Stochastic-offset-enhanced restricted slice excitation and 180° refocusing designs with spatially non-linear ΔB0 shim array fields

PURPOSE

Developing a general framework with a novel stochastic offset strategy for the design of optimized RF pulses and time-varying spatially non-linear ΔB0 shim array fields for restricted slice excitation and refocusing with refined magnetization profiles within the intervals of the fixed voxels.

METHODS

Our framework uses the decomposition property of the Bloch equations to enable joint design of RF-pulses and shim array fields for restricted slice excitation and refocusing with auto-differentiation optimization. Bloch simulations are performed independently on orthogonal basis vectors, Mx, My, and Mz, which enables designs for arbitrary initial magnetizations. Requirements for refocusing pulse designs are derived from the extended phase graph formalism obviating time-consuming sub-voxel isochromatic simulations to model the effects of crusher gradients. To refine resultant slice-profiles because of voxelwise optimization functions, we propose an algorithm that stochastically offsets spatial points at which loss is computed during optimization.

RESULTS

We first applied our proposed design framework to standard slice-selective excitation and refocusing pulses in the absence of non-linear ΔB0 shim array fields and compared them against pulses designed with Shinnar-Le Roux algorithm. Next, we demonstrated our technique in a simulated setup of fetal brain imaging in pregnancy for restricted-slice excitation and refocusing of the fetal brain.

CONCLUSIONS

Our proposed framework for optimizing RF pulse and time-varying spatially non-linear ΔB0 shim array fields achieve high fidelity restricted-slice excitation and refocusing for fetal MRI, which could enable zoomed fast-spin-echo-MRI and other applications.

Shim Coils Tailored for Correcting B0 Inhomogeneity in the Human Brain (SCOTCH): Design Methodology and 48-Channel Prototype Assessment in 7-Tesla MRI

Increased static field inhomogeneities are a burden for human brain MRI at Ultra-High-Field. In particular they cause enhanced Echo-Planar image distortions and signal losses due to magnetic susceptibility gradients at air-tissue interfaces in the subject's head. In the past decade, Multi-Coil Arrays (MCA) have been proposed to shim the field in the brain better than the 2nd or 3rd order Spherical Harmonic (SH) coils usually offered by MRI manufacturers. Here we present a novel MCA, named SCOTCH, optimized for whole brain shimming. Based on a cylindrical structure, it features several layers of small coils whose shape, size and location are found from a principal component analysis of ideal stream functions computed from an internal 100-brain fieldmap database. From an Open-Access external database of 126 brains, our SCOTCH implementation is shown to be equivalent to a partial 7th-order SH system with unlimited power, outperforming all known existing MCA prototypes. This result is further confirmed by a low-cost  30-cm diameter SCOTCH prototype built with 48 coils on 3 layers, and tested on 7 volunteers at 7T with a parallel-transmit RF coil made to be inserted in SCOTCH. Echo-Planar images of the subject brains before and after SCOTCH shimming show large signal recoveries, especially in the prefrontal cortex.

Selective RF excitation designs enabled by time-varying spatially non-linear ΔB0 fields with applications in fetal MRI

PURPOSE

To demonstrate, through numerical simulations, novel designs of spatially selective radiofrequency (RF) excitations of the fetal brain by both a restricted 2D slice and 3D inner-volume selection. These designs exploit a single-channel RF pulse, conventional gradient fields, and the spatially non-linear ΔB₀ fields of a multi-coil shim array, using an auto-differentiation optimization algorithm.

METHODS

The design algorithm jointly optimizes the RF pulse and the time-varying ΔB₀ fields, which is produced by a 64-channel multi-coil ΔB₀ body array to augment the RF and the linear gradient fields, using an auto-differentiation approach. Two design targets were specified, one a 4-mm thick slice with a limited in-slice extent in one dimension ("restricted slice"), and the other a 3D inner-volume selection encompassing the fetal brain ("inner volume"). The RF duration was limited to 2 ms for the restricted slice excitation and 6 ms for the inner-volume excitation.

