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The influence of both preload and the presence of shim types on the mechanical properties of composite-aluminium single-bolt, single-lap joints were studied in this paper. The load-displacement curve and surface strain field of joints in different shim types and preloads were obtained through tensile experiments. A progressive damage model was established using the UMAT subroutine in ABAQUS. A hybrid failure criterion and a linear continuous degradation model were used to describe the progressive damage of composite laminates. The results show that for joints with no shim and for those with various types of shims, the tensile stiffness, peak load and initial damage load could be reduced when the preload is insufficient or too large. Compared with joints with no shims or with peelable fibreglass shims, joints with liquid shims required a larger preload to achieve the best mechanical properties. As the proportion of peelable fibreglass shim increased, the tensile stiffness and peak load continued to increase in joints with a mixed shim of liquid and peelable fiberglass shim. Shims can serve as tension bearings, but have little effect on the initiation and development of bearing failure.

A multi-bolt spring–mass method considering assembly gap and shim was developed to predict the shear stiffness and bolt stiffness of the single-lap composite joints. Based on the premise that the h...

PURPOSE To improve pseudo-continuous arterial spin labeling (pcASL) at 7T by exploiting a hybrid homogeneity- and efficiency-optimized B1+ -shim with adapted gradient strength as well as background suppression. METHODS The following three experiments were performed at 7T, each employing five volunteers: (1) A hybrid (ie, homogeneity-efficiency optimized) B1+ -shim was introduced and evaluated for variable-rate selective excitation pcASL labeling. Therefore, B1+ -maps in the V3 segment and time-of-flight images were acquired to identify the feeding arteries. For validation, a gradient-echo sequence was applied in circular polarized (CP) mode and with the hybrid B1+ -shim. Additionally, the gray matter (temporal) signal-to-noise ratio (tSNR) in pcASL perfusion images was evaluated. (2) Bloch simulations for the pcASL labeling were conducted and validated experimentally, with a focus on the slice-selective gradients. (3) Background suppression was added to the B1+ -shimmed, gradient-adapted 7T sequence and this was then compared to a matched sequence at 3T. RESULTS The B1+ -shim improved the signal within the labeling plane (23.6%) and the SNR/tSNR increased (+11%/+11%) compared to its value in CP mode; however, the increase was not significant. In accordance with the simulations, the adapted gradients increased the tSNR (35%) and SNR (45%) significantly. Background suppression further improved the perfusion images at 7T, and this protocol performed as well as a resolution-matched protocol at 3T. CONCLUSION The combination of the proposed hybrid B1+ -phase-shim with the adapted slice-selective gradients and background suppression shows great potential for improved pcASL labeling under suboptimal B1+ conditions at 7T.

Bearing Housing is the critical part in Air end assembly of Screw compressor, because it determines the Discharge end clearance (DEC) which in term determine the volumetric and adiabatic efficiency. For setting a DEC in air end assembly, it is important to maintain the shoulder thickness of bearing housing within close tolerance of 10 µm. If not maintained so, metallic shim to be added in assembly process to maintain the clearance. Metallic shim selection for each assembly involves iteration method which is time consuming process. However due to machining constraint the current tolerance which achieved is 100 µm but the requirement is 10microns. If the improvement is done to achieve the required tolerance, DEC setting time can be reduced by eliminating iteration shim setting process. The aim of this project is to achieve the linear & Geometric dimensions within deserved critical tolerance in milling thin wall areas of housings in CNC milling machine by optimizing the process parameters using Design of experiments (DOE) and Finite element analysis (FEA). DOE with cutting parameters using Response surface modeling (RSM) with face centered design is used to obtain the significance level of Cutting speed, feed and Depth of cut. Optimize the tool cutting parameters to minimize the tool and part deflection during machining. FEA is used to reach the optimum clamping area and force to minimize the part deflection & deformation.

During the past several decades, inexpensive compact nuclear magnetic resonance (NMR) instruments have been widely used for on-site detections of chemical identity and sample analyses in industrial applications. In general, via shim coils and control strategies, automatic search shimming methods are capable of improving the magnetic field uniformity so that optimized signals can be acquired and particularly suited for rapid detections in compact NMR instruments. However, because these methods inherently endure the inefficiency in multidimensional shims and the sensitivity of starting points, it is time-consuming to obtain acceptable results in a field infected with distortions and strays. Here, two remedies that improve the shimming speed for the search shimming method are proposed. First, analyzing and compensating for magnetic susceptibility effects, we relocate starting points closely to optima and reduce the complexity of global exploration. Second, modifying the simplex movement with adaptive scale factors and gradient-like centroid, we have improved the convergence of the simplex shimming algorithm in a multidimensional search space. Via theoretical analyses and lab experiments, we find that the proposed shimming method outperforms regular shimming methods in terms of handling complex magnetic fields and reducing shimming times and hope that our study can help further improve NMR shimming methods in the future.

It is necessary to develop a high homogeneous, low power consumption, high frequency and small-size shim coil for high precision and low-cost atomic spin gyroscope (ASG). To provide the shim coil, a multi-objective optimization design method is proposed. All structural parameters including the wire diameter are optimized. In addition to the homogeneity, the size of optimized coil, especially the axial position and winding number, is restricted to develop the small-size shim coil with low power consumption. The 0-1 linear programming is adopted in the optimal model to conveniently describe winding distributions. The branch and bound algorithm is used to solve this model. Theoretical optimization results show that the homogeneity of the optimized shim coil is several orders of magnitudes better than the same-size solenoid. A simulation experiment is also conducted. Experimental results show that optimization results are verified, and power consumption of the optimized coil is about half of the solenoid when providing the same uniform magnetic field. This indicates that the proposed optimal method is feasible to develop shim coil for ASG.

PURPOSE To introduce and investigate a method for free-breathing three-dimensional (3D) B1+ mapping of the human body at ultrahigh field (UHF), which can be used to generate homogenous flip angle (FA) distributions in the human body at UHF. METHODS A 3D relative B1+ mapping sequence with a radial phase-encoding (RPE) k-space trajectory was developed and applied in 11 healthy subjects at 7T. An RPE-based actual flip angle mapping method was applied with a dedicated B1+ shim setting to calibrate the relative B1+ maps yielding absolute B1+ maps of the individual transmit channels. The method was evaluated in a motion phantom and by multidimensional in vivo measurements. Additionally, 3D gradient echo scans with and without static phase-only B1+ shims were used to qualitatively validate B1+ shim predictions. RESULTS The phantom validation revealed good agreement for B1+ maps between dynamic measurement and static reference acquisition. The proposed 3D method was successfully validated in vivo by comparing magnitude and phase distributions with a 2D Cartesian reference. 3D B1+ maps free from visible motion artifacts were successfully acquired for 11 subjects with body mass indexes ranging from 19 kg/m2 to 34 kg/m2 . 3D respiration-resolved absolute B1+ maps indicated FA differences between inhalation and exhalation up to 15% for one channel and up to 24% for combined channels for shallow breathing. CONCLUSION The proposed method provides respiration-resolved absolute 3D B1+ maps of the human body at UHF, which enables the investigation and development of 3D B1+ shimming and parallel transmission methods to further enhance body imaging at UHF.

