The spin-splitting symmetries of altermagnets, such as d, g, and i wave, are strictly constrained by the spin point group symmetries of their underlying Euclidean crystal lattices. This raises a fundamental question: Can we transcend this crystallographic constraint to create entirely new magnetic symmetries by manipulating the intrinsic geometry of space? Here, we develop a general theoretical framework for introducing altermagnetic order on hyperbolic Bravais lattices, which are characterized by negative curvature. We discover spin-splitting patterns with exceptionally high-fold symmetries, such as fourfold g wave, 10-fold m wave, and even higher orders. The symmetries of these patterns are directly dictated by the non-Abelian Fuchsian groups describing hyperbolic translations and are unattainable in Euclidean space. Furthermore, we uncover an unexpected spin degeneracy in hyperbolic altermagnets. This degeneracy is not protected by symmetry but originates from flat bands induced by the unique connectivity of the hyperbolic lattice, which arise from the local destructive interference of wave functions. This work not only extends the concept of altermagnetism from flat to curved space but also opens an avenue for designing spin phases through geometry engineering, providing a theoretical platform to explore the profound connections among geometry, magnetism, and electronic properties.

Polynitride compounds have recently attracted great attention because of their unique properties, including exotic nitrogen motifs and high energy densities. Among them, metal-bearing nitrogen-rich compounds with pentazolate anion (cyclo-N5−) are highly desirable in basic research and applications. In this article, we conduct a comprehensive simulation of the pressurized germanium–nitrogen system based on the strategy of introducing a small amount of metal into nitrogen and designing a previously unknown unconventional stoichiometric material, GeN20, being stable at 49 GPa through a first-principles structure search method. Strikingly, the cyclo-N5− anionic unit, acting as an energy storage carrier, is revealed in Ge(N5)4, the formation mechanism of which is attributed to strong covalent N–N bonds and charge transfer from Ge to N. Furthermore, the robust dynamic, mechanical, and thermal stabilities of Ge(N5)4 predict its feasibility of synthesis in the future. In addition, Ge(N5)4 has a relatively high energy density (4.1 kJ g−1), which is comparable to the energy density of TNT (4.2 kJ g−1). Remarkably, Ge(N5)4 has a high detonation pressure (619 kbar) and a high explosion velocity (11.42 km s−1), which are approximately three times the detonation pressure of TNT (190 kbar) and twice the explosion velocity of TNT (6.9 km s−1), respectively, and it produces a more environmentally friendly detonation product composition. Moreover, Ge(N5)4 hosts an indirect bandgap of 3.0 eV and exhibits optical absorption performance. These findings have valuable implications for the rational design and synthesis of novel multifunctional nitrogen-based materials.

BACKGROUND:Cardiac cine imaging is routinely used in patient with suspected or known cardiac dysfunction. Water and fat (W/F) separated cardiovascular magnetic resonance (CMR) will be helpful to distinguish adipose tissue, blood, and myocardium. Inclusion of a multi-echo acquisition in the conventional balanced steady-state free precession (bSSFP) cine sequence can introduce artifacts and reduce temporal resolution. Spiral MRI is known for its signal-to-noise ratio (SNR) efficiency and has the potential to improve temporal efficiency for W/F separated cine imaging. The present work implements a spoiled gradient echo sequence (SPGR) with spiral trajectory to obtain W/F separated cine images simultaneously. METHODS:Three different sequences were performed for comparison, a Cartesian 2-TE bSSFP sequence, a Cartesian 3-TE bSSFP sequence, and the proposed spiral SPGR sequence. Five volunteers were recruited for the scans on a 1.5T scanner with spatial resolution 1.7×1.7×8.0mm3 over a 400×400mm2 FOV. In addition to qualitative comparisons, a quantitative measurement is performed in terms of the contrast-to-noise ratio (CNR). RESULTS:The proposed method to obtain W/F separated cine images provides better temporal efficiency and fewer artifacts compared to conventional Cartesian bSSFP sequences. The 2-TE bSSFP features the highest artifact level, including susceptibility artifacts and fat/water swaps. The proposed method reduces scan time by approximately 50% with similar spatial and temporal resolution with lower specific absorption rate (SAR). The contrast between the blood pool and myocardium is higher when using the spiral readout (p≤0.05). The results suggest that the presented sequence has potential to facilitate simultaneous imaging for water and fat components in a cine scan while shortening exam time and lowering SAR.

