Lutz Max, Flassbeck Sebastian, Aigner Christoph Stefan, Krueger Felix, Schaeffter Tobias, Schmitter Sebastian
Physikalisch-Technische Bundesanstalt, Braunschweig and Berlin, Germany.
Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University Grossman School of Medicine, New York, New York, USA.
NMR Biomed. 2025 Aug;38(8):e70080. doi: 10.1002/nbm.70080.
This work proposes a 3D multi-transmit channel B mapping approach based on magnetic resonance fingerprinting (MRF) for the human abdomen at 7 T. A stack-of-stars acquisition is employed to achieve motion-robust 3D encoding, along with a hybrid method where transmit (Tx) channel-wise B information is obtained through low flip angle GRE images. B mapping at ultra-high field (UHF) in the human abdomen is particularly challenging due to the large dynamic range of B , the extensive field of view (FOV), and the effects of respiratory motion. Few methods have been proposed to address these challenges, with a significant limitation being the relatively low RF power available at UHF, especially for pTx systems with a 8 × 1 kW power configuration. This limitation makes it difficult to achieve FAs greater than 30° in central body regions, which are required for accurate results with classical methods. In contrast, Tx channel-combined MRF-based B mapping has been validated as accurate for FAs greater than 6°, offering improved accuracy at low FAs. Here, two Tx channel-combined MRF-based B maps (B1-MRF) are acquired using two tailored complementary phase shims to obtain absolute B information across the entire FOV. The 3D hybrid approach was validated against a 2D reference using phantoms and in vivo free-breathing scans in three subjects with varying BMIs, where only one Tx channel was active at a time. The comparison showed strong agreement, with the 3D hybrid acquisition demonstrating improved performance in regions affected by flow, low FAs, or low signal-to-noise ratio compared to the 2D implementation. The higher accuracy and level of detail provided by the proposed method, in contrast to existing methods, are particularly relevant for several applications, including the validation of faster approaches, validation of electromagnetic simulations (which are safety-critical), and the creation of B map libraries for applications such as AI-based B mapping or universal pulse calculations.
这项工作提出了一种基于磁共振指纹（MRF）的三维多发射通道B1映射方法，用于7T磁场下的人体腹部成像。采用星状采集堆栈来实现对运动稳健的三维编码，并结合一种混合方法，即通过低翻转角梯度回波（GRE）图像获取每个发射（Tx）通道的B1信息。在人体腹部的超高场（UHF）进行B1映射尤其具有挑战性，这是因为B1的动态范围大、视野（FOV）广以及呼吸运动的影响。针对这些挑战，人们提出的方法很少，一个显著的限制是在UHF可用的射频功率相对较低，特别是对于具有8×1kW功率配置的多发射通道（pTx）系统。这种限制使得在人体中心区域难以实现大于30°的翻转角（FA），而传统方法要获得准确结果需要这样的翻转角。相比之下，基于Tx通道组合的MRF的B1映射已被验证在FA大于6°时是准确的，在低FA时具有更高的准确性。在此，使用两个定制的互补相位垫片获取基于Tx通道组合的两个MRF的B1图，以在整个FOV中获得绝对B1信息。使用体模以及对三名具有不同体重指数（BMI）的受试者进行的自由呼吸活体扫描，针对二维参考对三维混合方法进行了验证，其中每次只有一个Tx通道处于激活状态。比较结果显示出高度一致性，与二维成像相比，三维混合采集在受血流、低FA或低信噪比影响的区域表现出更好的性能。与现有方法相比，所提出方法提供的更高准确性和细节水平对于多种应用尤其重要，包括对更快方法的验证、电磁模拟（对安全性至关重要）的验证以及创建用于基于人工智能的B1映射或通用脉冲计算等应用的B1图库。
