Spatial resolution determines how "sharp" the image looks. Low resolution
will give either fuzzy edges, or a pixelly appearance to the image. Spatial
resolution is defined by the size of the imaging voxels. Since voxels are
three dimensional rectangular solids, the resolution is frequently different
in the three different directions. The size of the voxel and therefore the
resolution depends on matrix size, the field-of-view (FOV), and the slice
thickness. The matrix size is the number of frequency encoding steps, in
one direction; and the number of phase encoding steps, in the other direction
of the image plane. Assuming everything else is constant, increasing the
number of frequency encodings or the number of phase steps results in improved
resolution. The frequency encoding depends of how rapidly the FID signal
is sampled by the scanner. Increasing the sampling rate results in no time
penalty. Increasing the number of phase steps increases the time of the
acquisition proportionately. This is why you may see images that have fewer
phase encodings than frequency encodings, e.g., 128x256 or 192x256.
The FOV is the total area that the matrix of phase and frequency encoding cover. Dividing the FOV by the matrix size gives you the voxel size; hence, increasing the FOV in either direction increases the size of the voxels and decreases the resolution. Decreasing the FOV improves the resolution.
The depth of the voxel is determined by the slice thickness. This is almost always the largest dimension of the voxel. Therefore, the resolution perpendicular to the image plane is the poorest.