Purpose: Perfusion MRI reveals important tumor physiological and pathophysiologic information, making it a critical component in managing brain tumor patients. This study aimed to develop a dual-echo 3D spiral technique with a single-bolus scheme to simultaneously acquire both dynamic susceptibility contrast (DSC) and dynamic contrast-enhanced (DCE) data and overcome the limitations of current EPI-based techniques. Methods: A 3D spiral-based technique with dual-echo acquisition was implemented and optimized on a 3T MRI scanner with a spiral staircase trajectory and through-plane SENSE acceleration for improved speed and image quality, in-plane variable-density undersampling combined with a sliding-window acquisition and reconstruction approach for increased speed, and an advanced iterative deblurring algorithm. Four volunteers were scanned and compared with the standard of care (SOC) single-echo EPI and a dual-echo EPI technique. Two patients were scanned with the spiral technique during a preload bolus and compared with the SOC single-echo EPI collected during the second bolus injection. Results: Volunteer data demonstrated that the spiral technique achieved high image quality, reduced geometric artifacts, and high temporal SNR compared with both single-echo and dual-echo EPI. Patient perfusion data showed that the spiral acquisition achieved accurate DSC quantification comparable to SOC single-echo dual-dose EPI, with the additional DCE information. Conclusion: A 3D dual-echo spiral technique was developed to simultaneously acquire both DSC and DCE data in a single-bolus injection with reduced contrast use. Preliminary volunteer and patient data demonstrated increased temporal SNR, reduced geometric artifacts, and accurate perfusion quantification, suggesting a competitive alternative to SOC-EPI techniques for brain perfusion MRI.

PurposeTo develop a method that achieves simultaneous brain and neck time-of-flight (ToF) magnetic resonance angiography (MRA) within feasible scan timeframes.MethodsLocalized quadratic (LQ) encoding is efficient for both signal-to-noise ratio (SNR) and in-flow enhancement. We proposed a spiral multiband LQ method to enable simultaneous intracranial and carotid ToF-MRA within a single scan. To address the venous signal contamination that becomes a challenge with multiband (MB) ToF, tilt-optimized non-saturated excitation (TONE) and partial-Fourier slice selection (PFSS) were further introduced in the LQ framework to mitigate the venous signal and improve artery contrast. A sequential spiral MB and LQ reconstruction pipeline was employed to obtain the brain-and-neck image volumes.ResultsThe proposed MB method was able to achieve simultaneous brain and neck ToF-MRA within a 2:50-min scan. The complementarily boosted SNR-efficiency by MB and LQ acquisitions allows for the increased spatial coverage without increase in scan time or noticeable compromise in SNR. The incorporation of both TONE and PFSS effectively alleviated the venous contamination with improved small vessel sensitivity. Selection of scan parameters such as the LQ factor and flip angle reflected the trade-off among SNR, blood contrast, and venous suppression.ConclusionsA novel MB spiral LQ approach was proposed to enable fast intracranial and carotid ToF-MRA with minimized venous corruption. The method has shown promise in MRA applications where large spatial coverage is necessary.

Accurate fat fraction and T2* evaluation offers useful diagnostic information. A data driven B 0 -update algorithm in conjunction with the Two-Point Dixon method is illustrated in this work, offering efficient computation to improve B 0 accuracy to separate fat and water in the multiple TE acquisition. The individual fat and water signals are then used for T2* mapping. The reconstructed parametric maps have comparable quality to those of the existing methods. The results also show that the proposed method requires shorter computation time and is more stable in the presence of an inaccurate reference field map.

PurposeThe combination of SENSE and spiral imaging with fat/water separation enables high temporal efficiency. However, the corresponding computation increases due to the blurring/deblurring operation across the multi-channel data. This study presents two alternative models to simplify computational complexity in the original full model (model 1). The performances of the models are evaluated in terms of the computation time and reconstruction error. MethodsTwo approximated spiral MRI reconstruction models were proposed: the comprehensive blurring before coil operation (model 2) and the regional blurring before coil operation (model 3), respectively, by altering the order of coil-sensitivity encoding process to distribute signals among the multi-channel coils. Four subjects were recruited for scanning both fully sampled T-1- and T-2-weighted brain image data with simulated undersampling for testing the computational efficiency and accuracy on the approximation models. ResultsBased on the examples, the computation time can be reduced to 31%-47% using model 2, and to 39%-56% using model 3. The quality of the water image remains unchanged among the three models, whereas the primary difference in image quality is in the fat channel. The fat images from model 3 are consistent with those from model 1, but those from model 2 have higher normalized error, differing by up to 4.8%. ConclusionModel 2 provides the fastest computation but exhibits higher error in the fat channel, particularly in the high field and with long acquisition window. Model 3, an abridged alternative, is also faster than the full model and can maintain high accuracy in reconstruction.

