Franck Mauconduit

In vivo demonstration of whole‐brain multislice multispoke parallel transmit radiofrequency pulse design in the small and large flip angle regimes at 7 tesla

A multispoke specific absorption rate (SAR) ‐aware pulse design approach for homogeneous multiple‐slice small and large flip angle (FA) excitations with parallel transmission is proposed. The approach aims at optimizing in a slice‐specific manner the spokes locations and radiofrequency pulses.

Stacked magnetic resonators for MRI RF coils decoupling

Parallel transmission is a very promising method to tackle B1+ field inhomogeneities at ultrahigh field in magnetic resonant imaging (MRI). This technique is however limited by the mutual coupling between the radiating elements. Here we propose to solve this problem by designing a passive magneto-electric resonator that we here refer to as stacked magnetic resonator (SMR). By combining numerical and experimental methodologies, we prove that this novelty passive solution allows an efficient decoupling of elements of a phased-array coil. We demonstrate the ability of this technique to significantly reduce by more than 10dB the coupling preserving the quality of images compared to ideally isolated linear resonators on a spherical salty agar gel phantom in a 7T MRI scanner.

Signal‐domain optimization metrics for MPRAGE RF pulse design in parallel transmission at 7 tesla

Standard radiofrequency pulse design strategies focus on minimizing the deviation of the flip angle from a target value, which is sufficient but not necessary for signal homogeneity. An alternative approach, based directly on the signal, here is proposed for the MPRAGE sequence, and is developed in the parallel transmission framework with the use of the kT‐points parametrization.

Homogeneous non-selective and slice-selective parallel-transmit excitations at 7 Tesla with universal pulses: A validation study on two commercial RF coils

Parallel transmission (pTx) technology, despite its great potential to mitigate the transmit field inhomogeneity problem in magnetic resonance imaging at ultra-high field (UHF), suffers from a cumbersome calibration procedure, thereby making the approach problematic for routine use. The purpose of this work is to demonstrate on two different 7T systems respectively equipped with 8-transmit-channel RF coils from two different suppliers (Rapid-Biomed and Nova Medical), the benefit of so-called universal pulses (UP), optimized to produce uniform excitations in the brain in a population of adults and making unnecessary the calibration procedures mentioned above. Non-selective and slice-selective UPs were designed to return homogeneous excitation profiles throughout the brain simultaneously on a group of ten subjects, which then were subsequently tested on ten additional volunteers in magnetization prepared rapid gradient echo (MPRAGE) and multi-slice gradient echo (2D GRE) protocols. The results were additionally compared experimentally with the standard non-pTx circularly-polarized (CP) mode, and in simulation with subject-specific tailored excitations. For both pulse types and both coils, the UP mode returned a better signal and contrast homogeneity than the CP mode. Retrospective analysis of the flip angle (FA) suggests that the FA deviation from the nominal FA on average over a healthy adult population does not exceed 11% with the calibration-free parallel-transmit pulses whereas it goes beyond 25% with the CP mode. As a result the universal pulses designed in this work confirm their relevance in 3D and 2D protocols with commercially available equipment. Plug-and-play pTx implementations henceforth become accessible to exploit with more flexibility the potential of UHF for brain imaging.

New method to characterize and correct with sub-µs precision gradient delays in bipolar multispoke RF pulses.

Small gradient delays with respect to radiofrequency (RF) events can have disastrous effects on the performance of bipolar spokes RF pulses used in parallel transmission (pTx). In this work, we propose a new method to characterize and correct this delay with sub‐µs precision.

Compressed perovskite aqueous mixtures near their phase transitions show very high permittivities: New prospects for high-field MRI dielectric shimming

Perovskites are greatly used nowadays in many technological applications because of their high permittivity, more specifically in the form of aqueous solutions, for MRI dielectric shimming. In this study, full dielectric characterizations of highly concentrated CaTiO3/BaTiO3 water mixtures were carried out and new permittivity maxima was reached.

Design of universal parallel-transmit refocusing kT-point pulses and application to 3D T2-weighted imaging at 7T: Universal Pulse Design of 3D Refocusing Pulses

T2‐weighted sequences are particularly sensitive to the radiofrequency (RF) field inhomogeneity problem at ultra‐high‐field because of the errors accumulated by the imperfections of the train of refocusing pulses. As parallel transmission (pTx) has proved particularly useful to counteract RF heterogeneities, universal pulses were recently demonstrated to save precious time and computational efforts by skipping B1 calibration and online RF pulse tailoring. Here, we report a universal RF pulse design for non‐selective refocusing pulses to mitigate the RF inhomogeneity problem at 7T in turbo spin‐echo sequences with variable flip angles.

Simultaneous multi-parametric mapping of total sodium concentration, T1, T2 and ADC at 7 T using a multi-contrast unbalanced SSFP

PURPOSE Quantifying multiple NMR properties of sodium could be of benefit to assess changes in cellular viability in biological tissues. A proof of concept of Quantitative Imaging using Configuration States (QuICS) based on a SSFP sequence with multiple contrasts was implemented to extract simultaneously 3D maps of applied flip angle (FA), total sodium concentration, T1, T2, and Apparent Diffusion Coefficient (ADC). METHODS A 3D Cartesian Gradient Recalled Echo (GRE) sequence was used to acquire 11 non-balanced SSFP contrasts at a 6 × 6 × 6 mm3 isotropic resolution with carefully-chosen gradient spoiling area, RF amplitude and phase cycling, with TR/TE = 20/3.2 ms and 25 averages, leading to a total acquisition time of 1 h 18 min. A least-squares fit between the measured and the analytical complex signals was performed to extract quantitative maps from a mono-exponential model. Multiple sodium phantoms with different compositions were studied to validate the ability of the method to measure sodium NMR properties in various conditions. RESULTS Flip angle maps were retrieved. Relaxation times, ADC and sodium concentrations were estimated with controlled precision below 15%, and were in accordance with measurements from established methods and literature. CONCLUSION The results illustrate the ability to retrieve sodium NMR properties maps, which is a first step toward the estimation of FA, T1, T2, concentration and ADC of 23Na for clinical research. With further optimization of the acquired QuICS contrasts, scan time could be reduced to be suitable with in vivo applications.

Fast and unconditionally safe in vivo MR head protocol for home-made coil prototype assessment at 7T

Depending on the local IRB (Internal Regulation Board) regulations for safety reasons, homemade RF coil prototype assessment through an in vivo experiment can be painful and lengthy administrative process. It includes to document simulations and experimental validations on phantom before being able to proceed. The situation can be even worse, if for some reasons, once it has passed all the acceptance stages, the coil does not deliver as expected in vivo. The process to maybe then rebooted. In this work, we introduce the concept of unconditional safe MR protocol allowing to safely use homemade coil in vivo, at any step of the RF coil development, securing valuable information for the developer, and making sure that neither local nor global SAR limits will be ever reached anywhere in the organ to image. The protocol includes in particular a fast B1+ mapping, which is essential to assess coil behaviour. The strategy can be easily extended to more contrasts, other organs and other magnetic field strengths.

B1 artifact reduction in abdominal DCE-MRI using kT-points: First clinical assessment of dynamic RF shimming at 3T: KT-points Clinical Assessment: Abdomen

The excitation inhomogeneity artifact occurring at 3T in the abdomen can lead to dramatic loss of signal and contrast, thereby hampering diagnosis.