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Purpose: Sodium MRI can be used to quantify tissue sodium concentration (TSC) in vivo; however, UTE sequences are required to capture the rapidly decaying signal. 2D MRI enables high in‐plane resolution but typically has long TEs. Half‐sinc excitation may enable UTE; however, twice as many readouts are necessary. Scan time can be minimized by reducing the number of signal averages (NSAs), but at a cost to SNR. We propose using compressed sensing (CS) to accelerate 2D half‐sinc acquisitions while maintaining SNR and TSC. Methods: Ex vivo and in vivo TSC were compared between 2D spiral sequences with full‐sinc (TE = 0.73 ms, scan time ≈ 5 min) and half‐sinc excitation (TE = 0.23 ms, scan time ≈ 10 min), with 150 NSAs. Ex vivo, these were compared to a reference 3D sequence (TE = 0.22 ms, scan time ≈ 24 min). To investigate shortening 2D scan times, half‐sinc data was retrospectively reconstructed with fewer NSAs, comparing a nonuniform fast Fourier transform to CS. Resultant TSC and image quality were compared to reference 150 NSAs nonuniform fast Fourier transform images. Results: TSC was significantly higher from half‐sinc than from full‐sinc acquisitions, ex vivo and in vivo. Ex vivo, half‐sinc data more closely matched the reference 3D sequence, indicating improved accuracy. In silico modeling confirmed this was due to shorter TEs minimizing bias caused by relaxation differences between phantoms and tissue. CS was successfully applied to in vivo, half‐sinc data, maintaining TSC and image quality (estimated SNR, edge sharpness, and qualitative metrics) with ≥50 NSAs. Conclusion: 2D sodium MRI with half‐sinc excitation and CS was validated, enabling TSC quantification with 2.25 × 2.25 mm2 resolution and scan times of ≤5 mins. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the feasibility of motion correction for sodium (23Na) MRI based on interleaved acquired 3D proton (1H) navigator images. Methods: A 3D radial density‐adapted sequence for interleaved 23Na/1H MRI was implemented on a 7 Tesla whole‐body MRI system. The 1H data obtained during the 23Na acquisition were used to reconstruct 140 navigator image volumes with a nominal spatial resolution of (2.5 mm)3 and a temporal resolution of 6 s. The motion information received from co‐registration was then used to correct the 23Na image dataset, which also had a nominal spatial resolution of (2.5 mm)3. The approach was evaluated on six healthy volunteers, whose motion during the scans had different intensities and characteristics. Results: Interleaved acquisition of two nuclei did not show any relevant influence on image quality (SNR of 13.0 for interleaved versus 13.2 for standard 23Na MRI and 176.4 for interleaved versus 178.0 for standard 1H MRI). The applied motion correction increased the consistency between two consecutive scans for all examined volunteers and improved the image quality for all kinds of motion. The SD of the differences ranged between 2.30% and 6.96% for the uncorrected and between 2.13% and 2.67% for the corrected images. Conclusion: The feasibility of interleaved acquired 1H navigator images to be used for retrospective motion correction of 23Na images was successfully demonstrated. The approach neither affected the 23Na image quality nor elongated the scan time and can therefore be an important tool to improve the accuracy of quantitative 23Na MRI. [ABSTRACT FROM AUTHOR]

Purpose: To capture the multiquantum coherence (MQC) 23Na signal. Different phase‐cycling options and sequences are compared in a unified theoretical layout, and a novel sequence is developed. Methods: An open source simulation overview is provided with graphical explanations to facilitate MQC understanding and access to techniques. Biases such as B0 inhomogeneity and stimulated echo signal were simulated for 4 different phase‐cycling options previously described. Considerations for efficiency and accuracy lead to the implementation of a 2D Cartesian single and triple quantum imaging of sodium (CRISTINA) sequence employing two 6‐step cycles in combination with a multi‐echo readout. CRISTINA was compared to simultaneous single‐quantum and triple‐quantum‐filtered MRI of sodium (SISTINA) under strong static magnetic gradient. CRISTINA capabilities were assessed on 8 × 60 mL, 0% to 5% agarose phantom with 50 to 154 mM 23Na concentration at 7 T. CRISTINA was demonstrated subsequently in vivo in the brain. Results: Simulation of B0 inhomogeneity showed severe signal dropout, which can lead to erroneous MQC