Objective To compare integrated slice-specific dynamic shim (iShim) with distortion correction post-processing to conventional 3D volume shim for the reduction of artefacts and signal loss in 1.5 T whole-body diffusion-weighted imaging (WB-DWI). Methods Ten volunteers underwent WB-DWI using conventional 3D volume shim and iShim. Forty-eight consecutive patients underwent WB-DWI with either volume shim (n = 24) or iShim (n = 24) only. For all subjects, displacement of the spinal cord at imaging station interfaces was measured on composed b = 900 s/mm2 images. The signal intensity ratios, computed as the average signal intensity in a region of high susceptibility gradient (sternum) divided by the average signal intensity in a region of low susceptibility gradient (vertebral body), were compared in volunteers. For patients, image quality was graded from 1 to 5 (1 = Poor, 5 = Excellent). Signal intensity discontinuity scores were recorded from 1 to 4 (1 = 2 + steps, 4 = 0 steps). A p value of Results Spinal cord displacement artefacts were lower with iShim (p p p Conclusion iShim reduced the degree of spinal cord displacement artefact between imaging stations and susceptibility-gradient-induced signal loss.

The upscaling of turbomachinery using gas foil journal bearings (GFJBs) is limited because of their limited load capacity and dynamic stability. The improvement potential of shim foil inserted under the bump foil of such bearings is investigated in terms of better bearing performance. The arch height difference Δ hb between the shim foil and bump foil can be zero or not to attain the different effect. By considering the local hardening structural stiffness and an Initial installation clearance due to the shim foil, the static and dynamic characteristics of the novel bearing were calculated through the finite difference method (FDM) and perturbation method, respectively. In the analysis, a modified bump stiffness model considering the variable foil thickness was established, and a 2 D thick plate model was adopted for the top foil. The characteristics of novel GFJB with and without preload were compared with the traditional bearing. The results indicate that the load capacity and direct stiffness of the novel GFJB with shim foil can be increased largely, especially when there is a preload (Δ hb≠0). And the improvement is reinforced as the increment of Δ hb. Moreover, the stability threshold speed ( STS) of rotor supported by the novel GFJBs is enhanced by the preload, which means better stability. In addition, an air compressor test has also been conducted to verify the improved supporting performance of novel bearings. Based on this study it is convinced that the addition of shim foil under a GFJB's bump foil can be of practical interest in the quest of enhanced load capacity and dynamic stability. Moreover, the installation of shim foil is not affected by the working environment and could even be retrofited on the existing GFJBs.

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.

PURPOSE: To describe and implement a strategy for dynamic slice-by-slice and multiband B0 shimming using spherical harmonic shims in the human brain at 7T. THEORY: For thin axial slices, spherical harmonic shims can be divided into pairs of shims (z-degenerate and non-z-degenerate) that are spatially degenerate, such that only ½ of the shims (non-z-degenerate) are required for single slice optimizations. However, when combined, the pairs of shims can be used to simultaneously generate the same in-plane symmetries but with different amplitudes as a function of their z location. This enables multiband shimming equivalent to that achievable by single slice-by-slice optimization. METHODS: All data were acquired at 7T using a spherical harmonic shim insert enabling shimming up through 4th order with two additional 5th order shims (1st-4th+). Dynamic shim updating was achieved using a 10A shim power supply with 2 ms ramps and constrained optimizations to minimize eddy currents. RESULTS: In groups of eight subjects, we demonstrated that: 1) dynamic updating using 1st-4th+ order shims reduced the SD of the B0 field over the whole brain from 32.4 ± 2.6 and 24.9 ± 2 Hz with 1st-2nd and 1st-4th+ static global shimming to 15.1 ± 1.7 Hz; 2) near equivalent performance was achieved when dynamically updating only the non-z-degenerate shims (14.3 ± 1.5 Hz), or when a using multiband shim factor of 2, MBs = 2, and all shims (14.4 ± 2.0 Hz). CONCLUSION: High order spherical harmonics provide substantial improvements over static global shimming and enable dynamic multiband shimming with near equivalent performance to that of dynamic slice-by-slice shimming. This reduces distortion in echo planar imaging.

Purpose Inhomogeneities of the static magnetic B\(_{0}\) field are a major limiting factor in cardiac MRI at ultrahigh field (≥ 7T), as they result in signal loss and image distortions. Different magnetic susceptibilities of the myocardium and surrounding tissue in combination with cardiac motion lead to strong spatio‐temporal B\(_{0}\)‐field inhomogeneities, and their homogenization (B0 shimming) is a prerequisite. Limitations of state‐of‐the‐art shimming are described, regional B\(_{0}\) variations are measured, and a methodology for spherical harmonics shimming of the B\(_{0}\) field within the human myocardium is proposed. Methods The spatial B\(_{0}\)‐field distribution in the heart was analyzed as well as temporal B\(_{0}\)‐field variations in the myocardium over the cardiac cycle. Different shim region‐of‐interest selections were compared, and hardware limitations of spherical harmonics B\(_{0}\) shimming were evaluated by calibration‐based B0‐field modeling. The role of third‐order spherical harmonics terms was analyzed as well as potential benefits from cardiac phase–specific shimming. Results The strongest B\(_{0}\)‐field inhomogeneities were observed in localized spots within the left‐ventricular and right‐ventricular myocardium and varied between systolic and diastolic cardiac phases. An anatomy‐driven shim region‐of‐interest selection allowed for improved B\(_{0}\)‐field homogeneity compared with a standard shim region‐of‐interest cuboid. Third‐order spherical harmonics terms were demonstrated to be beneficial for shimming of these myocardial B\(_{0}\)‐field inhomogeneities. Initial results from the in vivo implementation of a potential shim strategy were obtained. Simulated cardiac phase–specific shimming was performed, and a shim term‐by‐term analysis revealed periodic variations of required currents. Conclusion Challenges in state‐of‐the‐art B\(_{0}\) shimming of the human heart at 7 T were described. Cardiac phase–specific shimming strategies were found to be superior to vendor‐supplied shimming.

