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通过使用越来越多数量的独立表面线圈来驱动的，以便最大化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的极大增加以及由于线圈元件的尺寸减小而主导的线圈耦合问题，这项工作正在以日益递减的回报进行。这项工作介绍了一种新的方法来更有效地并行成像(使用较少的线圈)，使用新的梯度编码方案，提供相对于接收线圈提供的最佳互补空间信息。该方法采用一阶和二阶球谐函数进行非线性梯度编码。介绍了一种计算给定接收器线圈阵列的最佳编码梯度集的方法。对于每个采集的回波，在读出期间施加来自该互补集合的不同梯度，并且丢弃传统的相位编码。这种方法的优点在于由接收线圈和梯度编码方案共同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。利用这种类型的读出梯度，读出中的过采样显著改善了加速采集，而不会造成时间损失。对于传统的笛卡尔采样，对于混叠，对读出进行过采样是没有好处的。该项目旨在建造一种能够产生一系列二次球谐梯度形状的高速屏蔽梯度插件。这个梯度集将被用来证明原理，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。这项工作的最终成果将是对执行高速加速并行成像的新方法的验证，允许在只有8个接收器线圈的情况下实现8倍或更高的加速系数，以及能够对人头进行成像的更大梯度插入线圈的设计。该项目通过加速数据获取过程对公众健康带来自然好处，从而通过应用零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。该提议也符合NIBIB在加速并行磁共振成像方面的发展目标。

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.