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Purpose: The SNR at the center of a spherical phantom of known electrical properties was measured in quasi‐identical experimental conditions as a function of magnetic field strength between 3 T and 11.7 T. Methods: The SNR was measured at the center of a spherical water saline phantom with a gradient‐recalled echo sequence. Measurements were performed at NeuroSpin at 3, 7, and 11.7 T. The phantom was then shipped to Maastricht University and then to the University of Minnesota for additional data points at 7, 9.4, and 10.5 T. Experiments were carried out with the exact same type of birdcage volume coil (except at 3 T, where a similar coil was used) to attempt at isolating the evolution of SNR with field strength alone. Phantom electrical properties were characterized over the corresponding frequency range. Results: Electrical properties were found to barely vary over the frequency range. Removing the influence of the flip‐angle excitation inhomogeneity was crucial, as expected. After such correction, measurements revealed a gain of SNR growing as B01.94 ± 0.16 compared with B02.13 according to ultimate intrinsic SNR theory. Conclusions: By using quasi‐identical experimental setups (RF volume coil, phantom, electrical properties, and protocol), this work reports experimental data between 3 T and 11.7 T, enabling the comparison with SNR theories in which conductivity and permittivity can be assumed to be constant with respect to field strength. According to ultimate SNR theory, these results can be reasonably extrapolated to the performance of receive arrays with greater than about 32 elements for central SNR in the same spherical phantom. [ABSTRACT FROM AUTHOR]

The pathophysiological processes underlying the development and progression of Alzheimer's disease (AD) on the neuronal level are still unclear. Previous research has hinted at metabolic energy deficits and altered sodium homeostasis with impaired neuronal function as a potential metabolic marker relevant for neurotransmission in AD. Using sodium (23Na) magnetic resonance (MR) imaging on an ultra‐high‐field 7 Tesla MR scanner, we found increased cerebral tissue sodium concentration (TSC) in 17 biomarker‐defined AD patients compared to 22 age‐matched control subjects in vivo. TSC was highly discriminative between controls and early AD stages and was predictive for cognitive state, and associated with regional tau load assessed with flortaucipir‐positron emission tomography as a possible mediator of TSC‐associated neurodegeneration. TSC could therefore serve as a non‐invasive, stage‐dependent, metabolic imaging marker. Setting a focus on cellular metabolism and potentially disturbed interneuronal communication due to energy‐dependent altered cell homeostasis could hamper progressive cognitive decline by targeting these processes in future interventions. [ABSTRACT FROM AUTHOR]

When using endorectal coils, local radiofrequency (RF) heating may occur in the surrounding tissue. Furthermore, most endorectal coils create a susceptibility artifact detrimental to both anatomical magnetic resonance imaging (MRI) and spectroscopy (MRS) acquisitions. We aimed at assessing the safety and MRS performance of a susceptibility‐matched endorectal coil for further rectal wall analysis. Experiments were performed on a General Electric MR750 3 T scanner. A variable number of miniaturized passive RF traps were incorporated in the reception cable. The assessment of RF heating and coil sensitivity was conducted on a 1.5% agar‐agar phantom doped with NaCl. Several susceptibility‐matched materials such as Ultem, perfluorocarbon and barium sulfate were then compared with an external coil. Finally, Ultem was used as a solid support for an endorectal coil and compared with a reference coil. Phantom experiments exhibited a complete suppression of both the RF heating phenomenon and the coil sensitivity artifact. Ultem was the material that produced the smallest image distortion. The full width at half maximum of MR spectra acquired using the susceptibility‐matched endorectal coil showed at least 30% narrowing compared with a reference endorectal coil. A susceptibility‐matched endorectal coil with RF traps incorporated was validated on phantoms. This coil appears to be a promising device for future in vivo experiments. [ABSTRACT FROM AUTHOR]