PURPOSE: The purpose is: (a) Relate magnetic resonance imaging (MRI) quality recommendations for radiation therapy (RT) to B(0) field homogeneity; (b) Evaluate manufacturer specifications of B(0) homogeneity for 34 commercial whole-body MRI systems based on the MRI quality recommendations and RT application; (c) Measure field homogeneity in five commercial MRI systems and one commercial MRI-Linac used in RT and compare the results with their B(0) homogeneity specifications. METHODS: Magnetic resonance imaging quality recommendations for spatial integrity, image blurring, fat saturation, and null banding in RT were developed based on the literature. Guaranteed (maximum) and typical B(0) field homogeneity specifications for various diameter spherical volumes (DSVs) were provided by GE, Philips, Siemens, and Canon. For each system, the DSV that conforms to each MRI quality recommendation and anatomical RT application was estimated based on the manufacturer specifications. B(0) field homogeneity was measured on six MRI systems including Philips (1.5 T), Siemens (1.5 and 3 T), and ViewRay MRI (0.35 T) systems using 24 and 35 cm DSV spherical phantoms. Two measurement techniques were used: (a) MRI using phase contrast field mapping to measure peak-to-peak (pk-pk), volume root mean square (VRMS), and standard deviation (SD); and (b) Magnetic resonance (MR) spectroscopy by acquiring a volumetric free induction decay (FID) to measure full width at half maximum (FWHM). The measurements were used to assess: (a) conformance with the manufacturer specifications; and (b) the relationship between the various field homogeneity measurement units. Measurements were made with and without gradient shimming (gradshim) or second-order active shimming. Multiple comparisons, analysis of variance (ANOVA), and Pearson correlations were performed to assess the dependence of pk-pk, VRMS, SD, and FWHM measurements of field homogeneity on shim volume, level of shim, and MRI system. RESULTS: For a 40 cm DSV, the B(0) homogeneity specifications ranged from 0.35 to 5 ppm (median = 0.75 ppm) VRMS for 1.5 T systems and 0.2 to 1.4 ppm (median = 0.5 ppm) VRMS for 3 T systems. The usable DSVs ranged from 16 to 49 cm (median = 35 cm) based on the image quality recommendations and the manufacturer specifications. There was general compliance between the six measured field homogeneities and manufacturer specifications although signal dephasing was observed in two systems at < 35 cm DSV. The relationships between pk-pk, VRMS, SD, and FWHM varied based on MRI system, shim volume, and quality of shim. However, VRMS and SD measurements were highly correlated. CONCLUSIONS: The delineation of the diseased lesion from organs at risk is the main priority for RT. Therefore, field homogeneity performance for RT must minimize image blurring and image artifacts (null bands and signal dephasing) while optimizing spatial integrity and fat saturation. Based on the specifications and recommendations for field homogeneity, some MRI systems are not well suited to meet the strict demands of RT particularly for the large imaging volumes used in body MRI. VRMS and SD measurements of B(0) field homogeneity tend to be more stable and sensitive to field inhomogeneities in RT applications than pk-pk and FWHM.

PURPOSE: Artifacts caused by large magnetic susceptibility differences between metallic needles and tissue are a persistent problem in many interventional MRI applications. The signal void caused by the needle can hide procedure targets and prevent accurate image‐based monitoring. In this paper, a solution to this problem is presented in the form of an active shim insert inspired from degaussing coils used in naval vessels, that is designed to correct the field disturbance (ΔB(0)) caused by the needle. METHODS: The ΔB(0) induced by a 10 gauge hollow single‐beveled titanium needle at 3T is modeled in different orientations. A set of 63 orthogonal coil pairs with unique tip paths are evaluated for shimming performance, and an optimal coil pair is chosen. Shimming performance and current demands are evaluated over a range of needle orientations. RESULTS: Robust correction of the titanium needle induced ΔB(0) is predicted using a flat no‐loop coil combined with an orthogonal 1½ turn loop coil angled at the bevel angle for most orientations, with currents well below 1 amp per coil. Reductions in ΔB(0) standard deviations with shimming ranged from ~49% to ~10% depending on needle orientation, with performance worsening as the needle is aligned more along B(0). CONCLUSION: Simulations predict that it is possible to minimize metallic probe induced ΔB(0) and signal losses using externally supplied direct current shim coil inserts in arbitrary orientations for potential benefits in many interventional MRI applications.

PURPOSE: Spatial encoding for MRI is generally based on linear x, y, and z magnetic field gradients generated by a set of dedicated gradient coils. We recently introduced the dynamic multicoil technique (DYNAMITE) for B(0) field control and demonstrated DYNAMITE MRI in a preclinical MR environment. In this study, we report the first realization of DYNAMITE MRI of the in vivo human head. METHODS: Gradient fields for DYNAMITE MRI were generated with a 28-channel multicoil hardware arranged in 4 rows of 7 coils on a cylindrical surface (length 359 mm, diameter 344 mm, maximum 5 A per coil). DYNAMITE MRIs of a resolution phantom and in vivo human heads were acquired with multislice gradient-echo, multislice spin-echo, and 3D gradient-echo sequences. The resultant image fidelity was compared to that obtained with conventional gradient coil technology. RESULTS: DYNAMITE field control enabled the realization of all imaging sequences with average gradient errors ≤ 1%. DYNAMITE MRI provided image quality and sensitivity comparable to conventional gradient technology without any obvious artifacts. Some minor geometric deformations were noticed primarily in the image periphery as the result of regional field imperfections. The imperfections can be readily approximated theoretically through numerical integration of the Biot-Savart law and removed through image distortion correction. CONCLUSION: The first realization of DYNAMITE MRI of the in vivo human head has been presented. The obtained image fidelity is comparable to MRI with conventional gradient coils, paving the way for full-fledged DYNAMITE MRI and B(0) shim systems for human applications.

