OPTIMIZING PHASE CORRECTION FOR MULTI-SHOT DWI

本子项目是利用由NIH/NCRR资助的中心赠款提供的资源的众多研究子项目之一。子项目和研究者（PI）可能已经从另一个NIH来源获得了主要资金，因此可以在其他CRISP条目中表示。列出的机构是中心的，不一定是研究者的机构。多镜头技术，如PROPELLER(1)和自导航交错螺旋（蜗牛）(2)，已经证明了高分辨率弥散加权成像（DWI）的巨大效用。然而，对于弹道设计参数如何影响其导航能力以及如何优化多发DWI的弹道，人们知之甚少。近年来，提出了一种校正生理运动引起的相位变化的共轭梯度（CG）方法，可以显著提高交错DWI的图像质量。此外，该技术可以很容易地与SENSE(3)重建相结合，进行并行成像。在存在k空间欠采样和相位图误差的情况下，利用CG方法可以评估相对于不同读出策略及其轨迹设计参数的相位校正能力。这使我们能够根据经验优化多枪DWI的轨迹。在本研究中，我们证明了对k空间中心进行过采样的k空间轨迹比严格采样的轨迹具有更有利的相位校正特性。良好平衡的中心k空间过采样增加了对轨迹缺陷（如涡流）和相位图误差的容错性，从而为多镜头DWI提供了更强的鲁棒性。结果：我们展示了在不同条件下为不同轨迹重建的具有代表性的图像集。第一行对应于具有完美相位图的情况；第二行对应相位图误差为5%的情况；第三行对应的是只有一半k空间数据的情况。令人惊讶的是，即使有完美的相位信息，传统的螺旋(？= 1)图像有很高的残余伪影，这可能是由于网格误差。比较了不同轨迹的重建误差。很明显，k空间中心的过采样因子的增加提高了轨迹对不完美相位映射和k空间轨迹的容忍度。即使缩小系数为2，带有？= 4图像质量较好，但EPI和常规螺旋图像存在严重的混叠现象。致谢：NIH-1R01NS35959， NIH-1R01EB002771， Lucas基金会，斯坦福大学先进磁共振技术中心（NCRR P41 RR 09784）Magn reason Med 2002；47:42-52。2)刘超，等。Magn reason Med 2004；52:1388 - 1396。3) Pruessmann KP等。马恩森医学2001；46:638 - 651。4) Butts K等。马恩森医学1996；35:763 - 770

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. INTRODUCTION : Multi-shot techniques, such as PROPELLER (1) and self-navigated interleaved spiral (SNAILS) (2), have demonstrated great utility for high resolution diffusion-weighted imaging (DWI). However, little is known about how trajectory design parameters affect their navigation capabilities or how to optimize trajectories for multi-shot DWI. Recently, a conjugate gradient (CG) method that corrects for phase variation induced by physiologic motion was introduced and helps to improve the image quality of interleaved DWI quite dramatically. Moreover, this technique can be easily combined with the SENSE (3) reconstruction for parallel imaging. With the CG method we can evaluate the phase correction capability relative to different readout strategies and their trajectory design parameters in the presence of k-space undersampling and phase map errors. This allows us to optimize empirically the trajectories for multi-shot DWI. In this study, we demonstrate that k-space trajectories that oversample the center of k-space have more favorable properties for phase correction than critically sampled trajectories. Well-balanced oversampling of the center k-space increases the tolerance for trajectory imperfection (e.g. due to eddy currents) and errors in phase map, thus offers more robustness for multi-shot DWI. RESULTS: We show a representative set of images reconstructed for different trajectories under various conditions. The first row corresponds to the case with perfect phase maps; the second row corresponds to the case with 5% error in the phase maps; and the third row corresponds to the case with only half k-space data. Surprisingly, even with perfect phase information, the conventional spiral (? = 1) image has high residual artifacts, which is likely due to gridding errors. It compares reconstruction errors for various trajectories. It is clear that an increasing oversampling factor at the center of k-space improves the trajectory¿s tolerance to imperfect phase map and k-space trajectory. Even with a reduction factor of 2, the image with ? = 4 demonstrates good quality, while severe aliasing artifacts can be seen in EPI and conventional spiral images. ACKNOWLEDGMENTS: NIH-1R01NS35959, NIH-1R01EB002771, Lucas Foundation, Center of Advanced MR Technology of Stanford (NCRR P41 RR 09784) REFERENCES: 1) Pipe JG, et al. Magn Reson Med 2002; 47:42-52. 2) Liu C, et al. Magn Reson Med 2004; 52:1388-1396. 3) Pruessmann KP et al. Magn Reson Med 2001; 46:638-651. 4) Butts K, et al. Magn Reson Med 1996; 35:763-770