Pushing the limit of MQMAS for low-γ quadrupolar nuclei in pharmaceutical hydrochlorides

Solid-state NMR of quadrupolar nuclei such as ³⁵Cl has become a useful tool to characterize polymorphism in pharmaceutical hydrochlorides. The two-dimensional multiple-quantum magic-angle spinning (MQMAS) experiment can achieve isotropic resolution, and separate quadrupolar line shapes for samples with multiple sites but the pulse sequence efficiency is often low, limiting applications due to the intrinsically low NMR signals and rf field from the low gyromagnetic ratios γ. The use of cosine low-power MQMAS pulse sequences and high magnetic fields is presented to push the limit of MQMAS for insensitive low-γ quadrupolar nuclei. The improved efficiency and fields up to 35.2 T enable the acquisition of MQMAS spectra for pharmaceutical samples with multiple ³⁵Cl sites, large quadrupolar couplings and/or in diluted dosage forms.

Accumulation of Lithium in the Hippocampus of Patients With Bipolar Disorder: A Lithium-7 Magnetic Resonance Imaging Study at 7 Tesla

BACKGROUND

Lithium (Li) is a first-line treatment for bipolar disorder (BD). To study its cerebral distribution and association with plasma concentrations, we used ⁷Li magnetic resonance imaging at 7T in euthymic patients with BD treated with Li carbonate for at least 2 years.

METHODS

Three-dimensional ⁷Li magnetic resonance imaging scans (N = 21) were acquired with an ultra-short echo-time sequence using a non-Cartesian k-space sampling scheme. Lithium concentrations ([Li]) were estimated using a phantom replacement approach accounting for differential T₁ and T₂ relaxation effects. In addition to the determination of mean regional [Li] from 7 broad anatomical areas, voxel- and parcellation-based group analyses were conducted for the first time for ⁷Li magnetic resonance imaging.

RESULTS

Using unprecedented spatial sensitivity and specificity, we were able to confirm the heterogeneity of the brain Li distribution and its interindividual variability, as well as the strong correlation between plasma and average brain [Li] ([Li]_B ≈ 0.40 × [Li]_P, R = .74). Remarkably, our statistical analysis led to the identification of a well-defined and significant cluster corresponding closely to the left hippocampus for which high Li content was displayed consistently across our cohort.

CONCLUSIONS

This observation could be of interest considering 1) the major role of the hippocampus in emotion processing and regulation, 2) the consistent atrophy of the hippocampus in untreated patients with BD, and 3) the normalization effect of Li on gray matter volumes. This study paves the way for the elucidation of the relationship between Li cerebral distribution and its therapeutic response, notably in newly diagnosed patients with BD.

A mechanical rotatable magnetic force microscope operated in a 7 T superconducting magnet

We present a mechanical rotatable magnetic force microscope (MFM) with precise angle control that can be operated in a 7 T superconducting magnet. An inertial piezoelectric motor called a SpiderDrive was used for the coarse approach because of its high compactness, high rigidity, and small size. Due to the mechanical rotation design, the MFM head can be rotated in a 7 T superconducting magnet with a bore size of 89 mm so that the direction of the magnetic field can be changed from 0° to 90° continuously. The highest in-plane magnetic field strength tested was 7 T. This is the first rotatable MFM ever reported. Using the homemade rotatable MFM, we investigated a 40 nm thick La_0.67Ca_0.33MnO₃ (LCMO) thin film on NdGaO₃ (100) substrate with anisotropy, determining that the charge-ordering insulating (COI) phase of the LCMO disappears as the direction of the magnetic field changed from 0° to 90°. Furthermore, the ferromagnetic pattern, appearing as bright and dark contrasts and similar to that formed by the S and N of a magnet, was seen parallel to the direction of the magnetic field. The rotatable MFM in this paper is expected to be widely used in studying the anisotropy of magnetic materials.

Perspectives on high-field and solid-state NMR methods of quadrupole nuclei

High magnetic field can dramatically increase the spectral resolution and sensitivity of quadrupole nuclei S > 1/2 by the reduction of the second-order quadrupole broadening. A brief overview and outlook on spectral acquisition, the importance of high magnetic field, inter-nuclei distance measurement, various 2D separation and correlation methods of quadrupole nuclei are presented. The complications and consequences of spin dynamics under rf irradiation for the (2S + 1) level system and level-crossing with the satellite transition frequencies under magic-angle spinning are discussed. There is a scaling down of (S + 1/2) to the efficiency of many experiments in comparison with a spin-1/2 due to the fact that only two central transition spin states out of the (2S + 1) levels contribute to polarization transfer and spin correlation.

