Simultaneous proton density, T1 , T2 , and flip-angle mapping of the brain at 7 T using multiparametric 3D SSFP imaging and parallel-transmission universal pulses.

PURPOSE: Performing simultaneous quantitative MRI at ultrahigh field is challenging, as B0 and B1+ heterogeneities as well as specific absorption rate increase. Too large deviations of flip angle from the target can induce biases and impair SNR in the quantification process. In this work, we use calibration-free parallel transmission, a dedicated pulse-sequence parameter optimization and signal fitting to recover 3D proton density, flip angle, T1 , and T2 maps over the whole brain, in a clinically suitable time. METHODS: Eleven optimized contrasts were acquired with an unbalanced SSFP sequence by varying flip-angle amplitude and RF phase-cycling increment, at a 1.0 × 1.0 × 3.0 mm3 resolution. Acquisition time was 16 minutes 36 seconds for the whole brain. Parallel transmission and universal pulses were used to mitigate B1+ heterogeneity, to improve the results' reliability over 6 healthy volunteers (3 females/3 males, age 22.6 ± 2.7 years old). Quantification of the physical parameters was performed by fitting the acquired contrasts to the simulated ones using the Bloch-Torrey equations with a realistic diffusion coefficient. RESULTS: Whole-brain 3D maps of effective flip angle, proton density, and relaxation times were estimated. Parallel transmission improved the robustness of the results at 7 T. Results were in accordance with literature and with measurements from standard methods. CONCLUSION: These preliminary results show robust proton density, flip angle, T1 , and T2 map retrieval. Other parameters, such as ADC, could be assessed. With further optimization in the acquisition, scan time could be reduced and spatial resolution increased to bring this multiparametric quantification method to clinical research routine at 7 T.

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.

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

PURPOSE: 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. METHODS: Permittivity measurements were done on aqueous solutions from 0%v/v to dry powder. The permittivity dependence with pressure was investigated. Scanning electron microscopy images were performed on a few representative solutions. BaTiO3 pressed pads of different thicknesses, permittivities, and distances to the head were compared in a 7T MRI scanner. RESULTS: Perovskite aqueous mixtures undergo a pressure-dependent phase transition in terms of permittivity, with increasing water content. A new relative permittivity maximum of 475 was achieved. Microscopic images revealed structural differences between phases. A B1+ improvement in the temporal lobe was obtained with thin, high permittivity BaTiO3 head. CONCLUSIONS: This new preparation method allows improved pad geometry and placement, as a result of the high relative permittivity values achieved. This method has great significance for medical applications of MRI dielectric shimming, being easy to replicate and implement on a large scale. Magn Reson Med 79:1753-1765, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.