Résumé: © 2019 Acoustical Society of America. Generation of elastic waves is a major issue in nondestructive testing. Structural health monitoring of a thin element can be achieved through the analysis of its resonance spectrum. A time reversal mirror (TRM) operating in the audible frequency range (1-10 kHz) is used to remotely excite thin resonant elastic elements. The generation of elastic waves is studied with respect to the geometry of the TRM. It is observed that the quality of focusing only weakly depends on the number of loudspeakers (LS) in the TRM. When the air/plate coupling is at its maximum, the energetic efficiency of the TRM is estimated to be about 0.02%. The TRM is shown to efficiently and selectively excite a small structure embedded in a complex environment such as a hollow cylinder. Finally, the results are discussed in light of the DORT method (French acronym for “decomposition of the time reversal operator”). In particular, the optimal LS placement and emission signals in this configuration to excite individual eigenmodes of a plate is determined.

Résumé: © 2019 The Authors Preclinical MR applications at 17.2 T can require field of views on the order of a few square centimeters. This is a challenging task as the proton Larmor frequency reaches 730 MHz. Most of the protocols at such frequencies are performed with surface transceiver coils for which the sensitive volume and the signal to noise ratio (SNR) is given by their size. Here we propose an approach based on metamaterials in order to enhance the sensitive volume of a commercial surface coil for small animal imaging at 17.2 T. We designed a passive resonator composed of four hybridized electric dipoles placed onto the floor of the MRI bed. Combining numerical and experimental results on a phantom and in vivo, we demonstrate a 20% increase of the sensitive volume in depth and 25% along the rostro-caudal axis while maintaining more than 85% of the local SNR right beneath the surface coil plane. Moreover, our solution gives the ability to double the average SNR in the region between 1.2 and 2 cm away from the loop using a single layer of 1 mm thick metallic wires easy to design and manufacture.