建了优化的套筒天线阵列，并测量了其 S 参数，以验证去耦策略的有效性。结果表明，改进的无源去耦方法可以很好地降低去耦元件对射频场的破坏性干扰。采用这种方法开发的套筒天线阵列具有更高的 B1+ 效率和更好的传输性能。

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引用次数: 0

Selective enhancement of 1H signal from water and oil in porous media at low field with Overhauser DNP 用 Overhauser DNP 在低场条件下选择性增强多孔介质中水和油的 1H 信号

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-01 DOI: 10.1016/j.jmr.2024.107793

Devin M. Morin , Naser Ansaribaranghar , Benjamin Nicot , Derrick Green , Bruce.J. Balcom

In porous media MR studies, discriminating between oil and water presents a challenge because MR lifetimes are often similar and spectra overlap. Low saturations might suggest an experimental strategy of increasing the static field for increased sensitivity, but susceptibility effects are exacerbated at higher field. Overhauser dynamic nuclear polarization, effective at low static field, was employed with water and oil-soluble nitroxide to selectively enhance water and oil signals. We employ a home-built 2 MHz ceramic magnet to achieve selective enhancement of water and oil, in bulk, and in a rock core. For imaging, we employ a 705 kHz ceramic magnet with a 4 gauss/cm constant gradient configuration to image the hyperpolarized signal. A rock core flooding experiment was undertaken to highlight the advantages of Overhauser enhancement. A simple phase cycling technique may be employed to cancel the thermally polarized ¹H signal to isolate the enhanced signal of interest.

在多孔介质磁共振研究中，区分油和水是一项挑战，因为磁共振寿命通常相似且光谱重叠。低饱和度可能建议采用增加静场以提高灵敏度的实验策略，但在较高的静场下，易感性效应会加剧。过豪泽尔动态核偏振在低静态场下有效，我们使用水溶性和油溶性硝基氧化物来选择性地增强水和油信号。我们使用自制的 2 MHz 陶瓷磁铁来选择性地增强岩芯中的水和油。在成像方面，我们采用 705 kHz 陶瓷磁体，以 4 高斯/厘米的恒定梯度配置对超极化信号进行成像。为了突出奥弗霍瑟增强技术的优势，我们进行了岩心充水实验。可以采用简单的相位循环技术来消除热极化 1H 信号，从而分离出感兴趣的增强信号。

引用次数: 0

Chemical shift prediction in 13C NMR spectroscopy using ensembles of message passing neural networks (MPNNs) 利用信息传递神经网络 (MPNN) 集合预测 13C NMR 光谱中的化学位移

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-01 DOI: 10.1016/j.jmr.2024.107795

D. Williamson , S. Ponte , I. Iglesias , N. Tonge , C. Cobas , E.K. Kemsley

This study reports a deep learning approach that utilises message passing neural networks (MPNNs) for predicting chemical shifts in ¹³C NMR spectra of small molecules. MPNNs were trained on two distinct datasets: one with approximately 4000 labelled structures and another with over 40,000. To reduce stochastic variation, an ensemble framework was implemented, which is simple to deploy on multiple nodes of a High-Performance Computing facility.

The results emphasise the critical role of training set size and diversity. While prediction performance was comparable on test sets drawn from each dataset, the ensemble trained on the larger dataset retained its accuracy when these sets were crossed over, and when applied to a further collection of approximately 12,000 previously unseen structures introduced after all development work had been completed. In contrast, the ensemble trained on the smaller dataset showed a notable decline in generalisation ability. This difference is attributed to the greater diversity of atomic environments captured in the larger dataset.

The larger dataset also enabled more robust modelling of various error properties, providing a quantitative foundation for spectral assignment and verification. This was achieved in two ways. First, a clear relationship was observed between prediction errors and the frequency of different node feature vectors in the training data, allowing error estimates to be associated with individual nodes based on their type. These estimates can be used as weights in a modified cityblock distance metric when assigning observed to predicted shifts. Second, the mean absolute prediction error calculated at the structure level is well-fitted by a Gaussian kernel cumulative distribution. This enabled a probabilistic assessment of whether the predicted shifts and assigned observations are consistent with originating from the same molecular structure.

本研究报告了一种利用消息传递神经网络（MPNN）预测小分子 13C NMR 光谱中化学位移的深度学习方法。MPNN 在两个不同的数据集上进行了训练：一个数据集包含约 4000 个标记结构，另一个数据集包含 40,000 多个标记结构。为了减少随机变化，我们实施了一个集合框架，该框架很容易部署在高性能计算设备的多个节点上。虽然从每个数据集抽取的测试集的预测性能相当，但在较大数据集上训练的集合在这些数据集交叉使用时，以及在所有开发工作完成后应用于约 12,000 个以前未见过的结构的进一步集合时，仍保持了其准确性。相比之下，在较小数据集上训练的集合的泛化能力明显下降。这种差异归因于较大数据集中捕捉到的原子环境更加多样化。较大数据集还能对各种误差属性进行更稳健的建模，为光谱分配和验证提供定量基础。这可以通过两种方式实现。首先，在预测误差与训练数据中不同节点特征向量的频率之间发现了明确的关系，从而可以根据节点类型将误差估计值与单个节点联系起来。在将观测到的偏移分配到预测的偏移时，这些估计值可用作修改后的城市街区距离度量的权重。其次，在结构层面计算出的平均绝对预测误差与高斯核累积分布拟合良好。这样就可以从概率上评估预测偏移和分配的观测值是否符合同一分子结构。

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引用次数: 0

Implementing a two-stage, shim field-calibrated superconducting shimming method on a 7 T cryogen-free small animal MRI magnet 在 7 T 无低温小型动物磁共振成像磁体上实施两阶段、垫片场校准超导垫片法。

