Interslice current change constrained B0 shim optimization for accurate high-order dynamic shim updating with strongly reduced eddy currents.
Interslice current change constrained B0 shim optimization for accurate high-order dynamic shim updating with strongly reduced eddy currents.
复制标题
片间电流变化约束 B0 匀场优化，可实现精确的高阶动态匀场更新，并大幅减少涡流。
DOI：
10.1002/mrm.27720
复制
发表时间：
2019
影响因子：
3.3
通讯作者：
Shah,NJon
中科院分区：
文献类型：
--
作者：
Schwerter,Michael;Hetherington,Hoby;Moon,ChanHong;Pan,Jullie;Felder,Jörg;Tellmann,Lutz;Shah,NJon
PurposeTo克服现有的挑战，在动态B 0匀场通过实施一个垫片优化算法，限制垫片电流幅度和它们的时间变化，通过应用的约束和regularization terms.Theory and MethodsSpherical谐波动态B 0匀场是复杂的涡流，不适定的优化，并需要强大的电源。基于涡流幅值与匀场电流变化的幅值成比例的事实，并假设切片方向上的B 0不均匀性变化的平滑性，实施了一种新的算法，以通过限制切片间匀场电流变化来减少涡流的产生。通过惩罚高解范数来另外解决垫片退化问题和由此产生的高电流振幅。所提出的算法的适用性进行了验证，在模拟和幻影和在vivo measurement.ResultsHigh-order动态匀场模拟和测量表明，绝对匀场电流幅度和它们的时间变化可以大大减少可忽略不计的损失，可实现的B 0均匀性。传统的动态匀场更新优化提高了B 0均匀性，平均而言，比二阶静态解提高了2.1倍，而我们提出的例程达到了2.0倍，同时提供了平均最大匀场电流变化的14倍减少。结论所提出的算法大大降低了匀场振幅及其时间变化，而仅轻微地影响可实现的B 0均匀性。因此，它有可能减轻动态B 0匀场中的剩余挑战，并有助于使其应用更容易获得。
PurposeTo overcome existing challenges in dynamic B0shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms.Theory and MethodsSpherical harmonic dynamic B0shimming is complicated by eddy currents, ill‐posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B0inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements.ResultsHigh‐order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B0homogeneity. Whereas conventional dynamic shim updating optimizations improve the B0homogeneity, on average, by a factor of 2.1 over second‐order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14‐fold reduction of the average maximum shim current changes.ConclusionsThe proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B0homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B0shimming and help in making its application more readily available.