measurement. Stimulated echo signal was highest at the time of triple‐quantum coherences signal maximum. However, stimulated echo signal is separated by Fourier Transform as an offset and did not interfere with MQC signals. The multi‐echo readout enabled capturing both single‐quantum coherences and triple‐quantum coherences signal evolution at once. Signal combination of 2 phase‐cycles with a corresponding B0 map was found to recover the signal optimally. Experimental results confirm and complement the simulations. Conclusion: Considerations for efficient MQC measurements, most importantly avoiding B0 signal loss, led to the design of CRISTINA. CRISTINA captures triple‐quantum coherences and single‐quantum coherences signal evolution to provide complete sodium signal characterization including T2∗ fast, T2∗ slow, MQC amplitudes, and sodium concentration. [ABSTRACT FROM AUTHOR]

Purpose: Sodium MRI shows great promise as a marker for cerebral metabolic dysfunction in stroke, brain tumor, and neurodegenerative pathologies. However, cerebral blood vessels, whose volume and function are perturbed in these pathologies, have elevated sodium concentrations relative to surrounding tissue. This study aims to assess whether this fluid compartment could bias measurements of tissue sodium using MRI. Methods: Density‐weighted and B1 corrected sodium MRI of the brain was acquired in 9 healthy participants at 4.7T. Veins were identified using co‐registered 1H T2∗‐weighted images and venous partial volume estimates were calculated by down‐sampling the finer spatial resolution venous maps from the T2∗‐weighted images to the coarser spatial resolution of the sodium data. Linear regressions of venous partial volume estimates and sodium signal were performed for regions of interest including just gray matter, just white matter, and all brain tissue. Results: Linear regression demonstrated a significant venous sodium contribution above the underlying tissue signal. The apparent venous sodium concentrations derived from regression were 65.8 ± 4.5 mM (all brain tissue), 71.0 ± 7.4 mM (gray matter), and 55.0 ± 4.7 mM (white matter). Conclusion: Although the partial vein linear regression did not yield the expected sodium concentration in blood (~87 mM), likely the result of point spread function smearing, this regression highlights that blood compartments may bias brain tissue sodium signals across neurological conditions where blood volumes may differ. [ABSTRACT FROM AUTHOR]

Purpose: To investigate and to reduce influences on the determination of the short and long apparent transverse relaxation times ( T 2 , s *, T 2 , l *) of 23Na in vivo with respect to signal sampling. Methods: The accuracy of T 2 * determination was analyzed in simulations for five different sampling schemes. The influence of noise in the parameter fit was investigated for three different models. A dedicated sampling scheme was developed for brain parenchyma by numerically optimizing the parameter estimation. This scheme was compared in vivo to linear sampling at 7T. Results: For the considered sampling schemes, T 2 , s * / T 2 , l * exhibit an average bias of 3% / 4% with a variation of 25% / 15% based on simulations with previously published T 2 * values. The accuracy could be improved with the optimized sampling scheme by strongly averaging the earliest sample. A fitting model with constant noise floor can increase accuracy while additional fitting of a noise term is only beneficial in case of sampling until late echo time > 80 ms. T 2 * values in white matter were determined to be T 2 , s * = 5.1 ± 0.8 / 4.2 ± 0.4 ms and T 2 , l * = 35.7 ± 2.4 / 34.4 ± 1.5 ms using linear/optimized sampling. Conclusion: Voxel‐wise T 2 * determination of 23Na is feasible in vivo. However, sampling and fitting methods have to be chosen carefully to retrieve accurate results. Magn Reson Med 80:571–584, 2018. © 2018 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose A multinuclei imaging setup with the capability to acquire both sodium (23Na) and proton (1H) signals at 9.4 Tesla is presented. The main objective was to optimize coil performance at the 23Na frequency while still having the ability to acquire satisfactory 1H images. Methods The setup consisted of a combination of three radio frequency (RF) coils arranged in three layers: the innermost layer was a 27-channel 23Na receive helmet which was surrounded by a four-channel 23Na transceiver array. The outer layer consisted of a four-channel 1H dipole array for B0 shimming and anatomical localization. Transmit and