MR susceptibility contrast imaging using a 2D simultaneous multi-slice gradient-echo sequence at 7T

PurposeTo develop a 7T simultaneous multi-slice (SMS) 2D gradient-echo sequence for susceptibility contrast imaging, and to compare its quality to 3D imaging.MethodsA frequency modulated and phase cycled RF pulse was designed to simultaneously excite multiple slices in multi-echo 2D gradient-echo imaging. The imaging parameters were chosen to generate images with susceptibility contrast, including T2*-weighted magnitude/phase images, susceptibility-weighted images and quantitative susceptibility/R2* maps. To compare their image quality with 3D gradient-echo imaging, both 2D and 3D imaging were performed on 11 healthy volunteers and 4 patients with multiple sclerosis (MS). The signal to noise ratio (SNR) in gray and white matter and their contrast to noise ratio (CNR) was simulated for the 2D and 3D magnitude images using parameters from the imaging. The experimental SNRs and CNRs were measured in gray/white matter and deep gray matter structures on magnitude, phase, R2* and QSM images from volunteers and the visibility of MS lesions on these images from patients was visually rated. All SNRs and CNRs were compared between the 2D and 3D imaging using a paired t-test.ResultsAlthough the 3D magnitude images still had significantly higher SNRs (by 13.0~17.6%), the 2D magnitude and QSM images generated significantly higher gray/white matter or globus pallidus/putamen contrast (by 13.3~87.5%) and significantly higher MS lesion contrast (by 5.9~17.3%).Conclusion2D SMS gradient-echo imaging can serve as an alternative to often used 3D imaging to obtain susceptibility-contrast-weighted images, with an advantage of providing better image contrast and MS lesion sensitivity.研究目的：开发一款用于磁敏感对比成像的7T同时多切片（simultaneous multi-slice, SMS）二维梯度回波序列，并将其成像质量与三维成像进行对比。 方法：本研究设计了一款经频率调制且相位循环的射频脉冲，可在多回波二维梯度回波成像中同时激发多个切片。成像参数的设置旨在生成具备磁敏感对比特性的图像，包括T2*加权幅值/相位图像、磁敏感加权图像以及定量磁敏感（quantitative susceptibility mapping, QSM）与R2*图谱。为对比其与三维梯度回波成像的成像质量，本研究对11名健康志愿者及4名多发性硬化（multiple sclerosis, MS）患者开展了二维与三维成像。基于本次成像的参数，对二维与三维幅值图像的灰白质信噪比（signal to noise ratio, SNR）及其对比度信噪比（contrast to noise ratio, CNR）进行了仿真计算。通过志愿者的幅值、相位、R2*及定量磁敏感成像图像，测量了灰白质与深部灰质结构的实验信噪比与对比度信噪比；同时由阅片者对患者上述图像中的多发性硬化病灶可视性进行评分。采用配对t检验对二维与三维成像的所有信噪比与对比度信噪比结果进行对比分析。 结果：尽管三维幅值图像的信噪比仍显著更高（提升13.0%~17.6%），但二维幅值图像与定量磁敏感成像图像的灰白质或苍白球/壳核对比度（提升13.3%~87.5%）以及多发性硬化病灶对比度（提升5.9%~17.3%）均显著更优。 结论：二维同时多切片梯度回波成像可作为临床常用三维成像的替代方案，用于获取磁敏感对比加权图像，其优势在于可提供更优异的图像对比度与多发性硬化病灶检出敏感性。