Multidimensional motion correction in MRI

背景：高场强磁共振成像(MRI)扫描仪的出现正在将分辨率推向更高的水平。然而，被摄体运动通常会通过模糊图像或引入伪像来抵消增加场强的好处。图像质量的下降是运动本身和运动引起的局部磁场均匀度改变的结果。因此，需要快速测量六自由度(DoF)中的运动并快速生成可用于主动匀场的磁场(B0)图。*检测整体运动的一种方法是导航器-回声法，它从k-空间中采集的MR信号测量运动。导航器k空间轨迹的几何特征和运动提取算法是开发基于导航器的技术的两个关键方面。我们已经开发了使用球面导航仪(SNAV)的采集和计算算法，通过将在移动位置采集的导航仪数据与在参考位置收集的导航仪模板配准来快速确定6个自由度的运动。*对于检测到大运动之后的实时匀场，需要快速的方法来测量3D的局部磁场。在高场强下，这样的测量通常是困难的，因为用于产生场图的相位图像有许多卷绕。我们已经开发了健壮且计算高效的相位展开和B0映射方法，使得准确和快速的B0映射成为可能。*目标：我们建议开发我们的SNAV技术的高效和健壮的实现，以在高场强MR成像期间提供6自由度运动确定以及实时垫片。技术发展和实验评估将侧重于将新的导航技术与实时场映射相结合，以补偿3T和7T的非自愿和生理运动。*方法：该计划将侧重于：*加速SNAV轨迹的优化*实时B0场映射技术的实施*对不同头部运动的B0变化进行建模，并将这些模型与实时图结合在一起，将新型匀场和运动校正技术集成到主动匀场算法中*集成了用于脑成像的超高分辨率脉冲序列内的新型匀场和运动校正技术，并优化了计时参数，以实现即时运动校正*优化分析技术，以最大限度地减少处理时间，同时扩大可以跟踪的运动范围*在专门构建的动态模型中进行评估*意义：这项工作的成功将导致能够使用基于MR的技术实时监控和校正6 DOF运动，同时还可以根据所做的运动测量执行主动匀场。拟议的技术发展有望对磁共振成像产生立竿见影的影响。

BACKGROUND: The availability of high field strength magnetic resonance imaging (MRI) scanners is driving resolution to higher levels. However subject motion can often negate the benefits of increasing field strength by blurring images or introducing artifacts. Degradation in image quality is a result of the motion itself and the change in local magnetic field uniformity resulting from the motion. Therefore methods for rapid measurement of motion in six-degrees of freedom (DoF) and rapid generation of a magnetic field (B0) map, which can be used for active shimming, are required.***An approach to detect bulk motion is the navigator-echo method, which measures motion from the MR signal acquired in k-space. The geometric features of a navigator k-space trajectory and the motion extraction algorithms are two key aspects in developing navigator-based techniques. We have developed acquisition and computational algorithms to determine motion in 6 DoF using a spherical navigator (SNAV) rapidly, by registering the navigator data acquired at the moved position with navigator templates collected at a reference position.***For real time shimming following the detection of large motion, a need exists for rapid methods to measure the local magnetic field in 3D. At high field strength such measurements are often difficult because of numerous wraps of the phase images used to generate the field map. We have developed robust and computationally efficient phase unwrapping and B0 mapping methods, enabling accurate and rapid B0 mapping.***OBJECTIVE: We propose to develop efficient and robust implementations of our SNAV technique to provide 6 DoF motion determination during MR imaging at high field strengths along with real time shimming. Technical developments and experimental evaluation will focus on combining the novel navigator technique with real time field mapping to compensate for involuntary and physiological motions at 3T and 7T.***APPROACH: The program will focus on:*** Optimization of the accelerated SNAV trajectories *** Implementation of real-time B0 field mapping technique *** Modeling of the changes of B0 for different head motions and incorporating these models, along with the real time maps, into an active shimming algorithm*** Integration of the novel shimming and motion correction techniques within ultra-high resolution pulse sequences for brain imaging and optimization of timing parameters to allow on-the fly motion correction*** Optimization of analysis techniques to minimize processing time, while expanding the range of motion that can be tracked*** Evaluation in purpose-built dynamic models***SIGNIFICANCE: The success of this work will lead to the ability to monitor and correct for 6 DoF motion in real time using an MR-based technique, while also performing active shimming based on the motion measurements made. The proposed technical developments promises to have immediate impact on MR imaging.**