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Purpose: To introduce quantitative rapid gradient‐echo (QRAGE), a novel approach for the simultaneous mapping of multiple quantitative MRI parameters, including water content, T1, T2*, and magnetic susceptibility at ultrahigh field strength. Methods: QRAGE leverages a newly developed multi‐echo MPnRAGE sequence, facilitating the acquisition of 171 distinct contrast images across a range of TI and TE points. To maintain a short acquisition time, we introduce MIRAGE2, a novel model‐based reconstruction method that exploits prior knowledge of temporal signal evolution, represented as damped complex exponentials. MIRAGE2 minimizes local Block‐Hankel and Casorati matrices. Parameter maps are derived from the reconstructed contrast images through postprocessing steps. We validate QRAGE through extensive simulations, phantom studies, and in vivo experiments, demonstrating its capability for high‐precision imaging. Results: In vivo brain measurements show the promising performance of QRAGE, with test–retest SDs and deviations from reference methods of < 0.8% for water content, < 17 ms for T1, and < 0.7 ms for T2*. QRAGE achieves whole‐brain coverage at a 1‐mm isotropic resolution in just 7 min and 15 s, comparable to the acquisition time of an MP2RAGE scan. In addition, QRAGE generates a contrast image akin to the UNI image produced by MP2RAGE. Conclusion: QRAGE is a new, successful approach for simultaneously mapping multiple MR parameters at ultrahigh field. [ABSTRACT FROM AUTHOR]

Background: Given that changes in brain water content are often correlated with disease, investigating water content non‐invasively and in vivo could lead to a better understanding of the pathogenesis of several neurologic diseases. Purpose: To adapt a super‐resolution‐based technique, previously developed for humans, to the rat brain and report in vivo high‐resolution (HR) water content maps in comparison with ex vivo wet/dry methods. Study Type: Prospective. Animal Model: Eight healthy male Wistar rats. Field Strength/Sequence: 9.4‐T, multi‐echo gradient‐echo (mGRE) sequence. Assessment: Using super‐resolution reconstruction (SRR), a HR mGRE image (200 μm isotropic) was reconstructed from three low‐resolution (LR) orthogonal whole‐brain images in each animal, which was followed by water content mapping in vivo. The animals were subsequently sacrificed, the brains excised and divided into five regions (front left, front right, middle left, middle right, and cerebellum–brainstem regions), and the water content was measured ex vivo using wet/dry measurements as the reference standard. The water content values of the in vivo and ex vivo methods were then compared for the whole brain and also for the different regions separately. Statistical Tests: Friedman's non‐parametric test was used to test difference between the five regions, and Pearson's correlation coefficient was used for correlation between in vivo and ex vivo measurements. A P‐value <0.05 was considered statistically significant. Results: Water content values derived from in vivo MR measurements showed strong correlations with water content measured ex vivo at a regional level (r = 0.902). Different brain regions showed significantly different water content values. Water content values were highest in the frontal brain, followed by the midbrain, and lowest in the cerebellum and brainstem regions. Data Conclusion: An in vivo technique to achieve HR isotropic water content maps in the rat brain using SRR was adopted in this study. The MRI‐derived water content values obtained using the technique showed strong correlations with water content values obtained using ex vivo wet/dry methods. Level of Evidence: 1 Technical Efficacy: Stage 1 [ABSTRACT FROM AUTHOR]

Background Purpose Methods Results Conclusions In ovo MR presents a promising and viable alternative to traditional in vivo small animal experiments. Sodium MRI complements proton MRI by providing potential access to tissue cellular metabolism. Despite its abundance, sodium MRI is challenged by lower MR sensitivity and faster relaxation times compared to proton MRI. Ensuring a high signal‐to‐noise ratio and effective B0 shimming is essential. Double‐tuned coils combining 23Na and 1H are frequently employed to achieve structural imaging and efficient shim adjustment.This study introduces a novel, highly optimized, double‐tuned coil design, specifically for MR scans of chick embryos.A tapered‐spiral, double‐tuned coil was designed and constructed following careful consideration of design parameters. The performance of the coil was rigorously assessed through bench tests, and final validation was conducted on a 7 T MRI scanner using a chick embryo.Bench tests demonstrated that the return losses for both 1H and 23Na coils were better than − 30 dB, and isolation factors were better than − 21 dB, indicating that the double‐tuned coil was well‐set, with negligible coupling between channels. MR images of chick embryos, obtained using the coil, validated the feasibility of utilizing the design concept for in ovo applications.The innovative design of the proposed double‐tuned coil, characterized by its unique arrangement, offers improved performance. This design has the potential to significantly enhance the quality of in ovo 1H and 23Na measurements. [ABSTRACT FROM AUTHOR]

