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

描述(申请人提供)：超高磁场(UHF)磁共振扫描仪(7特斯拉或更高)具有很强的潜力，可以揭示在较低场强下无法获取的解剖、功能和化学信息，并可以进行更早的非侵入性诊断。然而，要在超高频获得高质量的数据，必须解决重要的障碍。可以说，最具挑战性的问题与超高频大生物样本中的射频传播有关。首先，射频穿透率的降低导致组织加热增加，这限制了可以在安全范围内进行的扫描类型。其次，传输的B1场严重失真和不均匀，破坏了图像和光谱采集。已经证明，射频问题可以通过B1垫片和并行激励等新技术来解决，这两种技术都需要多个发射射频线圈。这反过来又需要为每个RF线圈映射幅度B1，以及随后的两个主要问题：长扫描时间和高RF峰值功率。事实上，传统的B1标测不能用于多个线圈，因为它通常需要每个线圈几分钟的扫描时间。此外，传统方法需要大翻转角度，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)上产生大翻转角度将需要超出硬件和/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中仍然存在问题。在该方案中，我们引入了新的技术和算法，旨在在短时间内和有限的射频峰值功率下映射多个发射线圈的发射B1分布。第一种映射方法依赖于合并来自小翻转角(线性区域)和大翻转角(正弦区域)数据的信息。一种新的B1垫片算法将进行优化，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，近似的B1估计将在图像质量上提供足够的改善，则提出了一种额外的技术，允许在低翻转角度下更快地逼近B1。该技术依赖于在超高频收发机阵列中利用发送和接收B1信息的新形式。这项建议中的所有实验都将在配备多传输能力的7T和9.4T人类扫描仪上进行。预计这些新的B1标测方法将极大地促进B1垫片和超高频平行激发的临床应用。 公共卫生相关性：在这项建议中，我们介绍了在7特斯拉和9.4特斯拉下工作的人体扫描仪中校准多个射频线圈的发射轮廓的新方法。这些新方法预计将比传统方法更快，并且需要更少的射频峰值功率。这将极大地促进超高磁场磁共振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.