China Institute of Atomic Energy has developed a 230 MeV compact superconducting cyclotron CYCIAE-230 to meet the demands of proton therapy in China. Physical design requires a strict control of the average field error and first harmonic in the main field. A processing method of inclined 45° continuous cutting on the two pole sides is at the first time proposed to shim the field in compact superconducting cyclotron. For a spiral magnet pole, the field generated by the 45° continuous cutting on edges is relatively complicated to be calculated precisely by finite element method. In this paper, the analytical model of the linear equation for field shimming calculation is established. The shape function caused by the local cutting is solved by the magnetic saturation approximation. And then the shimming process is built with an iterative procedure based on the ridge regression method. The shimming algorithm is applied in CYCIAE-230, which achieve good results to reduce the shimming times and further the construction period. The process of the shimming algorithm and the corresponding shimming effect in CYCIAE-230 will be presented in detail.

Quadruple magnets are used as superconducting magnets in accelerator systems and to focus the accelerated beam particles. Therefore, studies are being conducted to implement HTS quadruple magnets in hot cell regions that possess high heat loads in accelerator systems. Additionally, studies on air-core HTS quadruple magnet (AHQM), which do not possess an iron yoke, are also being carried out. This magnet is suitable for the use in hot cell regions and generates a higher magnetic field. Further, magnetic field characteristic of the magnet are linear with respect to the operating current. However, the magnetic field of an AHQM can be the sensitively affected by manufacturing and assembly errors because the iron yoke is removed. After the manufacturing of the magnet, magnetic field shimming is used to compensate for magnetic field errors. This paper presents the optimum design and process for the copper wire bunch active shim (CBAS) method. Furthermore, a test model of the CBAS method is constructed based on the design, and the characteristics and applicability of the proposed model are analyzed.

Purpose: To interleave global and local higher order shimming for single voxel MRS. Single voxel MR spectroscopy requires optimization of the B0 field homogeneity in the region of the voxel to obtain a narrow linewidth and provide high data quality. However, the optimization of local higher order fields on a localized MRS voxel typically leads to large field offsets outside that volume. This compromises interleaved MR sequence elements that benefit from global field homogeneity such as water suppression, interleaved MRS-fMRI, and MR motion correction. Methods: A shimming algorithm was developed to optimize the MRS voxel homogeneity and the whole brain homogeneity for interleaved sequence elements, using static higher order shims and dynamic linear terms (HOS-DLT). Shimming performance was evaluated using 6 brain regions and 10 subjects. Furthermore, the benefits of HOS-DLT was demonstrated for water suppression, MRS-fMRI, and motion corrected MRS using fat-navigators. Results: The HOS-DLT algorithm was shown to improve the whole brain homogeneity compared to an MRS voxel-based shim, without compromising the MRS voxel homogeneity. Improved water suppression over the brain, reduced image distortions in MRS-fMRI, and improved quality of motion navigators were demonstrated using the HOS-DLT method. Conclusion: HOS-DLT shimming allowed for both local and global field homogeneity, providing excellent MR spectroscopy data quality, as well as good field homogeneity for interleaved sequence elements, even without the need for dynamic higher order shimming capabilities.

With few exceptions, a fresh magnetic resonance imaging (MRI) scanner may fail to achieve the magnetic field specifications, due to the existence of engineering errors or surrounding ferromagnetic materials. Excellent field homogeneity is one of the most important criteria for imaging quality; therefore, the shimming technique must be employed to bring the magnetic field homogeneity within the MRI requirements. The commonly used method is passive shimming (PS), which utilizes the shim pieces (iron pieces) with a different thickness in specific positions to generate a “corrective” magnetic field. In conventional PS, the inhomogeneous field over the measured volume (MV) is shimmed to be the best as much as possible. However, in fact, since the MV and the real imaging volumes (IVs) are different, a problem may exist as the field over the MV is good enough but may not be sufficiently qualified over some IVs. As a result, a multi-volumes PS strategy is proposed in this article, which pays more attention to the magnetic field over some potential IVs other than the MV. Compared with the conventional single-constraint method, the new method increases the shimming targets and constraints tailored to imaging, without increasing any computing burden and additional iron consumption. In addition, the PS test on a 1.5 T MRI scanner is performed using the conventional and proposed algorithms, respectively. The magnetic field obtained by a new strategy is obviously superior to that of the conventional method, evaluated from some specific volumes for parts of human body imaging, demonstrating the effectiveness of the multi-volumes shimming strategy.

MR spectroscopic imaging (MRSI) noninvasively maps the metabolism of human brains. In particular, the imaging of D-2-hydroxyglutarate (2HG) produced by glioma isocitrate dehydrogenase (IDH) mutations has become a key application in neuro-oncology. However, the performance of full field-of-view MRSI is limited by B0 spatial nonuniformity and lipid artifacts from tissues surrounding the brain. Array coils that multiplex RF-receive and B0 -shim electrical currents (AC/DC mixing) over the same conductive loops provide many degrees of freedom to improve B0 uniformity and reduce lipid artifacts. AC/DC coils are highly efficient due to compact design, requiring low shim currents (

Fluorescent proteins (FPs) are biotags of choice for second-harmonic imaging microscopy (SHIM). Because of their large size, computing their second-harmonic generation (SHG) response represents a great challenge for quantum chemistry. In this contribution, we propose a new all-atom quantum mechanics methodology to compute SHG of large systems. This is now possible because of two recent implementations: the tight-binding GFN2-xTB method to optimize geometries and a related version of the simplified time-dependent density functional theory (sTD-DFT-xTB) to evaluate quadratic response functions. In addition, a new dual-threshold configuration selection scheme is introduced to reduce the computational costs while retaining overall similar accuracy. This methodology was tested to evaluate the SHG of the proteins iLOV and bacteriorhodopsin (bR). In the case of bR, quantitative agreement with respect to experiment was reached for the out-of-resonance low-energy part of the βHRS frequency dispersion. This work paves the way toward an accurate prediction of the SHG of large structures-a requirement for the design of new and improved SHIM biotags.

PurposeTo develop an RF coil with an integrated commercial field camera for ultra-high field (7 T) neuroimaging. The RF coil will operate within a head-only gradient coil and be subject to the corresponding design constraints. The RF coil can thereafter be used for subject-specific correction of k-space trajectories—notably in gradient-sensitive sequences such as single-shot spiral imaging.MethodsThe transmit and receive performance was evaluated before and after the integration of field probes, while field probes were evaluated when in an optimal configuration external to the coil and after their integration. Diffusion-weighted EPI and single-shot spiral acquisitions were employed to evaluate the efficacy of correcting higher order field perturbations and the consequent effect on image quality.ResultsField probes had a negligible effect on RF-coil performance, including the transmit efficiency, transmit uniformity, and mean SNR over the brain. Modest reductions in field-probe signal lifetimes were observed, caused primarily by non-idealities in the gradient and shim fields of the head-only gradient coil at the probe positions. The field monitoring system could correct up to second-order field perturbations in single-shot spiral imaging.ConclusionThe integrated RF coil and field camera was capable of concurrent field monitoring within a 7T head-only scanner and facilitated the subsequent correction of k-space trajectories during spiral imaging.