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-11-01 DOI: 10.1016/j.jmr.2024.107787

Jinhao Liu , Yaohui Wang , Miutian Wang , Wenchen Wang , Gang Yang , Weimin Wang , Qiuliang Wang , Feng Liu

Ultrahigh field systems (

\geq

7 T) can increase the signal-to-noise ratio of magnetic resonance imaging (MRI), improving imaging performance compared to systems with lower fields. However, these enhancements heavily rely on a high

B_{0}

magnetic field homogeneity level, which can be achieved through superconducting shimming. This paper presents a novel two-stage superconducting shimming method designed to achieve precise shimming for a 7 T MRI superconducting magnet. In the initial stage, detailed measurements and fittings were conducted to determine the current polarity and the axial or circumferential positions of the shim fields. Subsequently, an optimization strategy was implemented to determine the optimal shim currents with a flexible target field. The second stage involves an iterative process to fine-tune the current of a specific shim coil, identified as having the most significant impact on field homogeneity. The overall fitness of 99.5% underscores the precision in determining the current polarity and position of the shim fields. Significantly, the calibrated shim system substantially improves the peak-to-peak and Root Mean Square Error (RMSE) field homogeneities from 107.42 ppm and 37.00 ppm to 11.12 ppm and 3.26 ppm, respectively, representing improvements of 89.65% and 91.19%. Furthermore, the simulation results of the fine-tuning stage demonstrate additional enhancements in peak-to-peak field homogeneity, to 9.9 ppm by reducing the current of the Z2 shim coil by 51.3 mA. Additionally, the shimmed magnetic field exhibited high time stability, with a maximum variation of only 27

µ T

observed within 48 h. Thus, the proposed two-stage superconducting shimming framework effectively addresses the challenge of imperfect

B_{0}

magnetic fields, enhancing peak-to-peak and RMSE field homogeneity. The stepwise optimized approach also mitigates deviations caused by shim-to-shim coupling, demonstrating its efficacy in achieving precise shimming in ultrahigh-field MRI systems.

与低磁场系统相比，超高磁场系统（≥ 7 T）可提高磁共振成像（MRI）的信噪比，改善成像性能。然而，这些改进在很大程度上依赖于高 B0 磁场均匀性水平，而这可以通过超导垫片来实现。本文介绍了一种新颖的两阶段超导垫片方法，旨在实现 7 T MRI 超导磁体的精确垫片。在初始阶段，进行了详细的测量和装配，以确定电流极性和垫片磁场的轴向或圆周位置。随后，实施了优化策略，以确定具有灵活目标磁场的最佳垫片电流。第二阶段包括一个迭代过程，对特定垫片线圈的电流进行微调，确定其对磁场均匀性的影响最大。总体合格率为 99.5%，凸显了确定电流极性和垫片场位置的精确性。值得注意的是，经过校准的垫片系统极大地改善了峰峰值和均方根误差（RMSE）场均匀性，分别从 107.42 ppm 和 37.00 ppm 降至 11.12 ppm 和 3.26 ppm，即分别改善了 89.65% 和 91.19%。此外，微调阶段的模拟结果表明，通过将 Z2 垫片线圈的电流降低 51.3 mA，峰-峰场均匀性进一步提高到 9.9 ppm。此外，微调后的磁场具有很高的时间稳定性，在 48 小时内观察到的最大变化仅为 27 µT。因此，所提出的两阶段超导微调框架有效地解决了 B0 磁场不完美的难题，提高了峰峰值和均方根值磁场的均匀性。分步优化方法还能减轻垫片间耦合造成的偏差，证明了它在超高场磁共振成像系统中实现精确垫片的功效。

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引用次数: 0

Neural net analysis of NMR spectra from strongly-coupled spin systems 强耦合自旋系统核磁共振波谱的神经网络分析

IF 2 3区化学 Q3 BIOCHEMICAL RESEARCH METHODS

Journal of magnetic resonance

Pub Date : 2024-10-22 DOI: 10.1016/j.jmr.2024.107792

James H. Prestegard , Geert-Jan Boons , Pradeep Chopra , John Glushka , John H. Grimes Jr. , Bernd Simon

Extracting parameters such as chemical shifts and coupling constants from proton NMR spectra is often a first step in using spectra for compound identification and structure determination. This can become challenging when scalar couplings between protons are comparable in size to chemical shift differences (strongly coupled), as is often the case with low-field (bench top) spectrometers. Here we explore the potential utility of AI methods, in particular neural networks, for extracting parameters from low-field spectra. Rather than seeking large experimental sets of spectra for training a network, we chose quantum mechanical simulation of sets, something that is possible with modern software packages and computer resources. We show that application of a network trained on 2-D J-resolved spectra and applied to a spectrum of iduronic acid, shows some promise, but also meets with some obstacles. We suggest that these may be overcome with improved pulse sequences and more extensive simulations.

从质子 NMR 光谱中提取化学位移和耦合常数等参数通常是利用光谱进行化合物鉴定和结构确定的第一步。当质子间的标量耦合与化学位移差异（强耦合）大小相当时，这可能会变得具有挑战性，低场（台式）光谱仪通常就是这种情况。在此，我们探讨了人工智能方法（尤其是神经网络）在从低场光谱中提取参数方面的潜在用途。我们没有寻求大型实验光谱集来训练网络，而是选择了量子力学模拟光谱集，这在现代软件包和计算机资源条件下是可行的。我们的研究表明，在二维 J 分辨光谱上训练的网络可应用于依度酸的光谱，该网络的应用显示了一些前景，但也遇到了一些障碍。我们建议通过改进脉冲序列和更广泛的模拟来克服这些障碍。

引用次数: 0