Null Space Imaging - A Novel Approach to Accelerating MR Imaging

描述（由申请人提供）：最近并行成像性能的改进是由于使用了越来越多的独立表面线圈，以便最大限度地解开沿线性相位编码方向发生的混叠的能力，并且由于与所有接收器通道相关的费用大大增加，并且由于线圈耦合问题，随着线圈元件尺寸的减小，这项工作的收益不断减少。这项工作引入了一种新的方法来更有效的并行成像（使用更少的线圈），使用新颖的梯度编码方案，提供相对于接收线圈提供的最佳互补空间信息。该方法利用一阶和二阶球谐波引入非线性梯度编码。介绍了给定接收线圈阵列的最优编码梯度集的计算公式。对于每个采集到的回波，在读出期间应用该互补集的不同梯度，并丢弃传统的相位编码。该方法的优点在于接收线圈和梯度编码方案的互补空间编码。另一个优点是使用跨越xy平面的频率编码梯度，从而在单个回波中提供2D信息。在读出中使用这种类型的读梯度过采样显著地提高了加速采集，而且没有时间损失。对于传统的笛卡尔采样，对于混叠来说，对读出进行过采样是没有好处的。本课题旨在构建一种能够产生一系列二阶球面谐波梯度形状的高速屏蔽梯度插入。该梯度集将用于原理证明，同时我们将进一步发展零空间成像理论（如何计算最优梯度集）以及重建方法。这项工作的最终产品将验证一种执行高加速并行成像的新方法，允许加速因子8或更多，只需8个接收器线圈，并设计出能够成像人类头部的更大梯度插入线圈。该项目通过加速数据采集进程，从而通过应用这种零空间成像方法，实现更高的分辨率，或对现有平台进行更彻底的检查，自然有利于公共卫生。该提案也符合NIBIB的目标，即加速并行磁共振成像的发展。

DESCRIPTION (provided by applicant): Recent improvements in parallel imaging performance have been driven by the use of ever-greater numbers of independent surface coils placed so as to maximize the ability to unwrap the aliasing that occurs along a linear phase encode direction, and this work is proceeding with ever diminishing returns due to both the greatly increasing expense associated with all of the receiver channels, and due to coil coupling problems which dominate as the size of the coil elements decreases. This work introduces a new approach to more efficient parallel imaging (with fewer coils) using novel gradient encoding schemes that provide optimally complementary spatial information relative to that provided by the receiver coils. This approach introduces nonlinear gradient encoding using 1st and 2nd order spherical harmonics. A formalism is introduced for calculating the optimal set of encoding gradient for a given receiver coil array. For each acquired echo, a different gradient from this complementary set is applied during the readout and conventional phase encoding is discarded. The advantage of this approach lies in the complementary spatial encoding contributed by both the receiver coils and the gradient encoding scheme. A further advantage arises with the use of frequency encoding gradients that span the xy-plane and thus provides 2D information in a single echo. With this type of read gradient oversampling in the readout significantly improves the accelerated acquisition with no time penalty. With conventional Cartesian sampling there is no benefit, with respect to aliasing, to over-sampling the read-out. This project is aimed at building a high-speed shield gradient insert capable of generating a series of 2nd order spherical harmonic gradient shapes. This gradient set will be used for proof of principle, and we will simultaneously further develop the theory of Null Space Imaging (how to calculate the optimal gradient set) as well as the reconstruction methodology. The end product from this work would be both a validation of a new methodology for performing highly accelerated parallel imaging, allowing acceleration factors of 8 or more with as few as 8 receiver coils and a design for a larger gradient insert coil capable of imaging the human head. The project carries a natural benefit to public health through acceleration of the data acquisition process, thus allowing for higher resolution, or more thorough examinations on existing platforms, through the application of this Null Space Imaging approach. The proposal also fits with the goals of NIBIB for developments in accelerated parallel MR imaging. PUBLIC HEALTH RELEVANCE: Current methods in accelerating MRI image acquisitions have focused on receiver coil arrays with more and more elements to improve acceleration. This work represents a paradigm shift in parallel imaging by designing the gradient encoding to be complementary to the coil encoding increasing maximum achievable accelerations by more than a factor of 2. The project carries a natural benefit to the public health through acceleration of the data acquisition process, allowing for higher resolution, more thorough examinations on existing platforms through the application of this Null Space Imaging approach.