[论文解读] High-resolution diffusion-weighted imaging at 7 Tesla: single-shot readout trajectories and their impact on signal-to-noise ratio, spatial resolution and accuracy

Sajjad Feizollah, Christine Tardif|arXiv (Cornell University)|Jul 15, 2022

Advanced Neuroimaging Techniques and Applications参考文献 63被引用 5

一句话总结

本研究在7T场强下探究了单次采集扩散加权成像的k空间轨迹——EPI、部分傅里叶EPI和螺旋轨迹，以优化信噪比（SNR）、空间分辨率与图像准确度之间的权衡。EPI在有效分辨率和空间特异性方面表现最佳，而螺旋轨迹则显著提升了SNR，从而在超高场强下实现了更优的有效分辨率，尽管其空间特异性较低。

ABSTRACT

Diffusion MRI (dMRI) is a valuable imaging technique to study the brain in vivo. However, the resolution of dMRI is limited by the low signal-to-noise ratio (SNR) of this technique. Various acquisition strategies have been developed to achieve high resolutions, but they require long scan times. Imaging at ultra-high fields (UHF) could further increase the SNR of single-shot dMRI; however, the shorter T2* and the greater field non-uniformities will degrade image quality. In this study, we investigated the trade-off between the SNR and resolution of different k-space trajectories, including echo planar imaging (EPI), partial Fourier EPI, and spiral, over a range of resolutions at 7T. The effective resolution, spatial specificity and sharpening effect were measured from the point spread function (PSF) of the simulated diffusion sequences for a nominal resolution range of 0.6-1.8 mm. In-vivo scans were acquired using the three readout trajectories. Field probes were used to measure dynamic magnetic fields up to the 3rd order of spherical harmonics. Using a static field map and the measured trajectories image artifacts were corrected, leaving T2* effects as the primary source of blurring. The effective resolution was examined in fractional anisotropy (FA) maps. In-vivo scans were acquired to calculate the SNR. EPI trajectories had the highest specificity, effective resolution, and image sharpening effect, but also had substantially lower SNR. Spirals had significantly higher SNR, but lower specificity. Line plots of the in-vivo scans in phase and frequency encode directions showed ~0.2 units difference in FA values between the different trajectories. The difference between the effective and nominal resolution is greater for spirals than for EPI. However, the higher SNR of spiral trajectories at UHFs allows us to achieve higher effective resolutions compared to EPI and PF-EPI trajectories.

研究动机与目标

• 评估不同k空间轨迹在7T高分辨率单次采集扩散加权成像中对信噪比（SNR）、空间分辨率与图像准确度的影响。
• 通过点扩散函数（PSF）分析，量化EPI、部分傅里叶EPI与螺旋轨迹在有效分辨率与空间特异性方面的表现。
• 通过FA图与SNR测量，评估在体内的轨迹性能，同时考虑磁场不均匀性与T2*模糊效应的影响。
• 研究名义分辨率与有效分辨率之间的差异，特别是螺旋轨迹的情况。
• 利用场探针与静态场图校正动态磁场效应，将T2*确定为图像模糊的主要来源。

提出的方法

• 在名义分辨率0.6–1.8 mm范围内，模拟EPI、部分傅里叶EPI与螺旋k空间轨迹的扩散序列。
• 基于每种轨迹的点扩散函数（PSF）计算有效分辨率与空间特异性。
• 在健康受试者中采集所有三种轨迹的7T在体DWI扫描数据。
• 使用场探针测量至三阶球谐函数的动态磁场。
• 应用静态场图与测量到的轨迹，校正几何畸变与失速效应。
• 在消除场相关伪影后，将T2*效应确定为图像模糊的主要来源。

实验结果

研究问题

• RQ1 在7T场强下，EPI、部分傅里叶EPI与螺旋轨迹在有效分辨率与空间特异性方面如何比较？
• RQ2 T2*衰减对图像模糊的影响如何？不同轨迹类型之间有何差异？
• RQ3 螺旋轨迹更高的SNR在多大程度上可补偿其较低的空间特异性，从而实现更高的有效分辨率？
• RQ4 名义分辨率与有效分辨率之间的差异在不同轨迹中如何变化，特别是在高空间分辨率下？
• RQ5 在在体扫描中，不同轨迹的分数各向异性（FA）值存在多大定量差异？这对图像准确度意味着什么？

主要发现

• EPI轨迹表现出最高的空间特异性和有效分辨率，在在体相位与频率编码方向上与螺旋轨迹相比，FA值差异约为0.2单位。
• 螺旋轨迹的SNR显著高于EPI与部分傅里叶EPI，使其即使在空间特异性较低的情况下，仍能实现更高的有效分辨率。
• 螺旋轨迹的名义分辨率与有效分辨率之间的差异大于EPI，表明螺旋轨迹的分辨率保真度退化更严重。
• 尽管SNR较低，EPI仍因更高的特异性而保持更优的图像锐化与空间准确性。
• 在校正磁场不均匀性后，T2*效应仍是图像模糊的主要来源，尤其对螺旋轨迹影响显著。
• 由于其卓越的SNR，螺旋轨迹的有效分辨率在超高场强下超越了EPI，凸显了其在7T高分辨率dMRI中的关键优势。