Purpose: To propose two innovations to existing eddy current characterization techniques, which include (1) an efficient spatio‐temporal sampling scheme and (2) a model‐based fitting of spherical harmonic eddy current components. Theory and Methods: This work introduces a three‐plane 2D image‐based acquisition scheme to efficiently sample eddy current fields. Additionally, a model‐based spherical harmonic decomposition is presented, which reduces fitting noise using a rank minimization to impose an exponential decay on the eddy current amplitude evolution. Both techniques are applied in combination and analyzed in simulations for their applicability in reconstructing suitable pre‐emphasis parameters. In a proof‐of‐concept measurement, the routine is tested for its propriety to correctly quantify user‐defined field dynamics. Furthermore, based on acquired precompensation and postcompensation eddy current data, the suitability of pre‐emphasis parameters calculated based on the proposed technique is evaluated. Results: Simulation results derived from 500 data sets demonstrate the applicability of the acquisition scheme for the spatio‐temporal sampling of eddy current fields. Compared with a conventional data processing strategy, the proposed model‐based approach yields pre‐emphasis parameters that reduce the average maximum residual field offset within a 10‐cm‐diameter spherical volume from 3.17 Hz to 0.58 Hz. Experimental data prove the proposed routine to be suitable to measure and effectively compensate for eddy currents within 10 minutes of acquisition time. Conclusion: The proposed framework was found to be well‐suited to efficiently characterize and compensate for eddy current fields in a one‐time calibration effort. It can be applied to facilitate pre‐emphasis implementations, such as for dynamic B0 shimming applications. [ABSTRACT FROM AUTHOR]

Purpose: To overcome existing challenges in dynamic B0 shimming by implementing a shim optimization algorithm which limits shim current amplitudes and their temporal variation through the application of constraints and regularization terms. Theory and Methods: Spherical harmonic dynamic B0 shimming is complicated by eddy currents, ill‐posed optimizations, and the need for strong power supplies. Based on the fact that eddy current amplitudes are proportional to the magnitude of the shim current changes, and assuming a smoothness of the B0 inhomogeneity variation in the slice direction, a novel algorithm was implemented to reduce eddy current generation by limiting interslice shim current changes. Shim degeneracy issues and resulting high current amplitudes are additionally addressed by penalizing high solution norms. Applicability of the proposed algorithm was validated in simulations and in phantom and in vivo measurements. Results: High‐order dynamic shimming simulations and measurements have shown that absolute shim current amplitudes and their temporal variation can be substantially reduced with negligible loss in achievable B0 homogeneity. Whereas conventional dynamic shim updating optimizations improve the B0 homogeneity, on average, by a factor of 2.1 over second‐order static solutions, our proposed routine reached a factor of 2.0, while simultaneously providing a 14‐fold reduction of the average maximum shim current changes. Conclusions: The proposed algorithm substantially reduces the shim amplitudes and their temporal variation, while only marginally affecting the achievable B0 homogeneity. As a result, it has the potential to mitigate the remaining challenges in dynamic B0 shimming and help in making its application more readily available. [ABSTRACT FROM AUTHOR]

Describes a design procedure to determine the circuit elements for optimum operation of the Class-E amplifier using state space methods. Description of the Class-E amplifier; Suitability of the amplitude modulation characteristic of the amplifier for high-efficiency amplifier systems; Consideration of finite feed inductance and quality factor of the resonant circuit.

Parallel radiofrequency (RF) transmission, either in the form of RF shimming or pulse design, has been proposed as a solution to the B1+ inhomogeneity problem in ultra high field magnetic resonance imaging. As a prerequisite, accurate B1+ maps from each of the available transmit channels are required. In this work, four different encoding methods for B1+ mapping, namely 1-channel-on, all-channels-on-except-1, all-channels-on-1-inverted and Fourier phase encoding, were evaluated using dual refocusing acquisition mode (DREAM) at 9.4 T. Fourier phase encoding was demonstrated in both phantom and in vivo to be the least susceptible to artefacts caused by destructive RF interference at 9.4 T. Unlike the other two interferometric encoding schemes, Fourier phase encoding showed negligible dependency on the initial RF phase setting and therefore no prior B1+ knowledge is required. Fourier phase encoding also provides a flexible way to increase the number of measurements to increase SNR, and to allow further reduction of artefacts by weighted decoding. These advantages of Fourier phase encoding suggest that it is a good choice for B1+ mapping in parallel transmit systems at ultra high field. [ABSTRACT FROM AUTHOR]