The application of reverse-time migration (RTM) on a large scale has become affordable and has led to significant advances in seismic depth imaging in complex areas, especially for sub-salt imaging. For imaging in anisotropic media with vertically transverse isotropy (VTI), RTM is usually based on an acoustic VTI approximation of the elastic wave equation by setting the vertical shear-wave velocity to zero. However, these equations generate artificial pseudo shear-waves that spoil any seismic imaging process. By studying the wave modes associated to that equation, we identify that a second mode exists for the anelliptic case, i.e., when the Thomsen parameters ϵ is different from δ. This mode generates the spurious pseudo shear-waves. We show that as long as the simulation of the P-wave propagation is consistent with the usual marine seismic wavelengths, the spurious pseudo-shear wave is not generated or very weak compared to the P-wave mode. We illustrate this fact, for the case of a homogeneous medium modelling and for the more realistic case of a 2D Salt body modelling.