Terahertz wave (T-ray) scanning applications are one of the most promising tools for nondestructive evaluation. T-ray scanning applications use a T-ray technique to measure the thickness of both thin Shim stock films and GFRP (glass fiber-reinforced plastics) composites, of which the samples were selected because the T-ray method could penetrate the non-conducting samples. Notably, this method is nondestructive, making it useful for analyzing the characteristics of the materials. Thus, the T-ray thickness measurement can be found for both non-conducting Shim stock films and GFRP composites. In this work, a characterization procedure was conducted to analyze electromagnetic properties, such as the refractive index. The obtained estimates of the properties are in good agreement with the known data for poly methyl methacrylate (PMMA) for acquiring the refractive index. The T-ray technique was developed to measure the thickness of the thin Shim stock films and the GFRP composites. Our tests obtained good results on the thickness of the standard film samples, with the different thicknesses ranging from around 120 μm to 500 μm. In this study, the T-ray method was based on the reflection mode measurement, and the time-of-flight (TOF) and resonance frequencies were utilized to acquire the thickness measurements of the films and GFRP composites. The results showed that the thickness of the samples of frequency matched those obtained directly by time-of-flight (TOF) methods.

High-field magnetic resonance imaging (MRI, 3.0T and above) offers numerous advantages for imaging the human body over lower-field strengths. However, it suffers from unwanted fast spatially-varying main (B0 ) fields caused by the susceptibility mismatch at the tissue interfaces. When this is combined with the anatomical complexity of the human body, undesirable image artifacts can become damaging as they can compromise potential image contrasts, limit the use of accelerated imaging, and interfere with clinical interpretation. Consequently, these limitations restrict the effective utilization of high-field body MRI and emphasize the need for a major improvement in B0 field homogeneity to take full advantage of the ever increased B0 field. Here we introduce a Unified shim-RF Coil (UNIC) to overcome this existing bottleneck by transcending the conventional low-efficiency, distantly located B0 shim coils. UNIC allows a shim array to be freely allotted and seamlessly integrated into a standard surface RF coil, thus maximizing both the performance of RF receive sensitivity and effective B0 shimming. We demonstrate the capacity of the UNIC approach through detailed characterization of the coil design, prototyping a body coil integrating the UNIC features, and conducting in-vivo imaging of deep organs adjacent to the lung. Our studies provide evidence that UNIC enables homogeneous B0 fields in the liver and the heart, where strong image artifacts are known to occur, and hence facilitate the acquisition of unprecedented image quality in a clinical 3.0T scanner. Further, UNIC’s design is practical as it overcomes one of the most, if not the most, critical limitations of the state-of-the-art high-field MRI with minimal changes to the current MRI hardware architecture. Accordingly, the proposed technique offers opportunities for major advancements in noninvasive imaging of deep organs with high-field imaging in a way it has not been possible thus far.

PURPOSE: Signal voids caused by metallic needles pose visualization and monitoring challenges in many MRI applications. In this work we explore a solution to this problem in the form of an active shim insert that fits inside a needle and corrects the field disturbance (ΔB(0)) caused by the needle outside of it. METHODS: The ΔB(0) induced by a 4mm OD Titanium needle at 3 Tesla is modeled and a 2-coil orthogonal shim set is designed and fabricated to shim the ΔB(0). Signal recovery around the needle is assessed in multiple orientations in a water phantom with four different pulse sequences. Phase stability around the needle is assessed in an ex-vivo porcine tissue dynamic gradient echo experiment with and without shimming. Additionally, heating of the shim insert is assessed under 8 minutes of continuous operation with 1A current and concurrent imaging. RESULTS: An average recovery of ~63% of lost signal around the needle across orientations is shown with active shimming with a maximum current of 1.172 A. Signal recovery and correction of the underlying ΔB(0) is shown to be independent of imaging sequence. Needle-induced phase gradients outside the perceptible signal void are also minimized with active shimming. Temperature rise of up to 0.9° Celsius is noted over 8 minutes of continuous 1A active shimming operation. CONCLUSION: A sequence independent method for minimization of metallic needle induced signal loss using an active shim insert is presented. The method has potential benefits in a range of qualitative and quantitative interventional MRI applications.

The primary excitatory and inhibitory neurotransmitters glutamate (Glu) and gamma-aminobutyric acid (GABA) are thought to be involved in the response of the brain to changes in glycemia. Therefore, their reliable measurement is critical for understanding the dynamics of these responses. The concentrations of Glu and GABA, as well as glucose (Glc) in brain tissue, can be measured in vivo using proton (1H) magnetic resonance spectroscopy (MRS). Advanced MRS methodology at ultrahigh field allows reliable monitoring of these metabolites under changing metabolic states. However, the long acquisition times needed for these experiments while maintaining blood Glc levels at predetermined targets present many challenges. We present an advanced MRS acquisition protocol that combines commercial 7T hardware (Siemens Scanner and Nova Medical head coil), BaTiO3 dielectric padding, optical motion tracking, and dynamic frequency and B0 shim updates to ensure the acquisition of reproducibly high-quality data. Data were acquired with a semi-LASER sequence [repetition time/echo time (TR/TE) = 5,000/26 ms] from volumes of interest (VOIs) in the prefrontal cortex (PFC) and hypothalamus (HTL). Five healthy volunteers were scanned to evaluate the effect of the BaTiO3 pads on B1+ distribution. Use of BaTiO3 padding resulted in a 60% gain in signal-to-noise ratio in the PFC VOI over the acquisition without the pad. The protocol was tested in six patients with type 1 diabetes during a clamp study where euglycemic (~100 mg/dL) and hypoglycemic (~50 mg/dL) blood Glc levels were maintained in the scanner. The new protocol allowed retention of all HTL data compared with our prior experience of having to exclude approximately half of the HTL data in similar clamp experiments in the 7T scanner due to subject motion. The advanced MRS protocol showed excellent data quality (reliable quantification of 11–12 metabolites) and stability (p > 0.05 for both signal-to-noise ratio and water linewidths) between euglycemia and hypoglycemia. Decreased brain Glc levels under hypoglycemia were reliably detected in both VOIs. In addition, mean Glu level trended lower at hypoglycemia than euglycemia for both VOIs, consistent with prior observations in the occipital cortex. This protocol will allow robust mechanistic investigations of the primary neurotransmitters, Glu and GABA, under changing glycemic conditions.

