Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA.
Magn Reson Med. 2010 Feb;63(2):419-26. doi: 10.1002/mrm.22234.
High-field (>or=3 T) cardiac MRI is challenged by inhomogeneities of both the static magnetic field (B(0)) and the transmit radiofrequency field (B(1)+). The inhomogeneous B fields not only demand improved shimming methods but also impede the correct determination of the zero-order terms, i.e., the local resonance frequency f(0) and the radiofrequency power to generate the intended local B(1)+ field. In this work, dual echo time B(0)-map and dual flip angle B(1)+-map acquisition methods are combined to acquire multislice B(0)- and B(1)+-maps simultaneously covering the entire heart in a single breath hold of 18 heartbeats. A previously proposed excitation pulse shape dependent slice profile correction is tested and applied to reduce systematic errors of the multislice B(1)+-map. Localized higher-order shim correction values including the zero-order terms for frequency f(0) and radiofrequency power can be determined based on the acquired B(0)- and B(1)+-maps. This method has been tested in 7 healthy adult human subjects at 3 T and improved the B(0) field homogeneity (standard deviation) from 60 Hz to 35 Hz and the average B(1)+ field from 77% to 100% of the desired B(1)+ field when compared to more commonly used preparation methods.
高磁场（>或=3T）心脏 MRI 受到静态磁场（B(0)）和发射射频场（B(1)+）不均匀性的挑战。不均匀的 B 场不仅要求改进调谐方法，还阻碍了零阶项（即局部共振频率 f(0)和产生预期局部 B(1)+场的射频功率）的正确确定。在这项工作中，双回波时间 B(0)-图和双翻转角 B(1)+-图采集方法相结合，在单次呼吸保持 18 次心跳时同时采集整个心脏的多层 B(0)-和 B(1)+-图。测试并应用了先前提出的依赖于激励脉冲形状的切片轮廓校正，以减少多层 B(1)+-图的系统误差。可以根据采集的 B(0)-和 B(1)+-图确定局部高阶调谐校正值，包括频率 f(0)和射频功率的零阶项。该方法已在 7 名健康成年人类受试者中在 3T 下进行了测试，与更常用的准备方法相比，将 B(0)场均匀性（标准偏差）从 60Hz 提高到 35Hz，将平均 B(1)+场提高到期望 B(1)+场的 77%至 100%。
