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用于高性能并行传输(PTX)的框架，该框架是跨 可广泛应用于MRI扫描仪，并应用于突破回波平面成像的空间编码极限 (EPI)7特斯拉。EPI是目前应用最广泛的用于快速功能、扩散和灌注的脉冲序列 成像，近年来一直是长足发展的焦点，以提高其速度和空间 决议。现在，人们强烈希望将EPI的空间分辨率降低到微观尺度。对于功能型 磁共振成像(FMRI)，这将使fine皮质和皮质下结构(层、柱和核)的成像成为可能 架构，同时更好地解决血流动力学响应。对于弥散磁共振成像(DMRI)，微尺度EPI将 使用分数各向异性改进大脑皮层和脑区fi神经元的表面和层状分析 灰质区域之间的差异，同时广泛减少部分体积效应。它将进一步使 EPI将被广泛应用于加速解剖扫描，这些扫描在几何上与fMRI和dMRI扫描相匹配。 然而，提高单次激发EPI的分辨率需要更长的读数，这会延长回波时间并重新- 在7特斯拉时，功能磁共振成像的DUCE功能对比度和dMRI的信噪比，同时增加了几何失真 而且很模糊。分段或多激发EPI是一种在不增加空间分辨率的情况下提高空间分辨率的经典方法 读出持续时间，但未得到充分利用，主要是因为其对运动和动态相变高度敏感 镜头之间会产生较大的图像伪影。 我们建议开发一种新的多激发EPI技术，称为快门EPI，它解决了LIM-EPI的问题。 通过在每个镜头中成像一组空间上不相交的快门，来尝试传统的多镜头EPI。百叶窗 由多维激励脉冲产生，并且在激发之间空间移位以覆盖整个 切成薄片。然而，对于薄片，激励脉冲的长度是不切实际的(20-100毫秒)。多线圈PTX(&GT； 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个特斯拉扫描仪只有8个 由于成本、占地面积、布线和其他限制，传输通道。在fiRst项目期间，我们率先开发了 称为数组压缩PTX(AcpTx)的技术克服了这一限制。使用acpTx，8传输通道- NEL可以控制任意数量的线圈，其中通道和线圈通过阵列连接 使用针对特定fic激励的RF脉冲进行优化的压缩网络。在这个项目中，我们将开发和 应用acpTx方法和硬件(多线圈磁头发射阵列和8通道对多线圈阵列) 按压网络)以在激励关闭所需的快门图案时实现可行的RF脉冲持续时间 肾上腺素。这些开发将在两个主要的7T扫描仪平台上实施，并以亚毫米为单位进行评估 (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.