PurposeAn accurate field map is essential to separate fat and water signals in a dual-echo chemical shift encoded spiral MRI scan. A rapid low-resolution B-0 map prescan is usually performed before each exam. Occasional inaccuracy in these field map estimates can lead to misclassification of the water and fat signals as well as blurring artifacts in the reconstruction. The present work proposes a self-consistent model to evaluate residual field offsets according to the image data to improve the reconstruction quality and facilitate the scan efficiency. Theory and MethodsThe proposed method compares the phase differences of the two-echo data after correcting for fat frequency offsets. A more accurate field map is approximated according to the phase discrepancies and improved image quality. Experiments were conducted with simulated off-resonance on a numerical phantom, five volunteer head scans, and four volunteer abdominal scans for validation. ResultsThe initial reconstruction of the demonstrated examples exhibit blurring artifacts and misregistration of fat and water because of the inaccuracy of the field map. The proposed method updates the field map to amend the fat and water estimation and improve image quality. ConclusionsThis work presents a model to improve the quality of fat-water imaging of the spiral MRI by estimating a better field map from the acquired data. It allows reducing the field map pre-scans before each spiral scan under normal circumstances to increase scan efficiency.

T2-weighted imaging typically employs either TSE or multi-pass 2D SE acquisitions, but using a thick slice and/or a slice gap is often necessary to overcome SNR inefficiencies of these methods, to achieve high-resolution scans in reasonable scan time. We studied efficient T2-weighted SE technique which employed localized quadratic encoding to realize SNR-efficient slice encoding scheme that produced contiguous volumetric coverage. Combined with long-readout spiral acquisitions, the proposed method, a hybrid of 2D and 3D imaging, demonstrated the expected SNR benefit compared to standard 2D scans and produced T2-weighted images with SNR equivalent as 2D-TSE scans but with larger, contiguous coverage.

PURPOSE:This work proposes a 2D/3D hybrid inflow MRA technique for fast scanning and high SNR and contrast-to-noise (CNR) efficiencies. METHODS:Localized quadratic (LQ) encoding was combined with a sliding-slice spiral acquisition. Inflow MRAs around the circle of Willis and the carotid bifurcations were collected on four healthy volunteers. Spiral images were deblurred without or with water-fat separation for sliding-slice LQ (ssLQ) out-of-phase (OP) and Dixon inflow MRAs, respectively. Results were compared to multiple overlapping thin slab acquisitions (MOTSA) and 2D OP inflow MRAs. Noise data were also acquired with RF and gradients turned off to compute maps of SNR and SNR efficiency. Quantitative assessment of relative contrast, CNR, and CNR efficiency for flow were performed in regions of interest. RESULTS:The sliding-slice spiral technique alone reduces scan time by 10% to 40% compared with a standard spiral acquisition scheme. The proposed spiral ssLQ OP achieves 50% higher scan speed than the spiral MOTSA with comparable SNR and CNR efficiencies, which are ∼100% higher than the Cartesian MOTSA for intracranial inflow MRAs. Spiral ssLQ Dixon inflow MRA provides better visibility for vessels around the fat compared to spiral ssLQ OP inflow MRA, with a trade-off of scan speed. Spiral ssLQ MRA with thinner slice thickness is two to five times faster than the 2D Cartesian inflow neck MRA around the carotid bifurcations, while also achieving higher SNR efficiency. CONCLUSION:The proposed spiral ssLQ is a fast and flexible MRA method with improved SNR and CNR efficiencies over traditional Cartesian inflow MRAs.

Conventionally, Cartesian pre-scans are collected over several seconds to create accurate coil sensitivity maps (CSM) for SENSE reconstruction of under-sampled MRI data, with body-coil images used for signal normalization. Since body coils have a larger sensitivity region than phased-array coils, these pre-scans typically employ a very large field-of-view (FOV) to avoid aliasing, leading to low-resolution CSM and potential small errors in subsequent reconstructions. This abstract proposes a new CSM pre-scan sequence using a spherically-distributed-spirals trajectory. The acquisition efficiency and incoherent aliasing offered by this trajectory allow for smaller FOV and higher resolution while maintaining similar scanning time as conventional pre-scans.

