Heidemann Robin M, Griswold Mark A, Seiberlich Nicole, Nittka Mathias, Kannengiesser Stephan A R, Kiefer Berthold, Jakob Peter M
Universität Würzburg, Physikalisches Institut, Würzburg, Germany.
Magn Reson Med. 2007 Jun;57(6):1037-46. doi: 10.1002/mrm.21227.
MRI with non-Cartesian sampling schemes can offer inherent advantages. Radial acquisitions are known to be very robust, even in the case of vast undersampling. This is also true for 1D non-Cartesian MRI, in which the center of k-space is oversampled or at least sampled at the Nyquist rate. There are two main reasons for the more relaxed foldover artifact behavior: First, due to the oversampling of the center, high-energy foldover artifacts originating from the center of k-space are avoided. Second, due to the non-equidistant sampling of k-space, the corresponding field of view (FOV) is no longer well defined. As a result, foldover artifacts are blurred over a broad range and appear less severe. The more relaxed foldover artifact behavior and the densely sampled central k-space make trajectories of this type an ideal complement to autocalibrated parallel MRI (pMRI) techniques, such as generalized autocalibrating partially parallel acquisitions (GRAPPA). Although pMRI can benefit from non-Cartesian trajectories, this combination has not yet entered routine clinical use. One of the main reasons for this is the need for long reconstruction times due to the complex calculations necessary for non-Cartesian pMRI. In this work it is shown that one can significantly reduce the complexity of the calculations by exploiting a few specific properties of k-space-based pMRI.
采用非笛卡尔采样方案的磁共振成像（MRI）具有内在优势。径向采集方式已知具有很强的鲁棒性，即使在大量欠采样的情况下也是如此。对于一维非笛卡尔MRI也是如此，其中k空间的中心被过采样或至少以奈奎斯特速率采样。折叠伪影表现更为宽松有两个主要原因：第一，由于中心的过采样，避免了源自k空间中心的高能折叠伪影。第二，由于k空间的非等距采样，相应的视野（FOV）不再明确界定。结果，折叠伪影在很宽的范围内变得模糊，看起来不那么严重。折叠伪影表现更为宽松以及中心k空间的密集采样使得这种类型的轨迹成为自动校准并行MRI（pMRI）技术（如广义自动校准部分并行采集（GRAPPA））的理想补充。虽然pMRI可以从非笛卡尔轨迹中受益，但这种组合尚未进入常规临床应用。其中一个主要原因是由于非笛卡尔pMRI所需的复杂计算，重建时间较长。在这项工作中表明，通过利用基于k空间的pMRI的一些特定属性，可以显著降低计算的复杂性。
