Anisotropy (the opposite of isotropy) is the property of being directionally dependent. Something which is anisotropic may appear different or have different characteristics in different directions. (Wikipedia)
A spherically symmetric Point Spread Function would be equal in all directions and would produce isotropic images.
Images from 3D Microscopes are anisotropic because the Point Spread Function is (at least in ideal conditions) cylindrically symmetric and larger along the optical axis.