Biomedical Imaging and Engineering Lab., Department of Brain and Cognitive Engineering, Korea University, Seoul, Republic of Korea.
Neuroimage. 2012 Jan 2;59(1):377-88. doi: 10.1016/j.neuroimage.2011.07.070. Epub 2011 Aug 5.
The thickness of the human cerebral cortex, which provides valuable information in the studies of normal and abnormal neuroanatomy, is commonly estimated using high-resolution, volumetric magnetization-prepared rapid gradient echo (MP-RAGE) magnetic resonance imaging due to its strong T1-weighted contrast and high signal-to-noise ratio. However, the accuracy of cortical thickness estimates using MP-RAGE is potentially contaminated by susceptibility-induced signal loss particularly at regions in close proximity to air-filled cavities. The purpose of this work is to investigate the feasibility of susceptibility-resistant variable-flip-angle (VFA) three-dimensional turbo/fast spin echo imaging for reliable estimation of cortical thickness of the human brain, wherein 1) radio-frequency (RF) pulse refocuses susceptibility-induced spin de-phasing, 2) the VFA refocusing pulse train is applied for a tissue-specific prescribed signal evolution along the echo train, 3) the desired T1-weighted contrast is achieved by composite restore pulses at the end of the refocusing pulse train, and 4) blood signals are suppressed using the VFA scheme combined with increasing moments of flow-sensitizing gradients while dura mater signals are attenuated due to short T2 relaxation time, which alleviates potential failure in brain segmentation. Numerical simulations of the Bloch equation are performed in both MP-RAGE and the proposed method for comparison. In vivo studies are performed in 14 healthy volunteers at 3 T. Image processing is then performed using the Freesurfer, resulting in mean and standard deviations of cortical thickness for the entire cortical surfaces. Statistical analysis demonstrates that particularly in the inferior prefrontal and temporal regions heavily affected by susceptibility-induced signal loss conventional MP-RAGE, if compared with the proposed method, significantly under-estimates cortical thickness. It is expected that the proposed pulse sequence, which is resistant to susceptibility-induced signal loss and attenuates the signal intensity of blood and dura mater, can be a potentially promising alternative to conventional MP-RAGE in reliably estimating cortical thickness for the entire brain.
人类大脑皮层的厚度为研究正常和异常神经解剖学提供了有价值的信息，通常使用高分辨率、容积磁化准备快速梯度回波（MP-RAGE）磁共振成像进行估计，因为它具有很强的 T1 加权对比和高信噪比。然而，MP-RAGE 估计皮质厚度的准确性可能会受到顺磁性引起的信号损失的影响，特别是在靠近充满空气的腔室的区域。本工作旨在研究抗顺磁性可变翻转角（VFA）三维涡轮/快速自旋回波成像在可靠估计人脑皮质厚度方面的可行性，其中 1）射频（RF）脉冲重新聚焦顺磁性引起的自旋去相位，2）VFA 重聚焦脉冲串用于沿回波链对组织特异性规定的信号演化，3）在重聚焦脉冲串结束时使用复合恢复脉冲实现所需的 T1 加权对比，4）通过 VFA 方案结合增加流动敏感梯度的阶数来抑制血液信号，同时由于短 T2 弛豫时间，硬膜信号被衰减，这减轻了脑分割潜在的失败。在 MP-RAGE 和所提出的方法中对 Bloch 方程进行数值模拟以进行比较。在 3T 下对 14 名健康志愿者进行体内研究。然后使用 Freesurfer 进行图像处理，得到整个皮质表面的皮质厚度平均值和标准偏差。统计分析表明，特别是在受顺磁性信号损失影响较大的下额额和颞叶区域，与传统的 MP-RAGE 相比，如果与所提出的方法相比，传统的 MP-RAGE 显著低估了皮质厚度。预计该脉冲序列抗顺磁性信号损失并衰减血液和硬膜的信号强度，有望成为传统 MP-RAGE 可靠估计整个大脑皮质厚度的潜在有前途的替代方法。
