Saranathan Manojkumar, Tourdias Thomas, Bayram Ersin, Ghanouni Pejman, Rutt Brian K
Department of Radiology, Stanford, California, USA.
Magn Reson Med. 2015 May;73(5):1786-94. doi: 10.1002/mrm.25298. Epub 2014 May 29.
To optimize the white-matter-nulled (WMn) Magnetization Prepared Rapid Gradient Echo (MP-RAGE) sequence at 7 Tesla (T), with comparisons to 3T.
Optimal parameters for maximizing signal-to-noise ratio (SNR) efficiency were derived. The effect of flip angle and repetition time (TR) on image blurring was modeled using simulations and validated in vivo. A novel two-dimensional (2D) -centric radial fan beam (RFB) k-space segmentation scheme was used to shorten scan times and improve parallel imaging. Healthy subjects as well as patients with multiple sclerosis and tremor were scanned using the optimized protocols.
Inversion repetition times (TS) of 4.5 s and 6 s were found to yield the highest SNR efficiency for WMn MP-RAGE at 3T and 7T, respectively. Blurring was more sensitive to flip in WMn than in CSFn MP-RAGE and relatively insensitive to TR for both regimes. The 2D RFB scheme had 19% and 47% higher thalamic SNR and SNR efficiency than the 1D centric scheme for WMn MP-RAGE. Compared with 3T, SNR and SNR efficiency were higher for the 7T WMn regime by 56% and 41%, respectively. MS lesions in the cortex and thalamus as well as thalamic subnuclei in tremor patients were clearly delineated using WMn MP-RAGE.
Optimization and new view ordering enabled MP-RAGE imaging with 0.8-1 mm(3) isotropic spatial resolution in scan times of 5 min with whole brain coverage.
优化7特斯拉（T）场强下的白质抑制（WMn）磁化准备快速梯度回波（MP-RAGE）序列，并与3T场强下的序列进行比较。
推导了使信噪比（SNR）效率最大化的最佳参数。利用模拟对翻转角和重复时间（TR）对图像模糊的影响进行建模，并在体内进行验证。采用一种新颖的以二维（2D）为中心的径向扇形束（RFB）k空间分割方案来缩短扫描时间并改善并行成像。使用优化后的方案对健康受试者以及患有多发性硬化症和震颤的患者进行扫描。
发现在3T和7T场强下，WMn MP-RAGE序列分别采用4.5秒和6秒的反转重复时间（TS）时，SNR效率最高。在WMn序列中，模糊对翻转比对CSFn MP-RAGE序列更敏感，并且在两种情况下对TR相对不敏感。对于WMn MP-RAGE序列，2D RFB方案的丘脑SNR和SNR效率比1D中心方案分别高19%和47%。与3T场强相比，7T场强下的WMn序列的SNR和SNR效率分别高出56%和41%。使用WMn MP-RAGE序列可以清晰地勾勒出皮质和丘脑中的多发性硬化症（MS）病灶以及震颤患者丘脑中的亚核。
通过优化和新的视图排序，能够在5分钟的扫描时间内实现全脑覆盖的各向同性空间分辨率为0.8 - 1立方毫米的MP-RAGE成像。