RESULTS

Excitation profiles were achieved for both the restricted slice excitation task (one-minus-minimum magnitude, 8%) within the region of interest (ROI) and (maximum-minus-zero magnitude, 8%) in the suppressed regions and the fetal brain volume excitation task (13% and 9%, respectively).

CONCLUSIONS

The proposed joint design of RF and time-varying, spatially non-linear ΔB₀ fields achieves the target excitation profiles with short RF pulse durations and demonstrates the potential to enhance fetal MRI with multi-channel body shim arrays.

An integrated RF-receive/B0-shim array coil boosts performance of whole-brain MR spectroscopic imaging at 7 T

Metabolic imaging of the human brain by in-vivo magnetic resonance spectroscopic imaging (MRSI) can non-invasively probe neurochemistry in healthy and disease conditions. MRSI at ultra-high field (≥ 7 T) provides increased sensitivity for fast high-resolution metabolic imaging, but comes with technical challenges due to non-uniform B0 field. Here, we show that an integrated RF-receive/B0-shim (AC/DC) array coil can be used to mitigate 7 T B0 inhomogeneity, which improves spectral quality and metabolite quantification over a whole-brain slab. Our results from simulations, phantoms, healthy and brain tumor human subjects indicate improvements of global B0 homogeneity by 55%, narrower spectral linewidth by 29%, higher signal-to-noise ratio by 31%, more precise metabolite quantification by 22%, and an increase by 21% of the brain volume that can be reliably analyzed. AC/DC shimming provide the highest correlation (R2 = 0.98, P = 0.001) with ground-truth values for metabolite concentration. Clinical translation of AC/DC and MRSI is demonstrated in a patient with mutant-IDH1 glioma where it enables imaging of D-2-hydroxyglutarate oncometabolite with a 2.8-fold increase in contrast-to-noise ratio at higher resolution and more brain coverage compared to previous 7 T studies. Hence, AC/DC technology may help ultra-high field MRSI become more feasible to take advantage of higher signal/contrast-to-noise in clinical applications.

An orthogonal shim coil for 3T brain imaging

PURPOSE

We designed and implemented an orthogonal shim array consisting of shim coils with their planes perpendicular to the planes of neighboring RF coils. This shim coil improves the magnetic field homogeneity by minimizing the interference to RF coils.

METHODS

Using realistic off-resonance maps of the human brain, we first evaluated the performance of shim coils in different orientations. Based on simulations, we developed a 7-channel orthogonal shim array, whose coil plan was perpendicular to neighboring RF coils, at the forehead. A programmable open-source current driver supplied shim currents.

RESULTS

The 7-channel orthogonal shim array caused only marginal SNR loss to the integrated 32-channel RF-shim array. The 7-channel orthogonal shim array itself improved the magnetic field homogeneity by 30% in slice-optimized shimming, comparable to the baseline shimming offered by the scanner's 2nd order spherical harmonic shimming.

CONCLUSION

Orthogonal shim coils can improve the field homogeneity while maintaining high image SNR.

A 16-channel AC/DC array coil for anesthetized monkey whole-brain imaging at 7T

Functional magnetic resonance imaging (fMRI) in monkeys is important for bridging the gap between invasive animal brain studies and non-invasive human brain studies. To resolve the finer functional structure of the monkey brain, ultra-high-field (UHF) MR is essential, and high-performance, close-fitting RF receive coils are typically desired to fully leverage the intrinsic gains provided by UHF MRI. Moreover, static field (B0) inhomogeneity arising from the tissue susceptibility interface is more severe at UHF, presenting an obstacle to achieving high-resolution fMRI. B0 shim of the monkey head is challenging due to its smaller size and more complex sources of B0 offsets in multi-modal imaging tasks. In the present work, we have customized an array coil for lightly-anesthetized monkey fMRI in the 7T human scanner that combines RF and multi-coil (MC) B0 shim functionality (also referred to as AC/DC coils) to provide high imaging SNR and high-spatial-order, rapidly switchable B0-shim capability. Additional space was retained on the coil to render it compatible with monkey multi-modal imaging studies. Both MC global (whole-volume) and dynamic (slice-optimized) shim methods were tested and evaluated, and the benefits of MC shim for fMRI experiments was also studied. A minor reduction in RF coil performance was found after introducing additional B0 shim circuitry. However, the proposed RF coil provided higher image SNR and more uniform contrast compared to a commercially available coil for human knee imaging. Compared with static 2nd-order shim, the B0 inhomogeneity was reduced by 56.8%, and 95-percentile B0 offset was reduced to within 28.2 Hz through MC shim, versus 68.7 Hz with 2nd-order static shim. As a result, functional image quality could be improved, and brain activation can be better detected using the proposed AC/DC monkey coil.

