Novel, Fast B1 Mapping with Low SAR for Multiple RF Coils at Ultra High Field

描述（由申请人提供）：超高磁场（UHF）MR扫描仪（7特斯拉及更高）具有很强的潜力，可以揭示在较低场强下无法获得的解剖、功能和化学信息，并可以进行早期无创诊断。然而，重要的障碍，必须得到解决，以获得高质量的数据在特高频。可以说，最具挑战性的问题涉及在UHF的大型生物样品中的射频（RF）传播。首先，减少的RF穿透导致增加的组织加热，这限制了可以在安全限度内执行的扫描类型。其次，发射B1场严重失真且不均匀，破坏图像和光谱采集。已经证明，可以通过B1垫片和并行激励等新技术解决RF问题，这两种技术都需要多个发射RF线圈。这反过来又需要每个RF线圈的幅度B1标测，这有两个主要的后续问题：长扫描时间和高RF峰值功率。实际上，传统的B1标测不能与许多线圈一起使用，因为它通常需要每个线圈几分钟的扫描时间。此外，常规方法需要大的翻转角，这通常是不可行的，因为从单个发射线圈元件在大区域（头部或躯干）上生成大的翻转角将需要超过硬件和/或患者安全限制的RF功率水平。最近的方法允许更快的B1映射，但在具有多个通道的高场下仍然存在问题。在该提案中，我们介绍了新的技术和算法，旨在在短时间内以有限的RF峰值功率映射多个发射线圈的发射B1曲线。第一种映射方法依赖于合并来自小翻转角（线性状态）和大翻转角（正弦状态）数据的信息。将优化一种新的B1匀场算法，以最大限度地减少所需的大翻转角数据量。假设在许多临床应用中，近似B1估计将提供足够的图像质量改善，提出了一种额外的技术，允许在低翻转角状态下更快的B1近似。这种技术依赖于一种新的形式主义，利用发射和接收B1信息的收发器阵列在UHF。本提案中的所有实验将在配备多发射能力的7 T和9.4T人体扫描仪上进行。这些新的B1标测方法将极大地促进UHF下B1匀场和平行激励的临床应用。 公共卫生相关性：在这项提案中，我们介绍了新的方法来校准多个射频线圈在人体扫描仪工作在7特斯拉和9.4特斯拉的传输配置文件。预计这些新方法将比传统方法快得多，并且需要更少的RF峰值功率。这将极大地促进超高磁场MR扫描仪临床应用的发展。

DESCRIPTION (provided by applicant): Ultra High Magnetic Field (UHF) MR scanners (7 Tesla and higher) have strong potential to unveil anatomical, functional and chemical information that is not accessible at lower field strengths and could allow for earlier, non invasive diagnoses. Important obstacles, however, must be solved to obtain high quality data at UHF. Arguably, the most challenging problems relate to Radio Frequency (RF) propagation in large biological samples at UHF. First, the reduced RF penetration leads to increased tissue heating, which limits the types of scans that can be performed within safety limits. Second, transmit B1 fields are severely distorted and inhomogeneous, corrupting images and spectral acquisitions. It has been shown that RF issues can be addressed with new technologies such as B1 Shim and Parallel Excitation, that both require multiple transmit RF coils. This, in turn, requires magnitude B1 mapping for each RF coil, with two main subsequent issues: long scan times and high RF peak power. Indeed, conventional B1 mapping cannot be used with numerous coils as it typically requires several minutes of scan time per coil. Furthermore, conventional approaches necessitate large flip angles, which is not generally feasible because generating a large flip angle over a large region (head or torso) from a single transmit coil element would require levels of RF power beyond hardware and/or patient safety limits. Recent approaches allow for faster B1 mapping but are still problematic at high field with multiple channels. In this proposal we introduce novel techniques and algorithms aiming at mapping transmit B1 profiles for multiple transmit coils in a short time and with limited RF peak power. The first mapping approach relies on merging information from small flip angle (linear regime) and large flip angle (sinusoidal regime) data. A new category of B1 shim algorithms will be optimized for minimizing the needed amount of large flip angle data. Assuming that in many clinical applications approximate B1 estimation will provide sufficient improvement in image quality, an additional technique is proposed, allowing for even faster B1 approximation in low flip angle regime. This technique relies on a new formalism utilizing transmit and receive B1 information in transceiver arrays at UHF. All experiments in this proposal will be conducted on human scanners operating at 7T and 9.4T equipped with multi transmit capability. It is expected that those novel B1 mapping methods will greatly facilitate clinical use of B1 shim and Parallel Excitation at UHF. PUBLIC HEALTH RELEVANCE: In this proposal we introduce new methods to calibrate the transmit profile of multiple radiofrequency coils in human scanners operating at 7Tesla and 9.4Tesla. Those new methods are expected to be significantly faster and to require less RF peak power than conventional approaches. This should greatly facilitate the development of clinical applications on ultra high magnetic field MR scanners.