Optical spectroscopy study of Ca3(Ru0.91Mn0.09)2O7 single crystal in high magnetic fields

The magneto-optical spectrum, with magnetic fields up to 42 T, of double layered ruthenates Ca₃(Ru_0.91Mn_0.09)₂O₇ (CRMO) single crystal is studied. Both the temperature and magnetic field induced insulator-to-metal transitions (IMTs) are observed via magneto-optical properties of the crystal. The critical magnetic field (H // c) of IMT for CRMO is found to be as large as 35 T at 5 K. The fine structure of optical spectra identified the antiferromagnetic/ferro-orbital-ordering configurations of Ru 4d orbitals at low temperatures. Meanwhile, the configuration of orbital polarization of such double-layer CRMO single crystal is discussed. These results suggest that the orbital degree of freedom plays an important role in the field induced IMT of multi-orbital system.

Magnetic field-induced electronic phase transition in the Dirac semimetal state of black phosphorus under pressure

Different instabilities have been confirmed to exist in the three-dimensional (3D) electron gas when it is confined to the lowest Landau level in the extreme quantum limit. The recently discovered 3D topological semimetals offer a good platform to explore these phenomena due to the small sizes of their Fermi pockets, which means the quantum limit can be achieved at relatively low magnetic fields. In this work, we report the high-magnetic-field transport properties of the Dirac semimetal state in pressurized black phosphorus. Under applied hydrostatic pressure, the band structure of black phosphorus goes through an insulator-semimetal transition. In the high pressure topological semimetal phase, anomalous behaviors are observed on both magnetoresistance and Hall resistivity beyond the relatively low quantum limit field, which is demonstrated to indicate the emergence of an exotic electronic state hosting a density wave ordering. Our findings bring the first insight into the electronic interactions in black phosphorus under intense field.

Development and application of 2.5 GPa-25 T high-pressure high-field electron spin resonance system using a cryogen-free superconducting magnet

We have developed a high-pressure electron spin resonance probe and successfully installed into the world's highest-field cryogen-free superconducting magnet having a maximum central field of 24.6 T. The high pressure of 2.5 GPa is achieved by the specially designed piston-cylinder pressure cell using THz-wave-transparent components. In the first application of this high-pressure high-field ESR system, we observed that the orthogonal dimer spin system SrCu₂(BO₃)₂ undergoes a quantum phase transition from the dimer singlet ground to the plaquette singlet ground states.

A general protocol of ultra-high resolution MR angiography to image the cerebro-vasculature in 6 different rats strains at high field

BACKGROUND

Differences in the cerebro-vasculature among strains as well as individual animals might explain variability in animal models and thus, a non-invasive method tailored to image cerebral vessel of interest with high signal to noise ratio is required.

NEW METHOD

Experimentally, we describe a new general protocol of three-dimensional time-of-flight magnetic resonance angiography to visualize non-invasively the cerebral vasculature in 6 different rat strains. Flow compensated angiograms of Sprague Dawley, Wistar Kyoto, Lister Hooded, Long Evans, Fisher 344 and Spontaneous Hypertensive Rat strains were obtained without the use of contrast agents. At 11.7T using a repetition time of 60ms, an isotropic resolution of up to 62μm was achieved; total imaging time was 98min for a 3D data set.

RESULTS

The visualization of the cerebral arteries was improved by removing extra-cranial vessels prior to the calculation of maximum intensity projection to obtain the angiograms. Ultimately, we demonstrate that the newly implemented method is also suitable to obtain angiograms following middle cerebral artery occlusion, despite the presence of intense vasogenic edema 24h after reperfusion.

COMPARISON WITH EXISTING METHODS

The careful selection of the excitation profile and repetition time at a higher static magnetic field allowed an increase in spatial resolution to reliably detect of the hypothalamic artery, the anterior choroidal artery as well as arterial branches of the peri-amygdoidal complex and the optical nerve in six different rat strains.

CONCLUSIONS

MR angiography without contrast agent can be utilized to study cerebro-vascular abnormalities in various animal models.

230/115 GHz Electron Paramagnetic Resonance/Double Electron-Electron Resonance Spectroscopy

Electron paramagnetic resonance (EPR) and double electron-electron resonance (DEER) spectroscopies are powerful and versatile tools for studying local structures and dynamic properties of biological molecules. Similar to nuclear magnetic resonance (NMR) spectroscopy, EPR/DEER spectroscopies become more advantageous at higher frequencies and higher magnetic fields because of better spectral resolution as well as higher spin polarization. Here, we describe development of a high-frequency (HF) EPR/DEER spectrometer operating in the frequency range of 107-120 and 215-240 GHz and in the magnetic field range of 0-12.1 T, which has unique experimental capabilities such as enabling the complete spin polarization and wide-band DEER spectroscopy. Emphasis is given on the application of HF EPR/DEER techniques, and specific examples of HF EPR spectroscopy to drastically increase spin coherence in nanodiamonds as well as HF DEER spectroscopy to extract spin concentration in a diamond crystal are presented.

