Non Destructive Testing of materials is the main application of Ultrasonic Reflection Tomography (URT). This method results from a linearization of the Inverse Acoustic Scattering Problem, named Inverse Born Approximation (IBA). URT allows perturbations (theoretically small) of a reference medium to be visualized. For media with weak inhomogeneities, one chooses the reference medium to be homogeneous: the mean medium. This leads to a “Constant Background” IBA method, whose practical solution results in regular angular scanning with broad-band pulses, allowing one to cover slice-by-slice the spatial frequency spectrum of the imaged object. This leads to “Reconstruction-From-Projections” algorithms like those used for X-ray Computed Tomography. For media with strong heterogeneities, the problem is quite non-linear and there is in general no single solution. However, for example, one is generally concerned only by flaws, which appear to be strong (but small and localized so that the result is a small disturbance) inhomogeneities in well known media, the part of component to be inspected. In this case, one can use a “Variable Background” IBA method — the reference background being the water-specimen set — to reconstruct the perturbation. URT fails when strong multiple scattering occurs (strong contrast and large object with respect to wavelength). In this case, one would guess that low frequency (less than 1 MHz) tomography will have a larger domain of validity than the classical one. But, the usual algorithm leads to poor resolution images, inappropriate for material imagery.