PURPOSE To optimize pseudo-continuous arterial spin labeling (pCASL) for 7 T, and to further improve the labeling efficiency with parallel RF transmission transmit B1 ( B1+ ) shimming. METHODS pCASL parameters were optimized based on B1+/B0 field distributions at 7 T with simulation. To increase labeling efficiency, the B1+ amplitude at inflowing arteries was increased with parallel RF transmission B1+ shimming. The "indv-shim" with shimming weights calculated for each individual subject, and the "univ-shim" with universal weights calculated on a group of 12 subjects, were compared with circular polarized (CP) shim. The optimized pCASL sequences with three B1+ shimming modes (indv-shim, univ-shim, and CP-shim) were evaluated in 6 subjects who underwent two repeated scans 24 hours apart, along with a pulsed ASL sequence. Quantitative metrics including mean B1+ amplitude, perfusion, and intraclass correlation coefficient were calculated. The optimized 7T pCASL was compared with standard 3T pCASL on 5 subjects, using spatial SNR and temporal SNR. RESULTS The optimal pCASL parameter set (RF duration/gap = 300/250 us, Gave=0.6mT/m,gRatio=10 ) achieved robust perfusion measurement in the presence of B1+/B0 inhomogeneities. Both indv-shim and univ-shim significantly increased B1+ amplitude compared with CP-shim in simulation and in vivo experiment (P < .01). Compared with CP-shim, perfusion signal was increased by 9.5% with indv-shim (P < .05) and by 5.3% with univ-shim (P = .35). All three pCASL sequences achieved fair to good repeatability (intraclass correlation coefficient ≥ 0.5). Compared with 3T pCASL, the optimized 7T pCASL achieved 78.3% higher spatial SNR and 200% higher temporal SNR. CONCLUSION The optimized pCASL achieved robust perfusion imaging at 7 T, while both indv-shim and univ-shim further increased labeling efficiency.

Purpose: Brain imaging exams typically take 10-20 min and involve multiple sequential acquisitions. A low-distortion whole-brain echo planar imaging (EPI)-based approach was developed to efficiently encode multiple contrasts in one acquisition, allowing for calculation of quantitative parameter maps and synthetic contrast-weighted images.Methods: inversion prepared spin- and gradient-echo EPI was developed with sliceorder shuffling across measurements for efficient acquisition with T-1, T-2, and T-2* weighting. A dictionary-matching approach was used to fit the images to quantitative parameter maps, which in turn were used to create synthetic weighted images with typical clinical contrasts. Dynamic slice-optimized multi-coil shimming with a B-0 shim array was used to reduce B-0 inhomogeneity and, therefore, image distortion by >50%. Multi-shot EPI was also implemented to minimize distortion and blurring while enabling high in-plane resolution. A low-rank reconstruction approach was used to mitigate errors from shot-to-shot phase variation.Results: The slice-optimized shimming approach was combined with in-plane parallel-imaging acceleration of 4x to enable single-shot EPI with more than eightfold distortion reduction. The proposed sequence efficiently obtained 40 contrasts across the whole-brain in just over 1 min at 1.2 x 1.2 x 3 mm resolution. The multishot variant of the sequence achieved higher in-plane resolution of 1 x 1 x 4 mm with good image quality in 4 min. Derived quantitative maps showed comparable values to conventional mapping methods.Conclusion: The approach allows fast whole-brain imaging with quantitative parameter maps and synthetic weighted contrasts. The slice-optimized multi-coil shimming and multi-shot reconstruction approaches result in minimal EPI distortion, giving the sequence the potential to be used in rapid screening applications.

This paper proposes a new concept of designing compact high precision millimeter-wave wideband variable phase shifters. The phase shifter is implemented with a stacked shim with extremely short length of 0.9 mm and two waveguide flange adaptors with length of 0.5 mm. High precision phase shifting is achieved over entire D-band (110-170 GHz) by rotating the shim 90 degrees from aligned to perpendicular with consistent impedance matching performance. In addition, a glide-symmetric holey electromagnetic bandgap (EBG) structure is adopted to avoid wave leakage from the gap between the shim and the flange adaptors. A proof-of-concept (PoC) demonstrator is designed, manufactured, and tested. The measured results show that the designed stacked shim phase shifter with embedded EBG structure ensures return loss higher than 10 dB across 110-170 GHz with a 75 μm airgap between waveguide flanges. The studied phase shifter provides a 0.88° phase shifting with each degree of mechanical rotation. The fabricated PoC phase shifter has a worst-case insertion loss of 0.92 dB and a return loss of 20 dB across the entire 110-170 GHz band and a maximum phase shift of 30°. At 10° phase shifting, the measured insertion loss is lower than 0.52 dB, and return loss is higher than 23 dB, respectively.

Local susceptibility variations result in B0 field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with increasing B0 magnetic field strength. Active shimming is used to generate corrective magnetic fields, which can be used to improve B0 field homogeneity. FASTMAP is an effective shimming technique for computing optimal coil currents, which uses data from six projection directions (or columns): this technique is routinely used for shimming cubic volumes of interest (VOIs). In this paper, we propose several improvements to FASTMAP at 4T. For each shim coil, using a modified 3D gradient-echo pulse sequence, we compute B0 inhomogeneity maps and project them onto eight 1st and 2nd order spherical harmonic functions. This process is repeated for shim currents between -15,000 to 15,000 with increments of 5000 Digital to Analog Converter (DAC) units, and is used to compute the gradient between spherical harmonic coefficients and DAC values for all 8 shim coils—along with the R2 values of linear fits. A method is proposed (based on R2 values) to further refine optimal shim currents in respective coils. We present an analysis that is numerically robust and completely flexible in the selection of the VOIs for shimming. Performance analyses, phantom results, and in vivo results of a human brain are presented, comparing our methods with the FASTMAP method.

