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Purpose: This work proposes "Spin-3/2 Bloch Equation" (SBE), a consolidated formalism for spin-3/2 dynamics in biological environments. The formalism encapsulates excitation, relaxation, and off-resonance with accessible matrix representation for a straightforward implementation with high computational efficiency., Theory: The SBE is derived using spherical tensor operators to encapsulate the spin-3/2 dynamics in biological systems in a single system matrix, a formalism akin to the well-known Bloch Equations (BE)., Methods: Using the proposed SBE, simulations of three classical 23 Na pulse sequences were performed to demonstrate the versatility and applicability of the model, returning the evolution of the 23 Na spin system during these experiments: soft rectangular and adiabatic inversion recovery (IR) and triple-quantum filtering. IR simulations were compared with two existing spin-3/2 simulators and the adaptive BE as a first-order approximation., Results: The proposed SBE is straightforward to implement and facilitates accurate and fast simulations of the underlying higher order coherence in sodium experiments of biological tissues. SBE simulations and comparison spin-3/2 simulators outperform the BE simulations as expected, with the SBE offering superior computational efficiency achieved by the single system matrix formalism., Conclusion: The proposed SBE enables comprehensive and accurate simulations for spin-3/2 systems in biological tissue. With a one-line call to an ordinary differential equation solver, it offers a computationally efficient and accessible method for use in 23 Na pulse sequence design., (© 2022 International Society for Magnetic Resonance in Medicine.)

Great attention is being paid to solving, or mitigating, the technical problems associated with MRI at ultrahigh field strengths of 7 T and higher. This paper explores the use of the semiadiabatic spin-echo (SA-SE) pulse sequence, which uses semiadiabatic radiofrequency (RF) pulses to remove and/or mitigate the effects of the nonuniform B 1 excitation field and B 0 inhomogeneity associated with the electromagnetic properties of the human brain. A semiadiabatic RF pulse version of the recently published serial transmit excitation pulse (STEP) RF pulse sequence is also presented that now incorporates semiadiabatic pulses, henceforth is called SA-STEP. As demonstrated by computer simulation, and confirmed using head imaging, both techniques can produce multislice SE MR imaging at 7 T. These new methods use relatively low RF power and achieve good coverage of the human brain in a single scan., (© 2021 John Wiley & Sons, Ltd.)

Ultra-high field MRI offers many opportunities to expand the applications of MRI. In order for this to be realized, the technical problems associated with MRI at field strengths of 7 T and greater need to be solved or mitigated. This paper explores the use of new variations of composite RF pulses, named serial transmit excitation pulses (STEP), in contrast to parallel pulse techniques, in order to remove and/or mitigate the effects of non-uniform B 1 excitation fields associated with the subject (eg the human brain). Several techniques based on STEP sequences are introduced and their application to human brain imaging is presented and evaluated., (© 2020 John Wiley & Sons, Ltd.)

23 Na provides the second strongest MR-observable signal in biological tissue and exhibits bi-exponential T 2 ∗ relaxation in micro-environments such as the brain. There is significant interest in developing 23 Na biomarkers for neurological diseases that are associated with sodium channel dysfunction such as multiple sclerosis and epilepsy. We have previously reported methods for acquisition of multi-echo sodium MRI and continuous distribution modelling of sodium relaxation properties as surrogate markers of brain microstructure. This study aimed to compare 23 Na T 2 ∗ relaxation times to more established measures of tissue microstructure derived from advanced diffusion MRI at 7 ​T. Six healthy volunteers were scanned using a 3D multi-echo radial ultra-short TE sequence using a dual-tuned 1 H/ 23 Na birdcage coil, and a high-resolution multi-shell, high angular resolution diffusion imaging sequence using a 32-channel 1 H receive coil. 23 Na T 2 ∗ relaxation parameters [mean T 2 ∗ (T 2 ∗ mean ) and fast relaxation fraction (T 2 ∗ ff )] were calculated from a voxel-wise continuous gamma distribution signal model. White matter (restricted anisotropic diffusion) and grey matter (restricted isotropic diffusion) density were calculated from multi-shell multi-tissue constrained spherical deconvolution. Sodium parameters were compared with white and grey matter diffusion properties. Sodium T 2 ∗ mean and T 2 ∗ ff showed little variation across a range of white matter axonal fibre and grey matter densities. We conclude that sodium T 2 ∗ relaxation parameters are not greatly influenced by relative differences in intra- and extracellular partial volumes. We suggest that care be taken when interpreting sodium relaxation changes in terms of tissue microstructure in healthy tissue., Competing Interests: Declaration of competing interest The authors report no conflicts of interest relating to this work., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Purpose: The clinical application of sodium MRI is hampered due to relatively low image quality and associated long acquisition times. Compressed sensing (CS) aims at a reduction of measurement time, but has been found to encompass quantitative estimation bias when used in low SNR x-Nuclei imaging. This work analyses CS in quantitative human brain sodium MRI from undersampled acquisitions and provides recommendations for tissue sodium concentration (TSC) estimation., Methods: CS reconstructions from 3D radial acquisitions of 5 healthy volunteers were investigated over varying undersampling factors (USFs) and CS penalty weights on different sparsity domains, Wavelet, Discrete Cosine Transform (DCT), and Identity. Resulting images were compared with highly sampled and undersampled NUFFT-based images and evaluated for image quality (i.e. structural similarity), image intensity bias, and its effect on TSC estimates in gray and white matter., Results: Wavelet-based CS reconstructions show highest image quality with stable TSC estimates for most USFs. Up to an USF of 4, images showed good structural detail. DCT and Identity-based CS enable good image quality, however show a bias in TSC with a reduction in estimates across USFs., Conclusions: The image intensity bias is lowest in Wavelet-based reconstructions and enables an up to fourfold acquisition speed up while maintaining good structural detail. The associated acquisition time reduction can facilitate a translation of sodium MRI into clinical routine., (© 2019 International Society for Magnetic Resonance in Medicine.)