Seed production of Lotus uliginosus cv. Sharnae in response to plant population density

Summary. An experiment was carried out under irrigation in the field to determine the influence of plant population density (7.5, 15, 30 and 60 plants/m2) on growth, development and yield of the pasture legume Lotus uliginosuscv. Sharnae. Reproductive development (flowering, pod set and stages of pod development) was recorded from 79 days after establishment and harvests were taken on 3 occasions during pod development: 118, 138 and 158 days after establishment. At each harvest, plants were divided into leaf, stem and pods. The pods were further separated into maturity classes on the basis of colour, and seed number and weight measured for each class. Harvesting late (i.e. 158 days after establishment) maximised seed yield (113 g/m2) and seed quality (27% immature, green pods). Seed yield per unit area up to 158 days after establishment was relatively insensitive to plant population up to 60 plants/m2 because the response of individual plants of lotus was highly plastic. At 158 days after establishment, dry matter per plant declined from 985 g at 7.5 plants/m2 to 713 g at 60 plants/m2. At the same time, seed number per plant and seed weight per plant were reduced from 27 400 to 3230 seeds and from 15.5 to 1.65 g, respectively, across the range of population densities. Despite the lack of a clear optimum plant population for maximum seed yield, some aspects of seed quality declined at high population density. For example, single seed weight fell by about 10% from 0.563 mg and the proportion of the harvest contributed by seeds from immature pods rose from 18 to 33%, as population density increased from 7.5 and 60 plants/m2. Seed quality and ease of management, rather than yield, may be the most important considerations when seed growers choose a sowing rate for a L. pedunculatus cv. Sharnae seed crop.

Domain structure of an N-ethylmaleimide-sensitive fusion protein involved in vesicular transport

N-Ethylmaleimide-sensitive fusion protein (NSF) is an essential component for protein transport between Golgi cisternae. Sequence analysis and proteolytic dissection reveal that NSF contains two tandem "ATP domains," each containing the consensus sequence for the binding of nucleotide. When Escherichia coli-produced Chinese hamster ovary NSF is purified, it exhibits a low, but significant, ATPase activity. The ATPase activity of NSF is sensitive to N-ethylmaleimide and influenced by monoclonal antibodies against recombinant NSF.

IL-1, IL-6, and PDGF mRNA expression in alveolar cells following stimulation with a tobacco-derived antigen

To test the hypothesis that inflammatory cytokine production might be an early event in the development of the disease associated with smoking, we used alveolar cells from healthy nonsmokers stimulated with TGP as a model system. TGP, a phenol-rich glycoprotein which is present in tobacco leaves and cigarette smoke condensate, activates the immune system. It stimulates polyclonal B cell differentiation, induces primarily an IgE response, and activates human leukocytes to produce IL-1. Using in situ nucleic acid hybridization we show that the steady-state levels of IL-1 alpha, IL-1 beta, IL-6, platelet-derived growth factor (PDGF)-A, and PDGF-B mRNAs are consistently elevated in the alveolar cells of all donors following TGP stimulation. The kinetics of mRNA expression suggest that IL-1 alpha and IL-1 beta mRNAs are independently regulated in alveolar cells, while the regulation of PDGF-A and PDGF-B mRNA seems to be similar. The activated cells also synthesize elevated levels of IL-1 and IL-6. These findings lend support to the suggestion that some clinical consequences of smoking might be initiated and enhanced by the production of inflammatory cytokines. Moreover, IL-6 could also activate a polyclonal B cell response, which could lead to the synthesis of autoantibodies and thus cause immune-mediated tissue injury.