Research Department Tomographic Imaging Systems, Philips Research Europe, Hamburg, Germany.
Magn Reson Med. 2014 Jan;71(1):246-56. doi: 10.1002/mrm.24667. Epub 2013 Feb 14.
To tailor and optimize the Dual Refocusing Echo Acquisition Mode (DREAM) approach for volumetric B1 (+) mapping of the brain at 7T.
A new DREAM echo timing scheme based on the virtual stimulated echo was derived to minimize potential effects of transverse relaxation. Furthermore, the DREAM B1 (+) mapping performance was investigated in simulations and experimentally in phantoms and volunteers for volumetric applications, studying and optimizing the accuracy of the sequence with respect to saturation effects, slice profile imperfections, and T1 and T2 relaxation. Volumetric brain protocols were compiled for different isotropic resolutions (5-2.5 mm) and SENSE factors, and were studied in vivo for different RF drive modes (circular/linear polarization) and the application of dielectric pads.
Volumetric B1 (+) maps with good SNR at 2.5 mm isotropic resolution were acquired in about 20 s or less. The specific absorption rate was well below the safety limits for all scans. Mild flow artefacts were observed in the large vessels. Moreover, a slight contrast in the ventricle was observed in the B1 (+) maps, which could be attributed to T1 and T2 relaxation effects.
DREAM enables safe, very fast, and robust volumetric B1 (+) mapping of the brain at ultrahigh fields.
针对 7T 脑部容积 B1(+)映射，定制和优化双聚焦回波采集模式（DREAM）方法。
基于虚拟激发回波，推导出一种新的 DREAM 回波时间方案，以最小化横向弛豫的潜在影响。此外，在仿真和体模与志愿者实验中，研究了 DREAM B1(+)映射性能，针对饱和效应、层面轮廓不完美、T1 和 T2 弛豫，研究并优化了序列的准确性。针对不同各向同性分辨率（5-2.5mm）和 SENSE 因子，编译了容积脑部协议，并针对不同射频驱动模式（圆形/线性极化）和介电垫的应用，在体内进行了研究。
在 2.5mm 各向同性分辨率下，可获得具有良好信噪比的容积 B1(+)图，大约 20 秒或更短时间即可完成采集。所有扫描的比吸收率均远低于安全限值。在大血管中观察到轻微的流动伪影。此外，在 B1(+)图中还观察到脑室有轻微的对比度，这可能归因于 T1 和 T2 弛豫效应。
DREAM 可在超高场安全、非常快速且稳健地进行脑部容积 B1(+)映射。
