Is a slice obtained with functional MRI (fMRI) always 3-dimensional, i.e., x-y-z? I thought that a typical fMRI scan's total imaged volume would be 4D, i.e., made up of 3D slices plus time.
I will focus on question #1:
Analysis of MRI scans is typically done using voxels (Fig. 1). Voxels have a volume defined by three dimensions (length, width and depth).
Voxel analysis. Source: Philips
The slice thickness plus the 2D in-plane resolution (pixel size) yields the voxel size. The time to scan a slice can be regarded to be negligible. Making a scan consisting of multiple slices takes time. Hence, the time dimension is not directly relevant for voxel size. Practically, voxel size depends mostly on slice size, as well as the capacity of the scanner. More Tesla's makes smaller voxels possible.