Background: [18F]‐2‐fluoro‐2‐deoxy‐D‐glucose (FDG) positron emission tomography (PET) is applied to delineate cerebral glucose metabolic patterns in Alzheimer's disease (AD). Sodium MR imaging (23Na‐MRI) can reveal alterations in tissue sodium concentration (TSC) in the brain, potentially reflecting mitochondrial dysfunction and impaired cellular energy state. This initial study aims at investigating disease‐specific topographical associations between glucose metabolism assessed via FDG‐PET and regional TSC from 23Na‐MRI, thus improving our understanding of neuroenergetics alteration in AD. Method: 20 patients with a biological biomarker‐based diagnosis of prodromal/mild AD (mean age 68±7.5; 8 females; CDR 0.75±0.57, MMSE 23.9±4.2) received an FDG‐PET, 23Na‐MRI and 1H‐MRI at 3T magnetic field (Siemens Prisma). A variable flip angle method was used to obtain TSC maps (Coste et al., 2019). To control for inter‐individual differences, FDG‐PET and TSC images were intensity‐normalized to the brainstem to create normalized glucose metabolism (nFDG) and normalized TSC maps (nTSC) (de Souza et al., 2011; Nugent et al., 2020), respectively. The Hammersmith and VolBrain Segmentation atlases were applied for region‐of‐interest analysis and to create correlation matrices between nFDG, nTSC and normalized local brain volume. Result: Regional nTSC negatively correlated with nFDG in AD patients mainly in temporal lobe structures and superior frontal gyri, indicating that decreased glucose metabolism was associated with increased TSC and vice versa. As expected, regional volume was positively correlated with nFDG in regions of the temporal lobe. When performing partial correlation controlling for volume, negative correlation clusters between nTSC and nFDG remained significant for superior frontal gyri and the anterior temporal lobes (Figure 1). Conclusion: For the first time, the association between local glucose consumption rates and sodium concentrations were investigated in AD using combined FDG‐PET and 23Na‐MRI at 3T. Consistently to our recent findings of increased TSC in AD patients (Haeger et al., 2021), the negative correlations between nTSC and nFDG values in temporal and frontal brain regions, are confirming the interplay between reduced energy metabolism and sodium increase in AD, the exact nature of which remains to be determined. References: Coste [...]. Magnetic Resonance Imaging 2019. Haeger [...]. Alzheimer's & Dementia 2021. Nugent [...]. Scientific Reports 2020. de Souza [...]. Brain 2011. [ABSTRACT FROM AUTHOR]

Abstract Purpose The state-of-the-art method to quantify sodium concentrations in vivo consists in a fully relaxed 3D spin-density (SD) weighted acquisition. Nevertheless, most sodium MRI clinical studies use short-TR SD acquisitions to reduce acquisition durations. We present a clinically viable implementation of the Variable Flip Angle (VFA) method for robust and clinically viable quantification of total sodium concentration (TSC) and longitudinal relaxation rates in vivo in human brain at 3 T. Methods Two non-Cartesian steady-state spoiled ultrashort echo time (UTE) scans, performed at optimized flip angles, repetition time and pulse length determined under specific absorption rate constraints, are used to simultaneously compute T 1 and total sodium concentration (TSC) maps using the VFA method. Images are reconstructed using the non-uniform Fast Fourier Transform algorithm and TSC maps are corrected for possible inhomogeneity of coil transmission and reception profiles. Fractioned acquisitions are used to correct for potential patient motion. TSC quantifications obtained using the VFA method are validated at first in comparison with a fully-relaxed SD acquisition in a calibration phantom. The robustness of similar VFA acquisitions are compared to the short-TR SD approach in vivo on seven healthy volunteers. Results The VFA method resulted in consistent TSC and T 1 estimates across our cohort of healthy subjects, with mean TSC of 38.1 ± 5.0 mmol/L and T 1 of 39.2 ± 4.4 ms. These results are in agreement with previously reported values in literature TSC estimations and with the predictions of a 2-compartment model. However, the short-TR SD acquisition systematically underestimated the sodium concentration with a mean TSC of 31 ± 4.5 mmol/L. Conclusion The VFA method can be applied successfully to image sodium at 3 T in about 20 min and provides robust and intrinsically T 1 -corrected TSC maps. [ABSTRACT FROM AUTHOR]

Abstract 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, T 1 , T 2 , 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, T 1 , T 2 , 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. [ABSTRACT FROM AUTHOR]