The Degree of Non-Linearity (DNL) of scattering problems and specifically of differential Microwave Imaging (dMWI) problems is addressed in this paper. The quantification of the DNL amounts to evaluate the validity range of the Born first order (linear) approximation. It is therefore a way to assess the difficulty of both forward and inverse scattering problems arising in Microwave Imaging. Several examples of 3D dMWI with various DNLs are considered to illustrate the interest of the proposed derivation.