Purpose: To assess the effect of radiofrequency (RF) shimming of a 3 Tesla (T) two‐port body coil on B₁+ uniformity, the local specific absorption rate (SAR), and the local temperature increase as a function of the thermoregulatory response.Methods: RF shimming alters induced current distribution, which may result in large changes in the level and location of absorbed RF energy. We investigated this effect with six anatomical human models from the Virtual Population in 10 imaging landmarks and four RF coils. Three thermoregulation models were applied to estimate potential local temperature increases, including a newly proposed model for impaired thermoregulation.Results: Two‐port RF shimming, compared to circular polarization mode, can increase the B₁+ uniformity on average by +32%. Worst‐case SAR excitations increase the local RF power deposition on average by +39%. In the first level controlled operating mode, induced peak temperatures reach 42.5°C and 45.6°C in patients with normal and impaired thermoregulation, respectively.Conclusion: Image quality with 3T body coils can be significantly increased by RF shimming. Exposure in realistic scan scenarios within guideline limits can be considered safe for a broad patient population with normal thermoregulation. Patients with impaired thermoregulation should not be scanned outside of the normal operating mode. Magn Reson Med 76:986–997, 2016.

A monopulse horn feed with a triple radiation pattern (sum, elevation difference and azimuthal difference) operating at 35 is presented. The device consists of three main parts: a spline profiled horn, a mode converter that generates the modal field combinations that produce each radiation pattern, and a very compact single-layer monopulse comparator network. Unlike other state-of-the-art designs, the novel feed is based on a triple radiation pattern horn with a mode converter excited by a comparator network implemented in a single level for improving the compactness and easing a quasi 2D manufacturing. The kernel of this network is a novel 180∘ H-plane waveguide coupler with in-line ports, which avoids any additional phase compensation in the routing waveguide sections used in other works. The design process of the Ka-band antenna feed is described, and simulated results for the three radiation patterns and their associated reflection coefficients are presented. The prototyping process is outlined and the obtained experimental results are compared with the simulated ones showing very good agreement and validating the design process.

A general method to obtain a matrix which allows the compensation of mutual coupling effects in transmitting arrays for the total field in all directions is introduced. This method is independent of the numerical method used in the analysis and it can include the effect of the antenna platform. The starting point can be the active element patterns or the spherical mode expansion from spherical near-field antenna measurements. Additionally, the spherical mode expansion is also used to find a matrix which allows the compensation of mutual coupling effects in receiving arrays. Through this theory, a simple relation between the compensation matrices of the transmitting and the receiving arrays is found. As a consequence, the scattering matrix of a circuit that allows the simultaneous compensation of mutual coupling effects for the transmission and the reception problem can be easily defined. Finally, it will be shown how the capabilities of the compensation in all directions depend strongly on the array element.

An accurate and efficient method is proposed to characterize waveguides with arbitrary cross sections exhibiting discrete rotational symmetries, known in the literature as $C_{N}$ symmetries. This new method is based on a novel tailored 2-D finite element method (FEM) formulation exploiting rotationally periodic boundary conditions involved in the minimum sector that defines the waveguide cross section. While retaining the proven accuracy and convergence rate of FEM, it allows for a reduction in dimensionality of the problem and classifies the modes with respect to their rotational symmetry. Most importantly, it discriminates reliably and unambiguously degenerate modes (no matter how high their order is) with the exact same computed cutoff frequency value. This is especially interesting in modal analysis of devices involving circular polarization, since the formulation inherently provides different mode sets for clockwise and counterclockwise circularly polarized modes. The method has been tested on a wide variety of waveguides and computed higher order modes have been compared against analytical solutions and general-purpose FEM.

This letter presents a flexible method to compensate the interelement mutual coupling (MC) effects that may degrade the field pattern of an array of real and coupled antennas. A closed-expression for a mutual coupling compensation matrix (MCCM) is derived. The MCCM is used to compensate the presence of the real individual elements' patterns and the interelement MC effects for any excitations obtained with an isotropic-based pattern synthesis method. The MCCM is calculated from the generalized scattering matrix (GSM) of an antenna array and the spherical mode expansion (SME) of its radiated field. For a given array, this MCCM has to be calculated only once since it only depends on the radiating and scattering characteristics of the antenna elements as well as on their location in the array. Conditions regarding null field pattern directions can also be reinforced in the MCCM. To compute the GSM of the array and the SME of the radiated field, a validated full-wave hybrid and modular methodology is used. Numerical results of synthesized patterns where the MC effects have been compensated are presented for arrays made up of dielectric resonator antennas.