Study Design-Biomechanical cadaveric study. Objective-Long anterior cervical plate and cage (APC) constructs have a risk of pseudarthrosis with minor bone resorption. Stand-alone cages (SACs) allow settling. The biomechanics of SAC have been investigated, but not multilevel, compression screw SAC. The purpose of this study is to evaluate the biomechanical safety of three-level SAC versus APC. Methods-Discectomies at three levels of five human cadaver spines (T1-C3) were fixed with SAC. A 0.18 mm thick shim was interposed between the cage and the superior endplate, and a pressure transducer map was placed between the cage and the inferior endplate. Tests were performed in flexion-extension and then repeated after removing the shims to simulate minor bone resorption. Subsequently, APC was applied and experiments were repeated. The pressure between each cage and endplate and motion of the implants were measured. Results-The range of motion (ROM) of SAC and APC constructs were comparable. The contact area and pressure between cage and endplate did not significantly change during motion with SAC. Shim removal did not significantly affect ROM, contact area, or average pressure measures. For APC, both contact area and pressure decreased from extension to flexion. Shim removal caused a significant loss of contact area and pressure. Conclusions-SAC provided comparable rigidity to the conventional APC construct while maintaining compression at the endplate-cage interface throughout flexion-extension and after minor bone resorption.

Two closed loop configurations for the axial flux tilt control practice are compared in terms of certain controllability measures. To this end, the core of a PWR is taken into account. The Morari resiliency index and the Condition Number are adopted as reliable controllability measures examined for the open loop plant to evaluate the selected input/output pairs. In one scenario, boron injection is accompanied by the control rod as common means for the concurrent regulation of the core power and the pertaining axial offset. In the other case however, the boron mechanism is replaced for another regulating rod which implies the mechanical shim strategy proposed in more recent designs (e.g. IRIS or APR). A two-region modeling of the reactor core is resorted in both cases and the resultant TITO model is controlled by a Davison tuned PI regulator. Simulations indicate that the chemical shim configuration better meets the attempted controllability indices and thereby predicates a more resilient closed loop control practice. A step input perturbation is effected on the core inlet temperature to further verify this result.

Rotary percussion drilling with pneumatic hammers is a promising, high-performance method widely used in the practice of geological exploration, the performance of which is determined mainly by pressure and the amount of cleaning agent supplied by the compressor. The parameters of commercially available high-pressure compressors do not allow drilling at a depth of more than 300 meters, which is one of the main limiting factors of its practical application in production conditions. One of the ways to improve the performance of the rotary-percussion drilling of wells, and at the same time the maximum depth of drilled wells is to improve the mechanism of rock destruction by applying eccentric impact pulses to the drilling tool, which will make it possible to implement more actively the tangential component of the impact pulse that affects the shape and the volumes of the fracture holes being formed, providing additional splitting of the rock in the direction of the rock face. The paper discusses the ways to perfect the known designs of bits for rotary-percussion drilling of wells implementing eccentric application of impact impulses, issues of geometric substantiation of the shape of an eccentric protrusion on a drill bit torus shim for rotary-percussion drilling and oscillations of torus shim when transmitting eccentric impact pulses from the point of view of enhancement durability of a construction and accuracy of transfer of eccentric impact pulses.

Purpose We assessed how improved static magnetic field (B0 ) homogeneity with a dynamic multicoil shimming can influence the blood oxygen level dependent (BOLD) contrast to noise when echo planar imaging (EPI) sequence is used for a motor task functional MRI study. We showed that a multicoil shim setup can be a proper choice for dynamic shimming of 2 spatially distant areas with different inhomogeneity distributions. Methods A 16-channel multicoil shim setup is used to provide improved B0 homogeneity by dynamic slice-wise shimming. The performance of dynamic B0 shimming was investigated in 2 distinct brain regions, the motor cortex and the cerebellum, in the same experiment during a finger-tapping task. Temporal SNR (tSNR), geometric distortion of the EPIs, and results of an analysis with a general linear model before and after shimming with the multicoil were compared. Results Reduced B0 deviation by 30% and 52% in the cerebellum and motor cortex, respectively, resulted in higher tSNR and a reduction of distortions in the EPI. Statistical analysis applied to the EPIs showed higher t values and increased number of voxels above significance threshold when shimming with the multicoil setup. Conclusions Improved B0 homogeneity leads to higher tSNR and enhances the detection of BOLD signal.

PURPOSE To validate the feasibility of localized B0 shimming based on B0 maps acquired with sodium (23 Na) MRI. METHODS A localized B0 shimming routine based on a constrained regularized algorithm in combination with 23 Na MRI data acquired with a 3D density-adapted radial readout scheme was implemented on a 7T MR system. Measurements were performed using a dual-tuned 23 Na/1 H head coil. The quality of B0 maps reconstructed from 23 Na images and the resulting shim values was examined depending on the acquisition duration between 10 minutes and 15 seconds to examine clinical applicability. The B0 shimming based on 23 Na B0 maps was performed both for phantom and human head of 6 healthy volunteers, and the resulting B0 homogeneity was compared with the vendor-provided 1 H MRI-based gradient-echo brain shimming routine. RESULTS The proposed 23 Na MRI-based shimming routine showed a reduction in B0 variation comparable to the vendor-provided shim both in phantom and in vivo measurements. Within the examined multicompartment phantom, the B0 variations could be reduced by up to 77% using the 23 Na MRI-based shim. In human head, B0 variations were reduced by approximately 50% using an acquisition time of 15 seconds for the 23 Na B0 maps and only 1 iteration of B0 shimming. CONCLUSION The 23 Na MRI-based localized B0 shimming is possible at 7 T within clinically acceptable acquisition durations (< 1 minute). It was shown that using the proposed 23 Na MRI-based shimming approach, the 23 Na image quality at ultrahigh field strength can be strongly improved.

The SC200 superconducting cyclotron is developed by the collaboration between ASIPP (Hefei, China) and JINR (Dubna, Russia). The SC200 cyclotron will deliver 200 MeV beam for proton therapy and research. The influence of the time-dependent magnetic field created by the acceleration system of SC200 should be taken into account in beam dynamics simulations and during the shimming procedure, as two conditions are present together: first — due to the spiral shape of the accelerating cavities, there is no vertical plane of symmetry through the middle of the dee; and second — second harmonic mode of acceleration in four-sector magnet structure imposes that the RF phase shift from beam entrance of the dee to its exit is not 180 degrees. Particle tracking was performed with 3D maps of electromagnetic fields obtained from computer modelling of the magnet system and the accelerating RF cavities using CST Studio Suite. We have calculated particle dynamics taking into account RF magnetic field and have estimated a value of additional mean field needed for the compensation of beam phase lag induced by RF magnetic field. And finally, we designed a shim that should form the required additional field.

