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特斯拉或更高）具有强大的潜力，可以揭示在较低场强下无法获得的解剖，功能和化学信息，并且可以允许早期，非侵入性诊断。然而，要获得高质量的超高频数据，必须解决一些重要的障碍。可以说，最具挑战性的问题与射频（RF）在超高频下在大型生物样本中的传播有关。首先，射频穿透的减少导致组织加热的增加，这限制了在安全范围内可以进行的扫描类型。其次，发射B1场严重扭曲和不均匀，破坏了图像和光谱采集。研究表明，射频问题可以通过B1 Shim和并联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。这反过来又需要对每个RF线圈进行B1级映射，随之而来的两个主要问题是：长扫描时间和高RF峰值功率。实际上，传统的B1扫描不能用于多个线圈，因为每个线圈通常需要几分钟的扫描时间。此外，传统的方法需要大的翻转角度，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）产生大的翻转角度将需要超出硬件和/或患者安全限制的射频功率水平。最近的方法允许更快的B1映射，但在多通道的高场仍然存在问题。在本提案中，我们介绍了新的技术和算法，旨在在短时间内以有限的射频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（线性区）和大翻转角（正弦区）数据的信息。将优化一类新的B1 shim算法，以最小化所需的大翻转角度数据量。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，允许在低翻转角度下更快地近似B1。该技术依赖于一种新的形式，利用超高频收发器阵列中的发射和接收B1信息。本提案的所有实验都将在具有多传输能力的7T和9.4T人体扫描仪上进行。预计这些新的B1定位方法将极大地促进B1片和UHF平行激发的临床应用。

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.