Fast iterative pre-emphasis calibration method enabling third-order dynamic shim updated fMRI
Fast iterative pre-emphasis calibration method enabling third-order dynamic shim updated fMRI
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DOI：
10.1002/mrm.25695
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发表时间：
2016-03-01
影响因子：
3.3
通讯作者：
Henning, Anke
中科院分区：
文献类型：
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作者：
Fillmer, Ariane;Vannesjo, Signe Johanna;Henning, Anke
目的为了在不延长扫描时间的情况下，将动态垫片更新应用于功能磁共振成像中，校准一种能够充分补偿涡流的预加重。方法利用三阶场相机，通过时空场监测来表征所有三阶至三阶的垫片项中的涡流效应。预加重设置是从测量中得出的，并反复评估和改进。将校准的预加重应用于层状动态垫片更新，结合动态激发频率(F0)的确定和用于活体回波平面成像和静息状态功能磁共振成像的层状B-0优化例程。结果所描述的预加重校准方法导致剩余涡流效应的建立时间小于2ms，允许将动态垫片更新应用于功能磁共振成像，而不延长扫描时间或诱导涡流相关伪影。动态F0确定补偿由不同填充场的叠加引起的频率移位，因此防止视场内的图像移位。由于硬件的限制，需要降低最大适用的B-0填充场幅度，从而限制填充场的性能。结论该方法能够实现准确的预加重校准，从而将动态填充场更新应用于功能磁共振成像。Magn Reson Med 75：1119-1131,2016。(C)2015年威利期刊公司。
PurposeTo calibrate a pre-emphasis to sufficiently compensate eddy currents for application of dynamic shim updating to fMRI without extension of scan times.MethodsEddy current effects induced into all shim terms up to third-order were characterized by spatiotemporal field monitoring, using a third-order field camera. Pre-emphasis settings were derived from the measurements and iteratively evaluated and refined. The calibrated pre-emphasis was applied to slice-wise dynamic shim updating in combination with a dynamic excitation frequency (F0) determination and a slice-wise B-0 optimization routine for in vivo echo planar imaging and resting-state functional MRI.ResultsThe described method for pre-emphasis calibration led to settling times of remaining eddy current effects below 2 ms, allowing for the application of dynamic shim updating to fMRI without extension of scan times or induction of eddy current related artifacts. A dynamic F0 determination compensates frequency shifts induced by the superposition of different shim fields, and therefore, prevents an image shift within the field of view. Hardware limitations necessitate the reduction of the maximum applicable B-0 shim field amplitudes and restrict the shim performance.ConclusionThe proposed method enables accurate pre-emphasis calibration, and therefore, the application of dynamic shim updating to fMRI. Magn Reson Med 75:1119-1131, 2016. (c) 2015 Wiley Periodicals, Inc.