MR Research Center, Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA.
Magn Reson Med. 2010 Mar;63(3):543-52. doi: 10.1002/mrm.22215.
High-resolution (approximately 0.22 mm) images are preferably acquired on whole-body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time ( approximately 12 min) in three-dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off-resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point-spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize off-resonance-induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole-body 7T scanner to show the performance of the proposed method. The three-dimensional brain images of human subjects were obtained at echo time = 1.18 ms, resolution = 0.22 mm (field of view = 220 mm, matrix size = 1024), and in-plane spiral shots = 128, using a home-developed ultrashort echo time sequence (acquisition-weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time = 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient-echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible.
高分辨率（约 0.22 毫米）图像最好在全身 7T 扫描仪上获取，以可视化人体大脑的微小解剖结构。它们通常需要在三维扫描中进行长时间采集（约 12 分钟），即使使用并行成像和部分傅里叶采样也是如此。然而，两种快速成像技术的结合使用会导致偶尔可见的欠采样伪影。螺旋成像在采集效率方面优于矩形采样，但由于在 7T 处存在强离共振效应，其实现受到限制，导致图像模糊。本研究提出了一种在保持螺旋效率的同时最小化图像模糊的解决方案。首先，使用计算机模拟和点扩散函数对 7T 处的图像模糊进行了定量研究。然后，采用多shot 螺旋和超短回波时间采集的组合来最小化离共振引起的图像模糊。在全身 7T 扫描仪上对体模和健康受试者进行了实验，以展示所提出方法的性能。使用自主开发的超短回波时间序列（螺旋采集的加权堆栈），在回波时间=1.18ms、分辨率=0.22mm（视野=220mm、矩阵大小=1024）和平面螺旋拍摄=128 的情况下，获得了人体受试者的三维大脑图像。在不使用并行成像和部分傅里叶采样的情况下，脉冲重复时间=100ms 时，60 个分区的总采集时间为 12.8 分钟。这些螺旋图像中的模糊度最小化到与矩形采集的梯度回波图像相当的水平，而螺旋采集效率保持在 8 倍。这些图像表明，7T 处的螺旋成像是可行的。
