Axial Super-Resolution in Ultrasound Imaging

A fundamental challenge in non-destructive evaluation using ultrasound is to accurately estimate the thicknesses of different layers or cracks present in the object being probed. This inherently corresponds to localizing the point-sources of the reflections from the received signal. Conventional signal processing techniques cannot resolve reflectors whose spacing is below the axial resolution limit, which is of the order of the wavelength of the probing pulse. The objective of this paper is to demonstrate axial super-resolution capability using both simulated and experimental ultrasound data. We show that the ultrasound reflections could be modelled effectively as FRI signals, which can be sampled at sub-Nyquist rates. The FRI sampling method brings the reconstruction problem within a parametric estimation framework, for which efficient high-resolution spectral estimation techniques are available. We experimentally demonstrate that the proposed technique is able to resolve the thicknesses of layers of custom designed Agarose phantoms that are up to 2.25 times below the conventional resolution limit.