Ultrasonic imaging of static objects through an aberrating layer using harmonic phase conjugation approach.

The main goal of this study is to develop a new image reconstruction approach for the ultrasonic detection of small objects (comparable to or smaller than the ultrasonic wavelength) behind an aberrating layer. Instead of conventional pulse-echo experimental setup we used through transmission, as the backscattered field after going twice through the layer becomes much weaker than the through-transmitted field. The proposed solution is based on the Harmonic Phase Conjugation (HPC) technique. The developed numerical model allows to calculate the amplitude and phase distributions of the through-transmitted acoustic field interacting with the objects and received by a linear transducer array either directly or after passing through an additional aberrating layer. Then, the digitized acoustic field received by the array is processed, phase-conjugated, and finally, numerically propagated back through the medium in order to reconstruct the image of the target objects. The reconstruction quality of the algorithm was systematically tested on a numerical model, which included a barrier, a medium behind it, and a group of three scatterers, by varying scatterer distances from the source transducer, their mutual arrangement, and the angle of the incident field. Subsequently, a set of laboratory experiments was conducted (at transmit frequency of 2 MHz) to verify the accuracy of the developed simulation. The results demonstrate feasibility of imaging multiple scattering objects through a barrier using the HPC method with better than 1mm accuracy. The results of these tests are presented, and the feasibility of implementing this approach for various biomedical and NDT imaging applications is discussed.