Lee Yoojin, Wilm Bertram J, Brunner David O, Gross Simon, Schmid Thomas, Nagy Zoltan, Pruessmann Klaas P
Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland.
Laboratory for Social and Neural Systems Research, University of Zurich, Zurich, Switzerland.
Magn Reson Med. 2021 Apr;85(4):1924-1937. doi: 10.1002/mrm.28554. Epub 2020 Dec 6.
Spiral readouts combine several favorable properties that promise superior net sensitivity for diffusion imaging. The purpose of this study is to verify the signal-to-noise ratio (SNR) benefit of spiral acquisition in comparison with current echo-planar imaging (EPI) schemes.
Diffusion-weighted in vivo brain data from three subjects were acquired with a single-shot spiral sequence and several variants of single-shot EPI, including full-Fourier and partial-Fourier readouts as well as different diffusion-encoding schemes. Image reconstruction was based on an expanded signal model including field dynamics obtained by concurrent field monitoring. The effective resolution of each sequence was matched to that of full-Fourier EPI with 1 mm nominal resolution. SNR maps were generated by determining the noise statistics of the raw data and analyzing the propagation of equivalent synthetic noise through image reconstruction. Using the same approach, maps of noise amplification due to parallel imaging (g-factor) were calculated for different acceleration factors.
Relative to full-Fourier EPI at b = 0 s/mm , spiral acquisition yielded SNR gains of 42-88% and 40-89% in white and gray matter, respectively, depending on the diffusion-encoding scheme. Relative to partial-Fourier EPI, the gains were 36-44% and 34-42%. Spiral g-factor maps exhibited less spatial variation and lower maxima than their EPI counterparts.
Spiral readouts achieve significant SNR gains in the order of 40-80% over EPI in diffusion imaging at 3T. Combining systematic effects of shorter echo time, readout efficiency, and favorable g-factor behavior, similar benefits are expected across clinical and neurosciences uses of diffusion imaging.
螺旋读出结合了多种有利特性，有望在扩散成像中实现更高的净灵敏度。本研究的目的是验证与当前的回波平面成像（EPI）方案相比，螺旋采集在信噪比（SNR）方面的优势。
使用单次激发螺旋序列和单次激发EPI的几种变体采集了三名受试者的活体脑扩散加权数据，包括全傅里叶和部分傅里叶读出以及不同的扩散编码方案。图像重建基于一个扩展的信号模型，该模型包括通过同步场监测获得的场动态。每个序列的有效分辨率与标称分辨率为1mm的全傅里叶EPI相匹配。通过确定原始数据的噪声统计并分析等效合成噪声在图像重建中的传播来生成SNR图。使用相同的方法，计算了不同加速因子下由于并行成像（g因子）导致的噪声放大图。
相对于b = 0 s/mm²时的全傅里叶EPI，根据扩散编码方案的不同，螺旋采集在白质和灰质中的SNR增益分别为42 - 88%和40 - 89%。相对于部分傅里叶EPI，增益分别为36 - 44%和34 - 42%。螺旋g因子图的空间变化比其EPI对应图更小，最大值更低。
在3T的扩散成像中，螺旋读出相对于EPI可实现40 - 80%的显著SNR增益。结合较短回波时间、读出效率和有利的g因子行为的系统效应，预计在扩散成像的临床和神经科学应用中都会有类似的益处。
