The major methodological difference between our study and previous ones using VBM is the use of the unified model that integrates segmentation, bias correction, and registration(19). Another technical difference is that MR images were acquired at 3T in this study compared to previous studies at 1.5T (10,11,35,14). Although this is not expected to significantly impact the results, it is worth noting that studies that have quantified gray matter-white matter contrast-to-noise ratio (CNR) found higher CNR at 3T compared to 1.5 T when parameters are optimized (36,37). Higher gray matter-white matter CNR would be expected to result in better tissue segmentation and more accurate VBM results for a given spatial resolution and signal to noise ratio.
