Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands.
Maastricht Centre for Systems Biology, Maastricht University, Maastricht, Netherlands.
Hum Brain Mapp. 2018 Jun;39(6):2412-2425. doi: 10.1002/hbm.24011. Epub 2018 Feb 18.
Determination of cortical thickness using MRI has often been criticized due to the presence of various error sources. Specifically, anatomical MRI relying on T contrast may be unreliable due to spatially variable image contrast between gray matter (GM), white matter (WM) and cerebrospinal fluid (CSF). Especially at ultra-high field (≥ 7T) MRI, transmit and receive B -related image inhomogeneities can hamper correct classification of tissue types. In the current paper, we demonstrate that residual B1+ (transmit) inhomogeneities in the T -weighted and quantitative T images using the MP2RAGE sequence at 7T lead to biases in cortical thickness measurements. As expected, post-hoc correction for the spatially varying B1+ profile reduced the apparent T values across the cortex in regions with low B1+, and slightly increased apparent T in regions with high B1+. As a result, improved contrast-to-noise ratio both at the GM-CSF and GM-WM boundaries can be observed leading to more accurate surface reconstructions and cortical thickness estimates. Overall, the changes in cortical thickness ranged between a 5% decrease to a 70% increase after B1+ correction, reducing the variance of cortical thickness values across the brain dramatically and increasing the comparability with normative data. More specifically, the cortical thickness estimates increased in regions characterized by a strong decrease of apparent T after B1+ correction in regions with low B1+ due to improved detection of the pial surface The current results suggest that cortical thickness can be more accurately determined using MP2RAGE data at 7T if B1+ inhomogeneities are accounted for.
使用 MRI 测量皮质厚度经常受到各种误差源的影响，因此受到批评。具体来说，基于 T 对比度的解剖 MRI 可能不可靠，因为灰质 (GM)、白质 (WM) 和脑脊液 (CSF) 之间的空间对比度变化。特别是在超高场 (≥7T) MRI 中，发射和接收 B 相关的图像不均匀性会阻碍组织类型的正确分类。在目前的论文中，我们证明了在 7T 下使用 MP2RAGE 序列，T 加权和定量 T 图像中残留的 B1+（发射）不均匀性会导致皮质厚度测量的偏差。正如预期的那样，对空间变化的 B1+ 分布进行事后校正，减少了低 B1+区域皮质表面的 T 值，而在高 B1+区域稍微增加了 T 值。因此，可以观察到 GM-CSF 和 GM-WM 边界的对比噪声比提高，从而导致更准确的表面重建和皮质厚度估计。总体而言，在 B1+校正后，皮质厚度的变化范围在 5%的降低到 70%的增加之间，大大降低了大脑中皮质厚度值的方差，并提高了与正常数据的可比性。更具体地说，在 B1+校正后 T 值明显降低的区域，由于脑回表面的检测得到改善，皮质厚度估计值增加。目前的结果表明，如果考虑到 B1+的不均匀性，使用 7T 下的 MP2RAGE 数据可以更准确地确定皮质厚度。
