A well sampled image is one in which the acquisition was done with Sample Sizes (Voxel sizes) at least as small as the Critical Sampling Distance in all directions of space.
It can be oversampled (this means that redundant information was recorded), but never undersampled.
See the Nyquist Calculator.
It can be oversampled (this means that redundant information was recorded), but never undersampled.
See the Nyquist Calculator.