Real-Time Magnetic Resonance Imaging Radial Gradient-Echo Sequences With Nonlinear Inverse Reconstruction
Real-Time Magnetic Resonance Imaging Radial Gradient-Echo Sequences With Nonlinear Inverse Reconstruction
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DOI：
10.1097/rli.0000000000000584
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发表时间：
2019-12-01
影响因子：
6.7
通讯作者：
Uecker, Martin
中科院分区：
文献类型：
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作者：
Frahm, Jens;Voit, Dirk;Uecker, Martin
目的 本研究的目的是评估一种实时磁共振成像 (MRI) 方法，该方法不仅具有高时空分辨率，而且在广泛的科学和临床应用中具有实际稳健性。材料和方法所提出的方法依赖于具有径向编码方案的高度欠采样梯度回波序列。串行图像重建过程解决了真正的数学任务，该任务表现为复杂图像和所有线圈灵敏度图作为未知数的非线性反问题。用于定量参数映射的基于模型的重建的扩展进一步增加了未知数的数量，例如，通过添加相衬流或 T1 松弛的参数。在所有情况下，通过使用迭代正则化高斯-牛顿方法来实现最小化相应成本函数的迭代数值解。例如，通过对前一帧的正则化来支持收敛，而通过与每个迭代步骤中的数据一致性项相比缩小正则化强度来确保时间保真度。高度并行化算法的实际实现是在具有多个图形处理单元的计算机上实现的。它“无形地”集成到商业 3-T MRI 系统中，以允许常规使用并提供常规 DICOM 图像系列的在线重建、显示和存储。结果根据应用，所提出的方法提供串行成像，即记录 MRI 影片，具有可变的空间分辨率和高达每秒 100 帧 (fps)，相当于 10 毫秒的图像采集时间。例如，通过使用两个关节以 2 x 10 fps（每帧 50 毫秒）同步双切片影片来可视化张嘴和闭嘴期间颞下颌关节的运动。心脏功能可以以 30 至 50 fps（33.3 至 20 毫秒）的速度进行研究，而关节过程通常需要 50 fps（20 毫秒）或以 2 x 25 fps（20 毫秒）的速度进行正交双切片采集。基于模型的重建的方法学扩展实现了各种临床场景中流速和 T1 弛豫时间的改进定量映射。结论 具有极端径向欠采样和非线性逆重建的实时梯度回波 MRI 可以直接监测任意生理过程和身体功能。在许多情况下，相关应用提供了迄今为止不可能的临床研究（例如，高分辨率吞咽动力学）或有可能取代现有的 MRI 程序（例如，心电图门控心脏检查）。因此，许多新的机遇需要改变基于 MRI 的放射学范式。在这个阶段，需要进行更广泛的临床试验。
Objective The aim of this study is to evaluate a real-time magnetic resonance imaging (MRI) method that not only promises high spatiotemporal resolution but also practical robustness in a wide range of scientific and clinical applications. Materials and Methods The proposed method relies on highly undersampled gradient-echo sequences with radial encoding schemes. The serial image reconstruction process solves the true mathematical task that emerges as a nonlinear inverse problem with the complex image and all coil sensitivity maps as unknowns. Extensions to model-based reconstructions for quantitative parametric mapping further increase the number of unknowns, for example, by adding parameters for phase-contrast flow or T1 relaxation. In all cases, an iterative numerical solution that minimizes a respective cost function is achieved with use of the iteratively regularized Gauss-Newton method. Convergence is supported by regularization, for example, to the preceding frame, whereas temporal fidelity is ensured by downsizing the regularization strength in comparison to the data consistency term in each iterative step. Practical implementations of highly parallelized algorithms are realized on a computer with multiple graphical processing units. It is "invisibly" integrated into a commercial 3-T MRI system to allow for conventional usage and to provide online reconstruction, display, and storage of regular DICOM image series. Results Depending on the application, the proposed method offers serial imaging, that is, the recording of MRI movies, with variable spatial resolution and up to 100 frames per second (fps)-corresponding to 10 milliseconds image acquisition times. For example, movements of the temporomandibular joint during opening and closing of the mouth are visualized with use of simultaneous dual-slice movies of both joints at 2 x 10 fps (50 milliseconds per frame). Cardiac function may be studied at 30 to 50 fps (33.3 to 20 milliseconds), whereas articulation processes typically require 50 fps (20 milliseconds) or orthogonal dual-slice acquisitions at 2 x 25 fps (20 milliseconds). Methodological extensions to model-based reconstructions achieve improved quantitative mapping of flow velocities and T1 relaxation times in a variety of clinical scenarios. Conclusions Real-time gradient-echo MRI with extreme radial undersampling and nonlinear inverse reconstruction allows for direct monitoring of arbitrary physiological processes and body functions. In many cases, pertinent applications offer hitherto impossible clinical studies (eg, of high-resolution swallowing dynamics) or bear the potential to replace existing MRI procedures (eg, electrocardiogram-gated cardiac examinations). As a consequence, many novel opportunities will require a change of paradigm in MRI-based radiology. At this stage, extended clinical trials are needed.