All the aforementioned methods rely on scattering models that express scattering amplitudes as a function of the
directions and polarizations of incident and observed waves. As a consequence, they require that ultrasonic fields are
approximated as plane waves or quasi-plane waves whose directions and polarizations can be identified. This
approximation can lead to significant inaccuracies in unfavorable cases, such as for wide probe apertures, outside of
the focal region, or for distortions of the beam due to irregular part geometries.