Open birdcage coil for head imaging at 7T Nikulin, A. V., A. Vignaud, N. I. Avdievich, D. Berrahou, J. De Rosny, and A. Ourir Magnetic Resonance in Medicine (2021)
Résumé: Purpose: To theoretically describe, design, and test the new geometry of the birdcage coil for 7 Tesla anatomical brain imaging, which includes a large window on top, without deliberately jeopardizing its homogeneity and efficiency. This opencage will not only improve patient comfort but also enable the volunteer to follow functional MRI stimuli. This design could also facilitate the tracking of patient compliance and enable better correction of the movement. Methods: Via the transfer matrix approach, a birdcagelike coil with a nonperiodic distribution of rungs is constructed with optimized currents in the coil rungs. Subsequently, the coil is adjusted in fullwave simulations. Then, the coil is assembled, finetuned, and matched on the bench. Finally, these results are confirmed experimentally on a phantom and in vivo. Results: Indeed, the computed isolation of −14.9 dB between the feeding ports of the coil and the symmetry of the circular polarized mode pattern transmit RF magnetic field ((Formula presented.)) showed that the coil was properly optimized. An experimental assessment of the developed coil showed competitive transmit efficiency and coverage compared with the conventional birdcage coil of similar size. Conclusion: The proposed opencage coil can be designed and work without a dramatic drop of performance in terms of the (Formula presented.) field homogeneity, transmit efficiency ((Formula presented.) / (Formula presented.)), peak local specific absorption rate ((Formula presented.)) and SAR efficiency ((Formula presented.) / (Formula presented.)).


Laboratory Landquakes: Insights From Experiments Into the HighFrequency Seismic Signal Generated by Geophysical Granular Flows Arran, M. I., A. Mangeney, J. De Rosny, M. Farin, R. Toussaint, and O. Roche Journal of Geophysical Research: Earth Surface 126, no. 5 (2021)
Résumé: Geophysical granular flows exert basal forces that generate seismic signals, which can be used to better monitor and model these severe natural hazards. A number of empirical relations and existing models link these signals' highfrequency components to a variety of flow properties, many of which are inaccessible by other analyses. However, the range of validity of the empirical relations remains unclear and the models lack validation, owing to the difficulty of adequately controlling and instrumenting fieldscale flows. Here, we present laboratory experiments investigating the normal forces exerted on a basal plate by dense and partially dense flows of spherical glass particles. We measured the power spectra of these forces and inferred predictions for these power spectra from the models for debris flows' seismic signals proposed by Kean et al. (2015, https://doi.org/10.1002/2015GL064811), Lai et al. (2018, https://doi.org/10.1029/2018GL077683), and Farin, Tsai, et al. (2019, https://doi.org/10.1002/esp.4677), using Hertz theory to extend Farin, Tsai, et al. (2019)'s models to higher frequencies. Comparison of our observations to these predictions, and to predictions derived from Bachelet (2018) and Bachelet et al. (2021)'s model for granular flows' seismic signals, shows those of Farin, Tsai, et al. (2019)'s “thinflow” model to be the most accurate, so we examine explanations for this accuracy and discuss its implications for geophysical flows' seismic signals. We also consider the normalization, by the mean force exerted by each flow, of the force's mean squared fluctuations, showing that this ratio varies by 4 orders of magnitude over our experiments, but is determined by the bulk inertial number of the flow.


On the PathLoss of Reconfigurable Intelligent Surfaces: An Approach Based on Green&#x2019;s Theorem Applied to Vector Fields Danufane, F. H., M. Di Renzo, J. De Rosny, and S. Tretyakov IEEE Transactions on Communications, 11 (2021)
Résumé: In this paper, we introduce a physicsconsistent analytical characterization of the freespace pathloss of a wireless link in the presence of a reconfigurable intelligent surface. The proposed approach is based on the vector generalization of Green’s theorem. The obtained pathloss model can be applied to twodimensional homogenized metasurfaces, which are made of subwavelength scattering elements and that operate either in reflection or transmission mode. The pathloss is formulated in terms of a computable integral that depends on the transmission distances, the polarization of the radio waves, the size of the surface, and the desired surface transformation. Closedform expressions are obtained in two asymptotic regimes that are representative of farfield and nearfield deployments. Based on the proposed approach, the impact of several design parameters and operating regimes is unveiled.


A distortion matrix framework for highresolution passive seismic 3D imaging: Application to the San Jacinto fault zone, California Touma, R., T. Blondel, A. Derode, M. Campillo, and A. Aubry Geophysical Journal International 226, no. 2, 780794 (2021)
Résumé: Reflection seismic imaging usually suffers from a loss of resolution and contrast because of the fluctuations of the wave velocities in the Earth's crust. In the literature, phase distortion issues are generally circumvented by means of a background wave velocity model. However, it requires a prior tomography of the wave velocity distribution in the medium, which is often not possible, especially in depth. In this paper, a matrix approach of seismic imaging is developed to retrieve a 3D image of the subsoil, despite a rough knowledge of the background wave velocity. To do so, passive noise crosscorrelations between geophones of a seismic array are investigated under a matrix formalism. They form a reflection matrix that contains all the information available on the medium. A set of matrix operations can then be applied in order to extract the relevant information as a function of the problem considered. On the one hand, the background seismic wave velocity can be estimated and its fluctuations quantified by projecting the reflection matrix in a focused basis. It consists in investigating the response between virtual sources and detectors synthesized at any point in the medium. The minimization of their crosstalk can then be used as a guide star for approaching the actual wave velocity distribution. On the other hand, the detrimental effect of wave velocity fluctuations on imaging is overcome by introducing a novel mathematical object: The distortion matrix. This operator essentially connects any virtual source inside the medium with the distortion that a wavefront, emitted from that point, experiences due to heterogeneities. A time reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each real geophone at the surface and each virtual geophone in depth. Phase distortions can then be compensated for any point of the underground. Applied to passive seismic data recorded along the Clark branch of the San Jacinto fault zone (SJFZ), the present method is shown to provide an image of the fault until a depth of 4 km over the frequency range 1020Hz with an horizontal resolution of 80 m. Strikingly, this resolution is almost one eighth below the diffraction limit imposed by the geophone array aperture. The heterogeneities of the subsoil play the role of a scattering lens and of a transverse waveguide which increase drastically the array aperture. The contrast is also optimized since most of the incoherent noise is eliminated by the iterative time reversal process. Beyond the specific case of the SJFZ, the reported approach can be applied to any scales and areas for which a reflection matrix is available at a spatial sampling satisfying the Nyquist criterion.