PURPOSE:To demonstrate T2 -weighted (single-echo) spin-echo (SE) imaging with near-optimal acquisition efficiency by applying SNR-efficient RF slice encoding and spiral readout. METHODS:A quadratic-phase (frequency swept) excitation RF pulse replaced the conventional excitation in T2 -weighted SE sequence to excite a thick slab that is internally spatially encoded by a variable phase along the slice direction. Highly overlapping slabs centered at every desired slice location were acquired in multiple passes, such that the entire imaging volume was excited by contiguous slabs in any given pass. Following 90° excitation, each slab was refocused with a conventional 180° RF to produce a SE signal, followed by a spiral in-out readout. A noise-insensitive reconstruction removed the quadratic phase in the spatial frequency domain, yielding desired slice resolution and improved SNR. RESULTS:Increasing the RF frequency sweep (hence, excitation width) allowed more frequent encoding of each slice over the multiple passes, improving final image SNR, until crosstalk ensued at excessive slab widths compared to their center-to-center spacing. With an optimized slab width, the proposed technique used all passes to acquire every prescribed slice, with substantially improved SNR over conventional SE or 2D-turbo-spin-echo (TSE) scans. Quantitative SNR measurements indicated similar SNR as 3D-TSE, but radiologist scoring favored 3D-TSE, mainly because of spiral-related artifacts and possibly because of regularized reconstructions in 3D-TSE. CONCLUSION:Using SNR-efficient slice excitation scheme and spiral readout helped eliminate SNR and temporal inefficiencies in conventional T2 -weighted imaging, yielding SNR independent of TR or number of passes.

PurposeRobust implementation of spiral imaging requires efficient deblurring. A deblurring method was previously proposed to separate and deblur water and fat simultaneously, based on image-space kernel operations. The goal of this work is to improve the performance of the previous deblurring method using kernels with better properties.MethodsFour types of kernels were formed using different models for the region outside the collected k-space as well as low-pass preconditioning (LP). The performances of the kernels were tested and compared with both phantom and volunteer data. Data were also synthesized to evaluate the SNR.ResultsThe proposed "square" kernels are much more compact than the previously used circular kernels. Square kernels have better properties in terms of normalized RMS error, structural similarity index measure, and SNR. The square kernels created by LP demonstrated the best performance of artifact mitigation on phantom data.ConclusionsThe sizes of the blurring kernels and thus the computational cost can be reduced by the proposed square kernels instead of the previous circular ones. Using LP may further enhance the performance.

A diverse range of topological quantum phenomena and potential applications of three-dimensional topological insulators (TIs) hinge on opening an energy gap of Dirac-cone surface states. Layered van der Waals (vdW) topological materials, especially the recently discovered MnBi 2 Te 4 family magnetic TIs, have aroused great attention, where the interlayer vdW gap is expected to play a crucial role in topological surface states. However, it remains a serious controversy whether the surface states are gapped or gapless for magnetic TI MnBi 2 Te 4 , which is a crucial issue for the prospect of various magnetic topological states. Here, a 3-Dirac-fermion approach is developed to generally describe surface states of nonmagnetic/magnetic vdW TIs under the interlayer vdW gap modulation. In particular, we apply this approach to solving controversial issues in the surface states of vdW antiferromagnetic (AFM) TIs. Remarkably, unexpected topologically protected gapless Dirac-cone surface states are found to arise due to the interlayer vdW gap expansion on the surface, when the surface ferromagnetic layer has a zero Chern number, while the surface states remain gapped for all other cases. These results are further confirmed by first-principles calculations on AFM TI MnBi 2 Te 4 . The unexpected gapless Dirac-cone states are invaluable in solving the puzzle of the observed gapless surface states in MnBi 2 Te 4 . This work also provides a promising way for experiments to realize intrinsic magnetic quantum anomalous Hall effect with a large energy gap in MnBi 2 Te 4 films.

Purpose The goal of this work is to present the implementation of 3D spiral high-resolution MPRAGE and to demonstrate that SNR and scan efficiency increase with the increment of readout time. Theory Simplified signal equations for MPRAGE indicate that the T-1 contrast can be kept approximately the same by a simple relationship between the flip angle and the TR. Furthermore, if T-1 contrast remains the same, image SNR depends on the square root of the product of the total scan time and the readout time. Methods MPRAGE spiral sequences were implemented with distributed spirals and spiral staircase on 3 Tesla scanners. Brain images of three volunteers were acquired with different readout times. Spiral images were processed with a joint water-fat separation and deblurring algorithm and compared to Cartesian images. Pure noise data sets were also acquired for SNR evaluation. Results Consistent T-1 weighting can be achieved with various spiral readout lengths, and between spiral MPRAGE imaging and the traditional Cartesian MPRAGE imaging. Noise performance analysis demonstrates higher SNR efficiency of spiral MPRAGE imaging with matched T-1 contrast compared to the Cartesian reference imaging. Conclusion Fast, high SNR MPRAGE imaging is feasible with long readout spiral trajectories.