Cortical thickness determination of the human brain using high resolution 3T and 7T MRI data

Purpose: The analysis of the human cerebral cortex and the measurement of its thickness based on MRI data can provide insight into normal brain development and neurodegenerative disorders. Accurate and reproducible results of the cortical thickness measurement are desired for sensitive detection. This study compares ultra-high resolution data acquired at 7T with 3T data for determination of the cortical thickness of the human brain. The impact of field strength, resolution, and processing method is evaluated systematically.

Methods: Five subjects were scanned at 3T (1 mm isotropic resolution) and 7T (1 mm and 0.5 mm isotropic resolution) with 3D MP-RAGE and 3D gradient echo methods. The inhomogeneous signal and contrast of the 7T data due to the B1 field was corrected by division of the MP-RAGE with the GE. ARCTIC, utilizing a voxel-based approach, and FreeSurfer, utilizing a surface-based approach, have been used to compute the cortical thickness of the high resolution 3T and 7T data and of the ultra-high resolution 7T data. FreeSurfer is not designed to process data with a spatial resolution other than 1mm and was modified to avoid this limitation. Additionally SPM and FSL have been used to generate segmentations which were further processed with ARCTIC to determine the cortical thickness.

Results and conclusion: At identical resolution, the cortical thickness determination yielded consistent results between 3T and 7T confirming the robustness of the acquisition and processing against potential field strength related effects. However, the ultra-high resolution 7T data resulted in significantly reduced values for the cortical thickness estimation compared to the lower resolution data. The reduction in thickness amounts approximately one sixth to one third, depending on the processing algorithm and software used. This suggests a bias in the gray matter segmentation due to partial volume effects and indicates that true cortical thickness is overestimated by most current MR studies using both a voxel-based or surface-based method and can be more accurately determined with high resolution imaging at 7T.