随着线圈到样品的距离增加，平行传输中的射频能量沉积和射频功率需求。
Radiofrequency energy deposition and radiofrequency power requirements in parallel transmission with increasing distance from the coil to the sample.
作者信息
Deniz Cem M, Vaidya Manushka V, Sodickson Daniel K, Lattanzi Riccardo
机构信息
Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York, USA.
The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, New York, USA.
出版信息
Magn Reson Med. 2016 Jan;75(1):423-32. doi: 10.1002/mrm.25646. Epub 2015 Mar 5.
PURPOSE
We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased.
METHODS
We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift-off, and tracked local SAR distributions associated with global SAR optima.
RESULTS
In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift-off, approaching UISAR, which was constant for all lift-offs. We observed a lift-off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements.
CONCLUSION
For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation.
目的
我们研究了在平行传输中，随着发射线圈与样品之间距离的增加，整体比吸收率（SAR）和射频（RF）功率需求的变化情况。
方法
我们计算了极限固有SAR（UISAR），它取决于物体的几何形状和电学特性，而不取决于线圈设计，并将其作为比较各种发射阵列性能的参考。我们研究了在固定线圈尺寸并增加线圈数量的同时将阵列移离样品的情况，以及固定线圈数量并缩放线圈尺寸的情况。我们还研究了作为提离距离函数的RF功率需求，并跟踪了与整体SAR最优值相关的局部SAR分布。
结果
在所有情况下，都实现了目标激发分布，并且整体SAR（以及相关的最大局部SAR）随着提离距离的增加而降低，接近UISAR，UISAR在所有提离距离下都是恒定的。我们观察到一个提离距离值，该值优化了整体SAR与线圈导体功率损耗之间的平衡。我们表明，使用平行传输时，对于具有足够数量发射元件的有限阵列，在超高场下整体SAR可以降低。
结论
对于平行传输，可以优化线圈与物体之间的距离，以降低SAR并最小化与均匀激发相关的RF功率需求。
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