A new framework for B 1 insensitive adiabatic pulse design is proposed, denoted Spin Lock Adiabatic Correction (SLAC), which counteracts deviations from ideal behaviour through inclusion of an additional correction component during pulse design. SLAC pulses are theoretically derived, then applied to the design of enhanced BIR-4 and hyperbolic secant pulses to demonstrate practical utility of the new pulses. At 7T, SLAC pulses are shown to improve the flip angle homogeneity compared to a standard adiabatic pulse with validation in both simulations and phantom experiments, under SAR equivalent experimental conditions. The SLAC framework can be applied to any arbitrary adiabatic pulse to deliver excitation with increased B 1 insensitivity., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Purpose: Parameter mapping in sodium MRI data is challenging due to inherently low SNR and spatial resolution, prompting the need to employ robust models and estimation techniques. This work aims to develop a continuum model of sodium T 2 * -decay to overcome the limitations of the commonly employed bi-exponential models. Estimates of mean T 2 * -decay and fast component fraction in tissue are emergent from the inferred continuum model., Methods: A closed-form continuum model was derived assuming a gamma distribution of T 2 * components. Sodium MRI was performed on four healthy human subjects and a phantom consisting of closely packed vials filled with an aqueous solution of varying sodium and agarose concentrations. The continuum model was applied to the phantom and in vivo human multi-echo 7T data. Parameter maps by voxelwise model-fitting were obtained., Results: The continuum model demonstrated comparable estimation performance to the bi-exponential model. The parameter maps provided improved contrast between tissue structures. The fast component fraction, an indicator of the heterogeneity of localised sodium motion regimes in tissue, was zero in CSF and high in WM structures., Conclusions: The continuum distribution model provides high quality, high contrast parameter maps, and informative voxelwise estimates of the relative weighting between fast and slow decay components., (© 2019 International Society for Magnetic Resonance in Medicine.)

Purpose: Fast bi-exponential transverse signal decay compounds sodium image quality. This work aims at enhancing image characteristics using a special case of ramped hybrid encoding (RHE). Zero-gradient-excitation (zG RF )-RHE provides (1) gradient-free excitation for high flip angle, artifact-free excitation profiles and (2) gradient ramping during dead-time for the optimization of encoding time (t enc ) to reduce T 2 * signal decay influence during acquisition., Methods: Radial zG RF -RHE and standard radial UTE were investigated over a range of receiver bandwidths in simulations, phantom and in vivo brain experiments. Central k-space in zG RF -RHE was acquired through single point measurements at the minimum achievable TE. T 2 * blurring artifacts and image SNR and CNR were assessed., Results: zG RF -RHE enabled 90° flip angle artifact-free excitation, whereas gradient pre-ramping provided greater t enc efficiency for any readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations (T RO ). Significant SNR improvements of up to 4.8% were observed for longer T RO ., Conclusion: zG RF -RHE allows for artifact-free high flip angle excitation with time-efficient encoding improving on image characteristics. This hybrid encoding concept with gradient pre-ramping is trajectory independent and can be introduced in any center-out UTE trajectory design., (©2018 International Society for Magnetic Resonance in Medicine.)

Purpose: This work demonstrates a 3D radial multi-echo acquisition scheme for time-efficient sodium ( 23 Na) MR-signal acquisition and analysis. Echo reconstructions were used to produce signal-to-noise ratio (SNR)-enhanced 23 Na-images and parameter maps of the biexponential observed transverse relaxation time ( T2*) decay., Methods: A custom-built sequence for radial multi-echo acquisition was proposed for acquisition of a series of 3D volumetric 23 Na-images. Measurements acquired in a phantom and in vivo human brains were analyzed for SNR enhancement and multi-component T2* estimation., Results: Rapid gradient refocused imaging acquired 38 echoes within a repetition time of 160 ms. Signal averaging of multi-echo time (TE) measurements showed an average brain tissue SNR enhancement of 34% compared to single-TE images across subjects. Phantom and in vivo measurements detected distinguishable signal decay characteristics for fluid and solid media. Mapping results were investigated in phantom and in vivo experiments for sequence timing optimization and signal decay analysis. The T2* mapping results were consistent with previously reported values and facilitated fluid-signal discrimination., Conclusion: The proposed method offers an efficient 23 Na-imaging scheme that extends existing 23 Na-MRI sequences by acquiring signal decay information with no increase in time or specific absorption rate. The resultant SNR-enhanced 23 Na-images and estimated T2* signal decay characteristics offer great potential for detailed investigation of tissue compartment characterization and clinical application. Magn Reson Med 79:1950-1961, 2018. © 2017 International Society for Magnetic Resonance in Medicine., (© 2017 International Society for Magnetic Resonance in Medicine.)