Homogenizing the B1+ field with fast online-customized (FOCUS) parallel transmission (pTx)

该项目是RU研究小组“超高磁场定量MR生物特征快速测绘”的一部分。它专注于开发用于磁共振成像（MRI）的射频（RF）脉冲，使其能够在超高磁场强度（如7 t）下均匀激发/饱和，在这些场强下，必须克服大量技术挑战才能获得高质量的MRI数据。特别是，发射磁场（B1+）的空间非均匀性导致翻转角（FAs）的空间变化，从而导致信号强度和图像对比度的空间变化。此外，主磁场（B0）的不均匀性会导致图像伪影。为了提高质子（1H） MRI的数据质量，我们将使用并行传输（pTx）系统，该系统可以通过不同的RF线圈元件同时传输不同的B1+场。我们将为扩散加权成像（DWI）和化学交换饱和转移（CEST）项目以及RU的临床项目开发和评估专门定制的pTx激发，反演和饱和脉冲。对于CEST MRI，我们计划开发一个全面的CEST饱和脉冲设计，提供高质量的CEST数据，从而可以省去通常需要的B0和B1+校正方法。设计的脉冲应提高所应用成像技术的整体数据质量、可重复性和定量准确性，同时将校准程序的额外时间要求限制在最低限度（< 1分钟）。为了实现这些目标，我们将利用我们之前发表的快速在线定制（FOCUS） pTx脉冲的概念，该概念将预先计算的通用脉冲与快速特定主题的pTx脉冲计算相结合。这些脉冲将用于大脑和乳房成像。对于后者，将使用新的4通道发射/ 16通道接收乳房射频线圈。与RU开发先进射频线圈技术的另一个子项目密切合作，将进行射频场模拟验证，以确保在pTx模式下安全运行。此外，FOCUS脉冲设计的概念也将应用于钠（23Na） MRI，以解决由比吸收率（SAR）限制和B0不均匀性引起的限制。为了加速图像采集，我们将23Na MRI和23Na反演恢复MRI合并为一个脉冲序列，以配合RU的其他方法和发展。为了实现这一目标，我们将180°反转脉冲拆分为两个90°脉冲。第二个脉冲的持续时间将增加，以降低SAR。为了补偿消相效应并实现高数据质量，有关B0不均匀性的信息将集成到脉冲设计中。

This project is part of the research unit (RU) “Fast Mapping of Quantitative MR biosignatures at Ultra-high Magnetic Field”. It focuses on the development of radiofrequency (RF) pulses for magnetic resonance imaging (MRI) that enable homogeneous excitation/ saturation at ultra-high magnetic field strengths such as 7 T. At these field strengths, there are substantial technical challenges that must be overcome to enable the acquisition of high-quality MRI data. In particular, the spatial inhomogeneity of the transmit magnetic field (B1+) results in spatially varying flip angles (FAs), leading to spatially varying signal intensity and image contrast. In addition, inhomogeneities of the main magnetic field (B0) can result in image artifacts. To improve data quality in proton (1H) MRI, we will make use of a parallel transmission (pTx) system that enables the simultaneous transmission of different B1+ fields by different RF coil elements. We will develop and evaluate specially tailored pTx excitation, inversion and saturation pulses for the diffusion weighted imaging (DWI) and chemical exchange saturation transfer (CEST) projects, as well as for the clinical projects of the RU. For CEST MRI, we plan to develop a comprehensive CEST saturation pulse design that provides high-quality CEST data so that typically required B0 and B1+ correction methods can be omitted. The designed pulses shall improve the overall data quality, repeatability, and quantitative accuracy of the applied imaging techniques, while limiting additional time requirements for calibration procedures to a minimum (< 1 min). To achieve these goals, we will make use of our previously published concept of fast online-customized (FOCUS) pTx pulses that combines precalculated universal pulses with a fast subject-specific calculation of the pTx pulses. The pulses will be designed for brain and breast imaging. For the latter, a new 4-channel transmit/ 16-channel receive breast RF coil will be used. In close collaboration with another subproject of the RU working on the development of advanced RF coil technology, validation of RF field simulations will be performed to ensure safe operation in pTx mode. In addition, the concept of FOCUS pulse design will also be applied to sodium (23Na) MRI to tackle limitations caused by specific absorption rate (SAR) restrictions and B0 inhomogeneities. In line with the other methods and developments of the RU aiming to accelerate image acquisition, we will combine 23Na MRI and 23Na inversion recovery MRI into one pulse sequence. To achieve this goal, we will split the 180° inversion pulse into two 90° pulses. The duration of the second pulse will be increased to reduce SAR. To compensate for dephasing effects and to achieve high data quality, information about B0 inhomogeneities will be integrated into the pulse design.