Evaluation of EPI distortion correction methods for quantitative MRI of the brain at high magnetic field

High field MRI has been applied to high-resolution structural and functional imaging of the brain. Echo planar imaging (EPI) is an ultrafast acquisition technique widely used in diffusion imaging, functional MRI and perfusion imaging. However, it suffers from geometric and intensity distortions caused by static magnetic field inhomogeneity, which is worse at higher field strengths. Such susceptibility artifacts are particularly severe in relation to the small size of the mouse brain. In this study we compared different distortion correction methods, including nonlinear registration, field map-based, and reversed phase-encoding-based approaches, on quantitative imaging of T1 and perfusion in the mouse brain acquired by spin-echo EPI with inversion recovery and pseudo-continuous arterial spin labeling, respectively, at 7 T. Our results showed that the 3D reversed phase-encoding correction outperformed other methods in terms of geometric fidelity, and that conventional field map-based correction could be improved by combination with affine transformation to reduce the bias in the field map. Both methods improved quantification with smaller fitting error and regional variation. These approaches offer robust correction of EPI distortions at high field strengths and hence could lead to more accurate co-registration and quantification of imaging biomarkers in both clinical and preclinical applications.

Structure and properties of Co-doped ZnO films prepared by thermal oxidization under a high magnetic field

The effect of a high magnetic field applied during oxidation on the structure, optical transmittance, resistivity, and magnetism of cobalt (Co)-doped zinc oxide (ZnO) thin films prepared by oxidizing evaporated Zn/Co bilayer thin films in open air was studied. The relationship between the structure and properties of films oxidized with and without an applied magnetic field was analyzed. The results show that the high magnetic field obviously changed the structure and properties of the Co-doped ZnO films. The Lorentz force of the high magnetic field suppressed the oxidation growth on nanowhiskers. As a result, ZnO nanowires were formed without a magnetic field, whereas polyhedral particles formed under a 6 T magnetic field. This morphology variation from dendrite to polyhedron caused the transmittance below 1,200 nm of the film oxidized under a magnetic field of 6 T to be much lower than that of the film oxidized without a magnetic field. X-ray photoemission spectroscopy indicated that the high magnetic field suppressed Co substitution in the ZnO lattice, increased the concentration of oxygen vacancies, and changed the chemical state of Co. The increased concentration of oxygen vacancies affected the temperature dependence of the resistivity of the film oxidized under a magnetic field of 6 T compared with that of the film oxidized without a magnetic field. The changes of oxygen vacancy concentration and Co state caused by the application of the high magnetic field also increase the ferromagnetism of the film at room temperature. All of these results indicate that a high magnetic field is an effective tool to modify the structure and properties of ZnO thin films.

A compact high field magnetic force microscope

We present the design and performance of a simple and compact magnetic force microscope (MFM), whose tip-sample coarse approach is implemented by the piezoelectric tube scanner (PTS) itself. In brief, a square rod shaft is axially spring-clamped on the inner wall of a metal tube which is glued inside the free end of the PTS. The shaft can thus be driven by the PTS to realize image scan and inertial stepping coarse approach. To enhance the inertial force, each of the four outer electrodes of the PTS is driven by an independent port of the controller. The MFM scan head is so compact that it can easily fit into the 52mm low temperature bore of a 20T superconducting magnet. The performance of the MFM is demonstrated by imaging a manganite thin film at low temperature and in magnetic fields up to 15T.

Compressed sensing fMRI using gradient-recalled echo and EPI sequences

Compressed sensing (CS) may be useful for accelerating data acquisitions in high-resolution fMRI. However, due to the inherent slow temporal dynamics of the hemodynamic signals and concerns of potential statistical power loss, the CS approach for fMRI (CS-fMRI) has not been extensively investigated. To evaluate the utility of CS in fMRI application, we systematically investigated the properties of CS-fMRI using computer simulations and in vivo experiments of rat forepaw sensory and odor stimulations with gradient-recalled echo (GRE) and echo planar imaging (EPI) sequences. Various undersampling patterns along the phase-encoding direction were studied and k-t FOCUSS was used as the CS reconstruction algorithm, which exploits the temporal redundancy of images. Functional sensitivity, specificity, and time courses were compared between fully-sampled and CS-fMRI with reduction factors of 2 and 4. CS-fMRI with GRE, but not with EPI, improves the statistical sensitivity for activation detection over the fully sampled data when the ratio of the fMRI signal change to noise is low. CS improves the temporal resolution and reduces temporal noise correlations. While CS reduces the functional response amplitudes, the noise variance is also reduced to make the overall activation detection more sensitive. Consequently, CS is a valuable fMRI acceleration approach, especially for GRE fMRI studies.