Global Applied Science Laboratory, GE Healthcare, Menlo Park, California, USA.
Magn Reson Med. 2012 Sep;68(3):857-62. doi: 10.1002/mrm.23271. Epub 2011 Dec 5.
The Bloch-Siegert (B-S) method of B ₁⁺ mapping has been shown to be fast and accurate, yet has high SAR and moderately long TE. These limitations can lengthen scan times and incur signal loss due to B(0) inhomogeneity, particularly at high field. The B-S method relies on applying a band-limited off-resonant B-S radiofrequency pulse to induce a B ₁⁺-dependent frequency-shift for resonant spins. A method for optimizing the B-S radiofrequency pulse is presented here, which maximizes B-S B ₁⁺ measurement sensitivity for a given SAR and T(2) . A 4-ms optimized pulse is shown to have 35% less SAR compared with the conventional 6-ms Fermi pulse while still improving B ₁⁺ map angle-to-noise ratio by 22%. The optimized pulse performance is validated both in phantom and in vivo brain imaging at 7 T.
Bloch-Siegert (B-S) 方法的 B₁⁺映射已被证明既快速又准确，但 SAR 较高且 TE 适中较长。这些限制可能会延长扫描时间，并由于 B(0)不均匀性而导致信号丢失，尤其是在高场下。B-S 方法依赖于施加带限的离共振 B-S 射频脉冲，以诱导共振自旋的 B₁⁺依赖性频移。这里提出了一种优化 B-S 射频脉冲的方法，该方法针对给定的 SAR 和 T(2)，使 B-S B₁⁺测量灵敏度最大化。与传统的 6-ms Fermi 脉冲相比，4-ms 优化脉冲的 SAR 降低了 35%，同时仍将 B₁⁺图的角度信噪比提高了 22%。在 7T 的体模和活体脑成像中验证了优化脉冲的性能。
