Vol 48, No 3 (2017)

Three imidazoline-type nitroxide biradicals of the similar composition R₅^NO–CH=N–N=CH–R₅^N, B1, R₅^NO–CH=N–N=C(CH₃)–R₅^N, B2, and R₅^N–C(CH₃)=N–N=C(CH₃)–R₅^N, B3, with R₅^N and R₅^NO denoting, respectively, the nitroxide rings 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N–oxide imidazoline, have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in these biradicals dissolved in ethanol and the room temperature ionic liquid bmimBF₄ were characterized as a function of temperature by means of the analysis of the EPR lines shape. Thermodynamic parameters of the conformational rearrangements in ethanol were calculated. Analyzing the EPR spectra of these biradicals in bmimBF₄, it was revealed that the two-conformational model does not describe their conformational transitions. Moreover, the observed EPR spectra are not central symmetric especially at low temperatures that cannot be described and explained in the framework of the current theory of the intramolecular spin exchange. Probable reasons of this “strange” behavior are discussed.

Conjugate gradient SENSE (CG-SENSE) is a parallel magnetic resonance imaging reconstruction algorithm which solves the inversion problem of SENSE iteratively. One major limitation of CG-SENSE is the appropriate choice of the number of iterations required for good reconstruction results. Fewer iterations result in aliasing artifacts and too many iterations result in an increased noise level. This paper proposes a novel method to define the stopping criterion of CG-SENSE algorithm which is based on the use of correlation measure between the line profiles of the reconstructed images in the current and the previous iterations. The results are compared with Bregman distance-stopping criterion. Artifact power and peak signal-to-noise ratio are used to quantify the quality of the reconstructed images. The results demonstrate that the line profile correlation measure acts as an effective stopping criterion in CG-SENSE.

The purpose of this study was to fabricate and test a multi-frequency human brain-mimicking phantom for magnetic resonance imaging (MRI) assessment purposes. An anatomically realistic human head phantom was elaborated, for different Larmor frequencies, which allows rapid quantification of \({\text{B}}_{1}^{ + }\). It is a simple alternative solution in time and cost as compared to numerical simulations to validate simulation when the coil geometry and components cannot be known as a unique solution. The permittivity \(\varepsilon^{{\prime }}\) and conductivity \(\sigma\) of sucrose/salt/agar aqueous solutions of varying concentrations were determined; a solution with these components and having the adequate concentration to obtain the brain’s dielectric properties at 3, 7 and 11.7T was manufactured. An anthropomorphic polymeric skull was filled with this mixture. To check the behavior of this phantom in a MRI configuration, both numerical and experimental validations were done: a \({\text{B}}_{1}^{ + }\) field distribution inside the phantom was calculated with CST Microwave Studio inside a birdcage coil at 7T; the same mapping was assessed in a 7T MRI. The feasibility of a multi-MRI static field phantom was demonstrated. A solution composed by 54.7 wt% of sucrose, 3.1 wt% of salt and 3.1 wt% of agar was fabricated with good permittivity and conductivity matching for 3, 7 and 11.7T. The results were confirmed by both numerical simulation and MRI acquisition. This work has shown the possibility of manufacturing a head phantom with accessible and cheap components for MRI evaluation, having an adequate \({\text{B}}_{1}^{ + }\) field distribution and the dielectric properties of the human brain.

The theoretically known degeneracy condition of the band-pass birdcage coil has rarely been exploited in transmit coil designs. We have created an eight-channel degenerate birdcage for the human limbs at 7 T, with dedicated Tx/Rx switches and a Butler matrix. The coil can be split into two half cylinders, as required for its application to patients with limited mobility. The design of the coil, the Butler matrix, and Tx/Rx switches relied on a combination of analytical, circuital, and numerical simulations. The birdcage theory was extended to the degenerate case. The theoretical and practical aspects of the design and construction of the coil are presented. The performance of the coil was demonstrated by simulations, workbench, and scanner measurements. The fully assembled prototype presents good performance in terms of efficiency, B₁ homogeneity, and signal-to-noise ratio, despite the asymmetry introduced by the splittable design. The first in vivo images of the knee are also shown. A novel RF coil design consisting of an eight-channel splittable degenerate birdcage has been developed, and it is now available for 7 T MRI applications of the human lower limbs, including high-resolution imaging of the knee cartilages and of the patellar trabecular structure.