Wu Xiaoping, Zhang Xiaotong, Tian Jinfeng, Schmitter Sebastian, Hanna Brian, Strupp John, Pfeuffer Josef, Hamm Michael, Wang Dingxin, Nistler Juergen, He Bin, Vaughan Thomas J, Ugurbil Kamil, Van de Moortele Pierre-Francois
University of Minnesota Medical School, Department of Radiology, Center for Magnetic Resonance Research, Minneapolis, MN, USA.
Department of Biomedical Engineering, University of Minnesota, Minneapolis, MN, USA.
NMR Biomed. 2015 Oct;28(10):1332-44. doi: 10.1002/nbm.3378. Epub 2015 Aug 30.
The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B1 (B1 (+)) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32-rung birdcage body coil (driven either in a fixed quadrature mode or a two-channel transmit mode), two single-ring stripline arrays (with either 8 or 16 elements), and two multi-ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B1 (+) homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multichannel transmit coils achieved up to about eightfold reduction in local and global SAR. When used for imaging head and cervical spine or imaging thoracic spine, the double-ring array outperformed all coils, including the single-ring arrays. While the advantage of the double-ring array became much less pronounced for pelvic imaging, with a substantially larger ROI, the pTx approach still provided significant gains over the quadrature birdcage coil. For all design scenarios, using the three-ring array did not necessarily improve the RF performance. Our results suggest that pTx pulses with multichannel transmit coils can reduce local and global SAR substantially for body coils while attaining improved B1 (+) homogeneity, particularly for a "z-stacked" double-ring design with coil elements arranged on two transaxial rings.
针对全身线圈，在3T场强下，从发射B1（B1(+)）均匀性和比吸收率（SAR）方面评估了多通道发射线圈布局及并行发射（pTx）射频脉冲设计。对五个特定线圈进行了建模和比较：一个32节的鸟笼式体线圈（以固定正交模式或双通道发射模式驱动）、两个单环带状线阵列（分别有8或16个元件）以及两个多环带状线阵列（由两个或三个相同的环组成，沿z轴堆叠，每个环包含八个方位分布的元件）。考虑了三个解剖学目标，每个目标由一个3D体积定义，该体积代表常规临床应用中有意义的感兴趣区域（ROI）。对于给定的解剖学目标，设计了全局或局部SAR控制的pTx脉冲，以使ROI内的射频激发均匀化。在正交驱动鸟笼式设计实现的B1(+)均匀性条件下，使用多通道发射线圈的pTx脉冲可使局部和全局SAR降低约八倍。当用于头部和颈椎成像或胸椎成像时，双环阵列的性能优于所有线圈，包括单环阵列。虽然对于盆腔成像，双环阵列的优势不太明显，因为其ROI大得多，但pTx方法仍比正交鸟笼式线圈有显著优势。对于所有设计方案，使用三环阵列不一定能提高射频性能。我们的结果表明，多通道发射线圈的pTx脉冲可在提高B1(+)均匀性的同时，大幅降低体线圈的局部和全局SAR，特别是对于“z轴堆叠”的双环设计，其线圈元件布置在两个横向环上。
