Lowering the B1 threshold for improved BEAR B1 mapping.
Lowering the B1 threshold for improved BEAR B1 mapping.
复制标题
降低 B1 阈值以改进 BEAR B1 映射。
DOI：
10.1002/mrm.25711
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发表时间：
2016
影响因子：
3.3
通讯作者：
Kerr,AdamB
中科院分区：
文献类型：
--
作者：
Jordanova,KalinaV;Nishimura,DwightG;Kerr,AdamB
目的对于磁共振成像应用来说，非均匀发射射频场的精确测量是必要的。射频场激励幅度（B1）通常通过获取B1map来获得。我们使用绝热重聚焦（BEAR）方法修改B1估计，以将其范围扩展到更低的B1幅度。理论与方法BEAR方法是一种基于相位的B1映射方法，其中双曲正割脉冲引起对B1的相位敏感性。由于脉冲的绝热阈值，可测量的 B1 范围受到限制。我们重新设计了该方法，以使用具有较低绝热阈值的平坦双曲正割脉冲。我们优化了平坦双曲正割参数，以最大限度地降低相位对频率变化的敏感性。结果我们通过仿真和 3T 体内验证了新方法的性能，并表明，对于 ，可以使用减少的标称峰值 B1 值来获取准确的 B1 图。结论 BEAR 方法的绝热阈值通过平坦双曲正割脉冲来降低，该脉冲针对频率范围内的精确相位到 B1 映射进行了优化，并允许较低的标称B1值。在 3T 时，标称 B1 下降 52%，而对 B1 的敏感性增加 3.8 倍。这可以提高该方法对于低B1测量的适用性。 Magn Reson Med 75:1262–1268, 2016。© 2015 Wiley periodicals, Inc.
PurposeAccurate measurement of the nonuniform transmit radiofrequency field is necessary for magnetic resonance imaging applications. The radiofrequency field excitation amplitude (B1) is often obtained by acquiring aB1map. We modify theB1estimation using adiabatic refocusing (BEAR) method to extend its range to lowerB1magnitudes.Theory and MethodsThe BEAR method is a phase‐basedB1mapping method, wherein hyperbolic secant pulses induce a phase sensitivity toB1. The measurableB1range is limited due to the adiabatic threshold of the pulses. We redesign the method to use flattened hyperbolic secant pulses, which have lower adiabatic thresholds. We optimize the flattened hyperbolic secant parameters to minimize phase sensitivity to frequency variations.ResultsWe validate the performance of the new method via simulation and in vivo at 3T, and show that for , accurateB1maps can be acquired using reduced nominal peakB1values.ConclusionThe adiabatic threshold for the BEAR method is reduced with flattened hyperbolic secant pulses, which are optimized for accurate phase‐to‐B1mapping over a frequency range, and allow for lower nominalB1values. At 3T, the nominalB1is decreased by 52% and the sensitivity toB1is increased by a factor of 3.8. This can improve the method's applicability for measurement of lowB1. Magn Reson Med 75:1262–1268, 2016. © 2015 Wiley Periodicals, Inc.