The most common analysis approach for MRI is voxel-based morphometry (VBM), which measures the volume of tissue (grey matter, GM; white matter, WM) in the brain. This method can be used to compare brain tissue across two groups, such as AU versus non-AU youth. The spatial resolution of MRI is higher than other types of brain scanning, but low compared with studying brain cells under a microscope: each voxel of WM contains thousands of axons, and each voxel of GM contains tens of thousands of neurons and millions of synapses. Thus, we cannot be sure what changes in WM or GM as seen in MRI correspond to at the level of the cell or the synapse, and this question is debated elsewhere (e.g., Paus et al., 2008).
