Alagappan Vijayanand, Nistler Juergen, Adalsteinsson Elfar, Setsompop Kawin, Fontius Ulrich, Zelinski Adam, Vester Markus, Wiggins Graham C, Hebrank Franz, Renz Wolfgang, Schmitt Franz, Wald Lawrence L
AA Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA.
Magn Reson Med. 2007 Jun;57(6):1148-58. doi: 10.1002/mrm.21247.
An eight-rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this "loop coil" basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B(1) field pattern, and the higher-order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one-, four-, six-, and eightfold-accelerated spatially tailored RF excitations with 2D and 3D k-space excitation trajectories. Transmit accelerations of up to six-fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B(1) transmit inhomogeneities using parallel excitation.
构建了一个八阶、3T的简并鸟笼线圈（DBC），并对其进行评估，以实现具有八个独立激励通道的头部加速并行激励。测试了两种模式配置。第一种配置中，鸟笼形成的八个回路中的每一个都单独进行激励，产生类似于环形线圈阵列的激励模式。在第二种配置中，巴特勒矩阵将这个“环形线圈”基集转换为代表鸟笼线圈正交模式的基集。在这种情况下，梯级电流在整个线圈周围呈正弦变化，并且八个模式中只有四个具有显著的激励能力（其他四个产生反圆极化（ACP）场）。最低有用模式产生常见的均匀B(1)场模式，高阶模式产生中心幅度零点和方位角相位变化。利用各个模式的测量幅度和相位激励轮廓，通过二维和三维k空间激励轨迹生成一倍、四倍、六倍和八倍加速的空间定制射频激励。在空间伪影水平可接受的情况下，实现了高达六倍的发射加速。当使用这些模式的正交子集通过并行激励来减轻B(1)发射不均匀性时，发现巴特勒矩阵提供的正交基集具有优势。
