Array-Compressed Parallel Transmission for High Resolution Neuroimaging at 7T

项目摘要 该项目的目标是开发一个高性能并行传输（pTx）的框架，该框架是跨 可用于广泛的MRI扫描仪，并将其应用于推动回波平面成像的空间编码极限 (EPI)7特斯拉EPI是迄今为止最广泛使用的脉冲序列，用于快速功能、扩散和灌注 成像，并已在近年来相当大的发展，以增加其速度和空间的焦点 分辨率现在有一个强烈的愿望，推动EPI的空间分辨率下降到微观尺度。用于功能 磁共振成像（功能磁共振成像），这将使成像的精细结构（层，列，和核）的皮质和皮质下 同时更好地解决血流动力学反应。对于扩散MRI（dMRI），微尺度EPI将 改进皮质中纤维的表面和层分析，以及使用分数各向异性的脑包裹 灰质区域之间的差异，同时广泛减少部分体积效应。这将进一步使 EPI将被广泛应用于加速与fMRI和dMRI扫描几何匹配的解剖扫描。 然而，增加单次激发EPI的分辨率需要更长的读出时间，这延长了回波时间并重新测量。 在7特斯拉的fMRI中减少功能对比度，在dMRI中减少信噪比，同时增加几何失真 和模糊。分段或多激发EPI是一种经典的方法，可以在不增加分辨率的情况下提高空间分辨率。 读出持续时间，但未得到充分利用，主要是由于其对运动和动态相位变化的高灵敏度 这会导致大的图像伪影。 我们建议开发一种新的多点EPI技术，称为快门EPI，它解决了极限， 通过在每次拍摄中对一组空间上不相交的快门进行成像，来实现常规多拍摄EPI的多个阶段。百叶窗 由多维激励脉冲产生，并且在发射之间空间移位以覆盖整个 切片然而，对于薄切片，激励脉冲的长度是不切实际的（20-100 ms）。多线圈pTx（> 8个线圈）可以将这些脉冲的长度缩短到可行的持续时间，但目前的7特斯拉扫描仪只有8个 由于成本、占地面积、布线和其他限制，在第一个项目期间，我们开创了一个 称为阵列压缩pTx（acpTx）的技术克服了这一限制。使用acpTx，8个发射通道， 通道可以控制任意数量的线圈，其中通道和线圈通过阵列连接 压缩网络，针对特定激励使用RF脉冲进行优化。在这个项目中，我们将开发和 应用acpTx方法和硬件（多线圈头部发射阵列和8通道对多线圈阵列COM）， 以在激励快门模式所需的快门模式时实现可行的RF脉冲持续时间 肾上腺素这些开发将在两个主要的7 T扫描仪平台上实现，并在亚毫米波环境下进行评估。 (600微米）fMRI和dMRI采集。总体而言，该项目包括射频脉冲的协同设计， 硬件，采集和重建，以实现空间编码的重大进步。

Project Summary The goal of this project is to develop a framework for high-performance parallel transmission (pTx) that is trans- ferable to a wide range of MRI scanners, and apply it to push the spatial encoding limits of echo planar imaging (EPI) at 7 Tesla. EPI is by far the most widely used pulse sequence for rapid functional, diffusion, and perfusion imaging, and has been the focus of considerable development in recent years to increase its speed and spatial resolution. Now there is a strong desire to push EPI's spatial resolution down to the micro scale. For functional MRI (fMRI), this would enable imaging of fine structures (layers, columns, and nuclei) of cortical and subcortical architecture while better resolving the hemodynamic response. For diffusion MRI (dMRI), micro scale EPI would improve surface and laminar analysis of fibers in the cortex, as well as brain parcelation using fractional anisotropy differences between gray matter regions, while broadly reducing partial volume effects. It would further enable EPI to be broadly applied to accelerate anatomic scans that are geometrically matched to fMRI and dMRI scans. However, increasing the resolution of single-shot EPI requires longer readouts which extend echo times and re- duce functional contrast in fMRI and signal-to-noise in dMRI at 7 Tesla, while increasing geometric distortions and blurring. Segmented or multishot EPI is a classic method to increase spatial resolution without increasing readout durations, but is underutilized, primarily due to its high sensitivity to motion and dynamic phase changes between shots which cause large image artifacts. We propose to develop a new multishot EPI technique called shuttered EPI, which addresses the lim- itations of conventional multishot EPI by imaging a set of spatially disjoint shutters in each shot. The shutters are produced by a multidimensional excitation pulse and are spatially shifted between shots to cover an entire slice. However, with thin slices the length of the excitation pulses are impractical (20-100 ms). Many-coil pTx (> 8 coils) can shorten the length of these pulses to feasible durations, but current 7 Tesla scanners have only 8 transmit channels due to cost, footprint, cabling, and other constraints. In the first project period we pioneered a technique called array-compressed pTx (acpTx) which overcomes this limitation. Using acpTx, 8 transmit chan- nels can control an arbitrarily large number of coils, where the channels and coils are connected via an array compression network that is optimized with RF pulses for specific excitations. In this project, we will develop and apply acpTx methods and hardware (a many-coil head transmit array and an 8 channel-to-many coil array com- pression network) to achieve feasible RF pulse durations when exciting the shutter patterns required for shuttered EPI. These developments will be implemented on two major 7T scanner platforms and evaluated in submillimeter (600 micron) fMRI and dMRI acquisitions. Overall, the project encompasses the synergistic design of RF pulses, hardware, acquisitions and reconstructions to achieve a major advance in spatial encoding.