Unit of Academic Radiology, University of Sheffield, Floor C, Royal Hallamshire Hospital, Glossop Road, Sheffield, UK, S10 2JF.
NMR Biomed. 2012 Feb;25(2):389-99. doi: 10.1002/nbm.1766. Epub 2011 Aug 11.
In hyperpolarised (3)He lung MRI with constant flip angles, the transverse magnetisation decays with each RF excitation imposing a k-space filter on the acquired data. For radial data acquired in an angularly-sequential order, this filter causes streaking, angular shading and loss of spatial resolution in the images. The main aim of this work was to reduce the effects of the RF depletion k-space filter in radial acquisitions. Two approaches are presented; (i) retrospective deconvolution of the k-space filter for sequentially-acquired data and (ii) golden angle acquisition order. Radial trajectories sample the centre of k-space with every projection, thereby self-tracking signal decay. The inverse of the signal decay function was used to retrospectively deconvolve RF depolarisation k-space filter effects and the method was demonstrated in 2D radial imaging in phantoms and human lungs. A golden angle radial acquisition was shown to effectively suppress artefacts caused by the RF depletion k-space filter. In addition, the average flip angle per slice was calculated from the signal decay and the values were found to correspond with conventional flip angle maps, providing a means of flip angle self-calibration.
在使用恒定翻转角的超极化（3）氦肺部 MRI 中，横向磁化强度随着每次 RF 激发而衰减，从而对采集到的数据施加了 k 空间滤波器。对于以角度顺序采集的径向数据，该滤波器会导致条纹、角度阴影和图像空间分辨率降低。这项工作的主要目的是减少径向采集时 RF 耗散 k 空间滤波器的影响。提出了两种方法；（i）对顺序采集的数据进行 k 空间滤波器的回溯反卷积和（ii）黄金角度采集顺序。径向轨迹通过每次投影对 k 空间的中心进行采样，从而自我跟踪信号衰减。使用信号衰减函数的倒数来回溯反卷积 RF 去极化 k 空间滤波器的影响，并在 2D 径向成像的体模和人肺中进行了演示。黄金角度的径向采集有效地抑制了由 RF 耗散 k 空间滤波器引起的伪影。此外，还从信号衰减中计算了每个切片的平均翻转角，并且发现这些值与传统的翻转角图相对应，为翻转角自校准提供了一种方法。
