Ohliger Michael A, Grant Aaron K, Sodickson Daniel K
Harvard-MIT Division of Health Sciences and Technology, Boston, Massachusetts 02215, USA.
Magn Reson Med. 2003 Nov;50(5):1018-30. doi: 10.1002/mrm.10597.
A method is described for establishing an upper bound on the spatial encoding capabilities of coil arrays in parallel MRI. Ultimate intrinsic signal-to-noise ratio (SNR), independent of any particular conductor arrangement, is calculated by expressing arbitrary coil sensitivities in terms of a complete set of basis functions that satisfy Maxwell's equations within the sample and performing parallel imaging reconstructions using these basis functions. The dependence of the ultimate intrinsic SNR on a variety of experimental conditions is explored and a physically intuitive explanation for the observed behavior is provided based on a comparison between the electromagnetic wavelength and the distance between aliasing points. Imaging at high field strength, with correspondingly short wavelength, is shown to offer advantages for parallel imaging beyond those already expected due to the larger available spin polarization. One-dimensional undersampling of k-space yields a steep drop in attainable SNR for more than a 5-fold reduction of scan time, while 2D undersampling permits access to much higher degrees of acceleration. Increased tissue conductivity decreases baseline SNR, but improves parallel imaging performance. A procedure is also provided for generating the optimal coil sensitivity pattern for a given acceleration, which will serve as a useful guide for future coil designs.
描述了一种用于确定并行MRI中线圈阵列空间编码能力上限的方法。通过用一组在样本内满足麦克斯韦方程组的基函数来表示任意线圈灵敏度，并使用这些基函数进行并行成像重建，计算出与任何特定导体排列无关的最终固有信噪比（SNR）。探讨了最终固有SNR对各种实验条件的依赖性，并基于电磁波长与混叠点之间距离的比较，为观察到的行为提供了直观的物理解释。结果表明，在高场强下成像，由于波长相应较短，除了因更大的可用自旋极化而已经预期的优势外，还为并行成像提供了额外优势。k空间的一维欠采样在扫描时间减少超过5倍时，可达到的SNR会急剧下降，而二维欠采样则允许更高程度的加速。组织电导率增加会降低基线SNR，但会改善并行成像性能。还提供了一种为给定加速度生成最佳线圈灵敏度模式的程序，这将为未来的线圈设计提供有用的指导。
