Man L C, Pauly J M, Macovski A
Department of Electrical Engineering, Stanford University, California, USA.
Magn Reson Med. 1997 Jun;37(6):906-13. doi: 10.1002/mrm.1910370616.
Non-2DFT k-space readout strategies are useful in fast imaging but prone to blurring when reconstructed off resonance. Field inhomogeneities or susceptibility variations, coupled with a long readout time, are the major sources of this artifact. Correction methods based on a priori off-resonance information such as an acquired field map have been proposed in the literature. An alternative approach estimates the spatially varying off-resonance frequency from the data itself before applying a correction. In this latter approach there is a trade-off between the extent of correction and the chance of increased artifact due to estimation error. This paper introduces an improved algorithm for field map estimation which is both faster and more robust than the existing method. It uses a multi-stage estimation of the field map, starting from a coarse estimate both in frequency and space and proceeds towards higher resolution. The new algorithm is applied to phantom and in vivo images acquired with radial and spiral sequences to give sharper images.
非二维傅里叶变换（Non-2DFT）k空间读出策略在快速成像中很有用，但在离共振状态下重建时容易出现模糊。场不均匀性或磁化率变化，再加上较长的读出时间，是这种伪影的主要来源。文献中已经提出了基于先验离共振信息（如采集的场图）的校正方法。另一种方法是在应用校正之前从数据本身估计空间变化的离共振频率。在后一种方法中，校正程度与由于估计误差导致伪影增加的可能性之间存在权衡。本文介绍了一种改进的场图估计算法，它比现有方法更快且更稳健。它使用场图的多阶段估计，从频率和空间上的粗略估计开始，然后朝着更高分辨率推进。新算法应用于通过径向和螺旋序列采集的体模和体内图像，以获得更清晰的图像。
