RF Encoding for Gradient-Free MRI

描述(申请人提供)：该项目的目标是开发新的射频(RF)梯度编码方法来对磁共振成像中的信号进行空间编码。这些方法将使静音、低成本的MRI系统成为可能，从而大幅降低成像成本，并提高患者的依从性和舒适性。在传统的磁共振成像中，接收的信号基于其时间频率被定位到其起源的空间位置，该时间频率是使用平行于扫描仪的主(B0)场并且在空间中线性变化的磁场来控制的。这些B0梯度场有许多问题：它们噪音大，会刺激周围神经，影响患者的舒适度；由于线圈的高电感，它们的切换时间相对较长；它们需要笨重的冷却系统和定制的放大器；它们很昂贵，约占临床扫描仪成本的20%-25%。这些问题的一个潜在解决方案是将B0梯度替换为RF梯度，后者是静音的、低成本的。不幸的是，尽管其潜在的射频梯度编码尚未成为临床或商业上的成功。这在很大程度上是因为没有现有的RF梯度编码方法提供对比度显影和由B0梯度带来的空间编码之间的正交性，或者没有直接的路径来将现有的基于B0梯度的MRI采集技术转换为使用RF编码。该项目中提出的方法将是第一个满足这些要求的方法，因此将代表第一个真正可行的基于射频梯度的成像方法。该项目的中心创新是使用Bloch-Siegert移位对MRI信号进行空间编码。与B0梯度一样，这种编码机制基于将相移直接应用于横向平面中的磁化，因此不调制横向磁化的大小，从而使图像对比度不受空间编码的影响。该项目的第一个目标是开发新的射频梯度线圈和其他射频硬件，以实现0.5特斯拉下的2D和3D笛卡尔成像，包括用于同时进行RF发送和接收的硬件策略，以实现Bloch-Siegert频移编码。第二个目标是开发基于Bloch-Siegert移位的新的RF编码脉冲序列，利用Bloch-Siegert相位编码的RF脉冲设计和基于RF梯度的切片选择性激发的RF脉冲设计的最新创新。第三个目标是开发稳健的算法，从射频编码的数据中重建图像。这些目标的成功实现将广泛地证明所提议的用于磁共振成像的RF编码方法的可行性，为翻译成人类铺平道路。

DESCRIPTION (provided by applicant): The goal of this project is to develop new radiofrequency (RF) gradient encoding methods to spatially encode signals in MRI. The methods would enable silent, low-cost MRI systems, leading to a substantial reduction in the cost of imaging and improved patient compliance and comfort. In conventional MRI, a received signal is localized to its spatial location of origin based on its temporal frequency, which is controlled using magnetic fields that are parallel to the main (B0) field of the scanner and vary linearly across space. There are many problems with these B0 gradient fields: they are loud and induce peripheral nerve stimulation, compromising patient comfort; they have relatively long switching times due to the high inductance of the coils; they require bulky cooling systems and customized amplifiers; and they are expensive, representing around 20-25% of the cost of a clinical scanner. A potential solution to these problems is to replace B0 gradients with RF gradients, which are silent and low-cost. Unfortunately, in spite of its potential RF gradient encoding has not yet become a clinical or commercial success. This is largely due to the fact that no existing RF gradient encoding method offers the orthogonality between contrast development and spatial encoding that is en- joyed by B0 gradients, or a straightforward path to convert existing B0 gradient-based MRI acquisition techniques to use RF encoding. The methods proposed in this project will be the first to meet these requirements, and will thus represent the first truly viable RF gradient-based imaging methods. The central innovation of this project is to use the Bloch-Siegert shift to spatially encode the MRI signal. As with B0 gradients, this encoding mechanism is based on the application of phase shifts to magnetization directly in the transverse plane, and therefore does not modulate the magnitude of the transverse magnetization, leaving image contrast unaffected by spatial encoding. The first Aim of the project is to develop new RF gradient coils and other RF hardware to enable 2D and 3D Cartesian imaging at 0.5 Tesla, including hardware strategies for simultaneous RF transmission and reception to enable frequency encoding by Bloch-Siegert shift. The second Aim is to develop new RF-encoded pulse sequences based on the Bloch-Siegert shift, leveraging recent innovations in RF pulse design for Bloch-Siegert phase encoding, and in RF pulse design for RF gradient-based slice-selective excitation. The third Aim is to develop robust algorithms to reconstruct images from RF-encoded data. Successful completion of these Aims would broadly prove the feasibility of the proposed RF encoding methods for MR imaging, paving the way for translation to humans.