Null Space Imaging - A Novel Approach to Accelerating MR Imaging

描述（申请人提供）：并行成像性能的最新改进是通过使用越来越多的独立表面线圈来驱动的，所述线圈被放置成使得最大化解开沿线性相位编码方向沿着发生的混叠的能力，并且由于与所有接收器通道相关联的费用大大增加，并且由于线圈耦合问题，该问题随着线圈元件尺寸的减小而占主导地位。这项工作介绍了一种新的方法，更有效的并行成像（更少的线圈），使用新的梯度编码方案，提供最佳的互补空间信息相对于接收器线圈提供的。该方法引入了使用1阶和2阶球谐函数的非线性梯度编码。介绍了一种用于计算给定接收器线圈阵列的编码梯度的最佳集合的形式主义。对于每个采集的回波，在读出期间应用来自该互补集合的不同梯度，并且丢弃常规相位编码。这种方法的优点在于由接收器线圈和梯度编码方案两者贡献的互补空间编码。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。利用这种类型的读取梯度，读出中的过采样显著改善了加速采集，而没有时间损失。利用常规笛卡尔采样，对于混叠，对读出进行过采样没有益处。本项目旨在构建一种能够产生一系列二阶球谐梯度形状的高速屏蔽梯度插入件。该梯度集将用于原理证明，同时我们将进一步发展三维空间成像理论（如何计算最佳梯度集）以及重建方法。这项工作的最终产品将是一种新的方法进行高度加速并行成像的验证，允许8个或更多的加速因子与少至8个接收器线圈和一个更大的梯度插入线圈能够成像的人的头部的设计。该项目通过加速数据采集过程，从而通过应用这一空间成像方法，在现有平台上进行更高分辨率或更彻底的检查，从而给公共卫生带来自然的好处。该提案也符合NIBIB在加速并行MR成像方面的发展目标。 公共卫生相关性：加速MRI图像采集的当前方法集中在具有越来越多的元件的接收器线圈阵列上以提高加速。这项工作通过将梯度编码设计为与线圈编码互补，将最大可实现的加速度增加2倍以上，代表了并行成像的范式转变。该项目通过加速数据采集过程，为公共卫生带来了自然的好处，通过应用这一空间成像方法，可以在现有平台上进行更高分辨率、更彻底的检查。

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.