Fillmer Ariane, Vannesjo Signe Johanna, Pavan Matteo, Scheidegger Milan, Pruessmann Klaas Paul, Henning Anke
Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland.
Clinic of Affective Disorders and General Psychiatry, University Hospital of Psychiatry Zurich, Zurich, Switzerland.
Magn Reson Med. 2016 Mar;75(3):1119-31. doi: 10.1002/mrm.25695. Epub 2015 May 7.
To calibrate a pre-emphasis to sufficiently compensate eddy currents for application of dynamic shim updating to fMRI without extension of scan times.
Eddy 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 B0 optimization routine for in vivo echo planar imaging and resting-state functional MRI.
The 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 B0 shim field amplitudes and restrict the shim performance.
The proposed method enables accurate pre-emphasis calibration, and therefore, the application of dynamic shim updating to fMRI.
校准预加重，以充分补偿涡流，从而在不延长扫描时间的情况下将动态匀场更新应用于功能磁共振成像（fMRI）。
使用三阶场相机通过时空场监测来表征高达三阶的所有匀场项中感应的涡流效应。预加重设置从测量结果中得出，并进行迭代评估和优化。校准后的预加重与动态激发频率（F0）测定以及用于体内回波平面成像和静息态功能磁共振成像的逐切片B0优化程序相结合，应用于逐切片动态匀场更新。
所描述的预加重校准方法使剩余涡流效应的稳定时间低于2毫秒，从而能够在不延长扫描时间或不产生涡流相关伪影的情况下将动态匀场更新应用于fMRI。动态F0测定补偿了不同匀场场叠加引起的频率偏移，因此可防止视野内的图像移位。硬件限制使得最大适用B0匀场场幅度减小，并限制了匀场性能。
所提出的方法能够实现准确的预加重校准，从而将动态匀场更新应用于fMRI。