receive performance of the 23Na arrays was compared to a single-tuned 23Na birdcage resonator. Results While the transmit efficiency of the 23Na transceiver array was comparable to the birdcage, the 23Na receive array provided substantial signal-to-noise ratio (SNR) gain near the surface and comparable SNR in the center. The utility of this customized setup was demonstrated by 23Na images of excellent quality. Conclusion High SNR, efficient transmit excitation and B0 shimming capability can be achieved for 23Na MRI at 9.4T using novel coil combination. This RF configuration is easily adaptable to other multinuclei applications at ultra high field (≥ 7T). Magn Reson Med 75:906-916, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

23Na magnetic resonance imaging and the delayed gadolinium-enhanced magnetic resonance imaging methods to investigate cartilage can be used to determine the fixed charge density of cartilage. The methods give results that differ by a factor of 2. In this study, we use Monte Carlo simulations on a model system of cartilage and find that the difference originates from the Coulombic intermolecular interactions between the ions in the cartilage, and in the synovial fluid. Those interactions are neglected in the standard Donnan analysis that generally is adopted to evaluate magnetic resonance imaging data. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the feasibility of motion correction for sodium ( 23 Na) MRI based on interleaved acquired 3D proton ( 1 H) navigator images., Methods: A 3D radial density-adapted sequence for interleaved 23 Na/ 1 H MRI was implemented on a 7 Tesla whole-body MRI system. The 1 H data obtained during the 23 Na acquisition were used to reconstruct 140 navigator image volumes with a nominal spatial resolution of (2.5 mm) 3 and a temporal resolution of 6 s. The motion information received from co-registration was then used to correct the 23 Na image dataset, which also had a nominal spatial resolution of (2.5 mm) 3 . The approach was evaluated on six healthy volunteers, whose motion during the scans had different intensities and characteristics., Results: Interleaved acquisition of two nuclei did not show any relevant influence on image quality (SNR of 13.0 for interleaved versus 13.2 for standard 23 Na MRI and 176.4 for interleaved versus 178.0 for standard 1 H MRI). The applied motion correction increased the consistency between two consecutive scans for all examined volunteers and improved the image quality for all kinds of motion. The SD of the differences ranged between 2.30% and 6.96% for the uncorrected and between 2.13% and 2.67% for the corrected images., Conclusion: The feasibility of interleaved acquired 1 H navigator images to be used for retrospective motion correction of 23 Na images was successfully demonstrated. The approach neither affected the 23 Na image quality nor elongated the scan time and can therefore be an important tool to improve the accuracy of quantitative 23 Na MRI., (© 2022 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Purpose: To investigate and to reduce influences on the determination of the short and long apparent transverse relaxation times ( T2,s*, T2,l*) of 23 Na in vivo with respect to signal sampling., Methods: The accuracy of T2* determination was analyzed in simulations for five different sampling schemes. The influence of noise in the parameter fit was investigated for three different models. A dedicated sampling scheme was developed for brain parenchyma by numerically optimizing the parameter estimation. This scheme was compared in vivo to linear sampling at 7T., Results: For the considered sampling schemes, T2,s* / T2,l* exhibit an average bias of 3% / 4% with a variation of 25% / 15% based on simulations with previously published T2* values. The accuracy could be improved with the optimized sampling scheme by strongly averaging the earliest sample. A fitting model with constant noise floor can increase accuracy while additional fitting of a noise term is only beneficial in case of sampling until late echo time > 80 ms. T2* values in white matter were determined to be T2,s* = 5.1 ± 0.8 / 4.2 ± 0.4 ms and T2,l* = 35.7 ± 2.4 / 34.4 ± 1.5 ms using linear/optimized sampling., Conclusion: Voxel-wise T2* determination of 23 Na is feasible in vivo. However, sampling and fitting methods have to be chosen carefully to retrieve accurate results. Magn Reson Med 80:571-584, 2018. © 2018 International Society for Magnetic Resonance in Medicine., (© 2018 International Society for Magnetic Resonance in Medicine.)