Stefan Enoch

Purpose: The design of radiofrequency (RF) coils tailored for the target anatomy is necessary to overcome the transmit field homogeneity and penetration issues at Ultra‐High‐Field (UHF). For the spinal cord, the optimal design has not yet been determined. This work intends to demonstrate the added value of a pair of passive dipoles placed under a conventional thoracic spine 4 × 2‐loop array. Methods: The conventional loop array produces a broad transmit field with unnecessary lateral spread. To address this, two passive dipoles were incorporated and tuned out‐of‐phase to actively suppress these lateral side‐lobes and increase field penetration the spinal cord region. The design was numerically optimised using a homogeneous phantom, followed by human voxel model simulations to evaluate performance and specific absorption rate (SAR). Finally, a prototype was tested in a 7T scanner using a homogeneous phantom to acquire experimental maps. Results: Human voxel model simulations demonstrated that the passive dipoles improve the transmit field distribution and predicted a 35% gain in SAR efficiency in the spinal cord region. Correspondingly, phantom experiments showed a 25% gain in transmit efficiency and 15% gain in homogeneity in a selected region of interest compared to the loop‐array‐only reference. Conclusion: Incorporating passive dipoles effectively suppresses unwanted lateral field spread, increasing penetration depth and efficiency in the thoracic spinal cord region. While homogeneous phantom results offer proof of concept, numerical simulations suggest reliable performance in realistic spine anatomy as well as substantial SAR reduction.

We demonstrate strongly collimated beam formation, at audible frequencies, in a three-dimensional acoustic phononic crystal where the wavelength is commensurate with the crystal elements; the crystal is a seemingly simple rectangular cuboid constructed from closely-spaced spheres, and yet demonstrates rich wave phenomena acting as a canonical three-dimensional metamaterial. We employ theory, numerical simulation and experiments to design and interpret this collimated beam phenomenon and use a crystal consisting of a finite rectangular cuboid array of 4 × 10 × 10 polymer spheres 1.38 cm in diameter in air, arranged in a primitive cubic cell with the centre-to-centre spacing of the spheres, i.e. the pitch, as 1.5 cm. Collimation effects are observed in the time domain for chirps with central frequencies at 14.2 kHz and 18 kHz, and we deployed a laser feedback interferometer or Self-Mixing Interferometer – a recently proposed technique to observe complex acoustic fields—that enables experimental visualisation of the pressure field both within the crystal and outside of the crystal. Numerical exploration using a higher-order multi-scale finite element method designed for the rapid and detailed simulation of 3D wave physics further confirms these collimation effects and cross-validates with the experiments. Interpretation follows using High Frequency Homogenization and Bloch analysis whereby the different origin of the collimation at these two frequencies is revealed by markedly different isofrequency surfaces of the sonic crystal.

The energy harvesting system was developed for preclinical 4.7 T MRI scanners. The system is composed of a commercial birdcage coil tuned and matched at 200.1 MHz, a harvesting coil and an RF-DC rectifier. The collected energy was used to power a respiratory pressure sensor. The efficiency of the harvesting system was optimized as function of the angular positioning of the harvesting coil in the birdcage. The experiments were carried out with a water phantom and a mouse in vivo. A peak voltage of 1.5V was harvested for 45V input voltage and delivered to a load of 350 Ω.

Comments about our published paper entitled “A Semi-Analytical Model of High-Permittivity Dielectric Ring Resonators for Magnetic Resonance Imaging, in IEEE Transactions on Antennas and Propagation, vol. 68, no. 8, pp.6317-6329, August 2020, doi: 10.1109/TAP.2020.2980771.” were proposed.We thank and reply to the authors of the comments about our model of high-permittivity dielectric ring resonators used as microscopy magnetic resonance probes. In this prospect, we reply part by part to the comments.

Metasurfaces and, in particular, metalenses have attracted large interest and enabled various applications in the near-infrared and THz regions of the spectrum. However, the metalens design in the visible range stays quite challenging due to the smaller nanostructuring scale and the limited choice of lossless CMOS-compatible materials. We develop a simple yet efficient design of a polarization-independent, broadband metalens suitable for many CMOS-compatible fabrication techniques and materials and implement it for the visible spectral range using niobium pentoxide (Nb₂O₅). The produced metalens demonstrates high transmittance and focusing ability as well as a large depth of focus, which makes it a promising solution for a new generation of silicon photomultiplier photodetectors with reduced fill factor impact on the performance and reduced electron–hole generation regions, which altogether potentially leads to improved photodetection efficiency and other characteristics.

Magnetic Resonance Imaging (MRI) is an imaging technique exploiting the magnetic resonance of specific nuclear spins, like protons. In this paper, MR probes based on dielectric ring resonators are investigated from a theoretical approach. We take advantage of the high-permittivity and low-losses properties of the ceramic material used for manufacturing these probes for microscopy applications. Magnetic Resonance Microscopy (MRM) aims at imaging tiny samples with a sufficient resolution to distinguish small details. In this framework, compact resonators, called volume probes, contain the investigated sample and are used for both signal transmission and reception. The new developed semi-analytical model enables estimation of the frequency of the first transverse electric mode of a cylindrical resonator. It also provides a method to compute the corresponding magnetic field distribution, the dielectric losses contributions from the probe and the sample, and Signal-to-Noise Ratio (SNR). The proposed approach aims at providing design guidelines for dielectric probes.