Background: Repeated glycoCEST MRI measurements on the same subject should produce similar results under the same environmental and experimental conditions. However, fluctuations in the static B 0 field, which may occur between and within measurements due to heating of the shim iron or subject motion, may alter results and affect reproducibility. Here we investigate the repeatability and reproducibility of glycoCEST measurements and examine the effectiveness of a real-time shim- and motion navigated chemical exchange saturation transfer (CEST) sequence to improve reproducibility. Methods: In nine subjects, double volumetric navigated (DvNav)-CEST acquisitions in the calf muscle were repeated five times in each of two sessions—the first without correction, and the second with real-time shim- and motion correction applied. In both sessions a dynamically changing field was introduced by running a 5-minute gradient intensive diffusion sequence. We evaluated the effect of the introduced B 0 inhomogeneity on the reproducibility of glycoCEST, where the small chemical shift difference between the hydroxyl and bulk water protons at 3 T makes CEST quantification extremely sensitive to magnetic field inhomogeneities. Results: With real-time shim- and motion correction, glycoCEST results were relatively consistent with mean coefficient of variation (CoV) 2.7%±1.4% across all subjects, whereas without correction the results were less consistent with CoV 84%±71%. Conclusions: Our results demonstrate that real-time shim- and motion correction can mitigate effects of B 0 field fluctuations and improve reproducibility of glycoCEST data. This is important when conducting longitudinal studies or when using glycoCEST MRI to assess treatment or physiological responses over time.

Purpose A multi-coil shim setup is designed and optimized for human brain shimming. Here, the size and position of a set of square coils are optimized to improve the shim performance without increasing the number of local coils. Utilizing such a setup is especially beneficial at ultrahigh fields where B0 inhomogeneity in the human brain is more severe. Methods The optimization started with a symmetric arrangement of 32 independent coils. Three parameters per coil were optimized in parallel, including angular and axial positions on a cylinder surface and size of the coil, which were constrained by cylinder size, construction consideration, and amplifiers specifications. B0 maps were acquired at 9.4T in 8 healthy volunteers for use as training data. The global and dynamic shimming performance of the optimized multi-coil were compared in simulations and measurements to a symmetric design and to the scanner's second-order shim setup, respectively. Results The optimized multi-coil performs better by 14.7% based on standard deviation (SD) improvement with constrained global shimming in comparison to the symmetric positioning of the coils. Global shimming performance was comparable with a symmetric 65-channel multi-coil and full fifth-order spherical harmonic shim coils. On average, an SD of 48.4 and 31.9 Hz was achieved for in vivo measurements after global and dynamic slice-wise shimming, respectively. Conclusions An optimized multi-coil shim setup was designed and constructed for human whole-brain shimming. Similar performance of the multi-coils with many channels can be achieved with a fewer number of channels when the coils are optimally arranged around the target.

PURPOSE To measure spatiotemporal B0 field changes in real time using FID navigators (FIDnavs) and to demonstrate the efficacy of retrospectively correcting high-resolution T2* -weighted images using a novel FIDnav framework. METHODS A forward model of the complex FIDnav signals was generated by simulating the effect of changes in the underlying B0 inhomogeneity coefficients, with spatial encoding provided by a multi-channel reference image. Experiments were performed at 3T to assess the accuracy of B0 field estimates from FIDnavs acquired from a 64-channel head coil under different shim settings and in 5 volunteers performing deep-breathing and nose-touching tasks designed to modulate the B0 field. Second-order, in-plane spherical harmonic (SH) inhomogeneity coefficients estimated from FIDnavs were incorporated into an iterative reconstruction to retrospectively correct 2D gradient-echo images acquired in both axial and sagittal planes. RESULTS Spatiotemporal B0 field changes measured from rapidly acquired FIDnavs were in good agreement with the results of second-order SH fitting to the measured field maps. FIDnav field estimates accounted for a significant proportion of the ΔB0 variance induced by deep breathing (64 ± 21%) and nose touching (67 ± 34%) across all volunteers. Ghosting, blurring, and intensity modulation artifacts in T2* -weighted images, induced by spatiotemporal field changes, were visibly reduced following retrospective correction with FIDnav inhomogeneity coefficients. CONCLUSIONS Spatially resolved B0 inhomogeneity changes up to second order can be characterized in real time using the proposed approach. Retrospective FIDnav correction substantially improves T2* -weighted image quality in the presence of strong B0 field modulations, with potential for real-time shimming.

International audience; Occasionally in proton high-resolution magic angle spinning nuclear magnetic resonance spectroscopy (1H HR-MAS NMR), the standard field shimming across the region-of-interest with the active shims is not sufficient in the presence of substantial magnetic susceptibility gradients. This can be ascribed to the presence of large air pocket within the sample, the proximity between sample and probehead (especially with micro-sized probes), or data acquisition at high magnetic fields. Herein, the study demonstrates a simple approach to enhance the capacity of the shim fields—by supplementing a specific passive ferro-shim—for unifying the field distributions across the sample. Qualitative field numerical analyses were carried out to illustrate the effectiveness of the applied passive shims together with the active shims.

An air-core high-temperature superconducting quadruple magnet (AHQM) is composed of high-temperature superconductor (HTS) coils without an iron yoke. An AHQM exhibits magnetic field characteristics that vary linearly with respect to changes in the operating current. Further, the AHQM is more suitable for use in high heat load regions. A conventional iron-core quadruple magnet (IQM) contains an iron yoke to achieve the specific shape and magnitude of the magnetic field. In the case of IQMs, the iron yoke generates most of the magnetic field. In the case of AHQMs, the magnetic fields are generated solely by HTS coils. Thus, an AHQM requires a larger number of coils to generate a magnetic field, because the iron yoke has been removed. Consequently, the influence of manufacturing errors in the AHQM is high; these errors deteriorate the designed magnetic field. Therefore, magnetic field correction is required to reduce the deterioration in the magnetic field of an AHQM. This paper, proposes a novel active shim method to compensate for the deteriorated magnetic field of an AHQM. This method improves the uniformity, but does not affect the gradient and effective length of the quadruple magnet. The effectiveness of this method is verified by simulation.

Purpose: B1+ and T1 corrections and dynamic multi-coil shimming approaches were proposed to improve the fidelity of high isotropic resolution Generalized slice dithered enhanced resolution (gSlider) diffusion imaging. Methods: An extended reconstruction incorporating B1+ inhomogeneity and T1 recovery information was developed to mitigate slab-boundary artifacts in short-TR gSlider acquisitions. Slab-by-slab dynamic B0 shimming using a multi-coil integrated {\Delta}B0/Rx shim-array, and high in-plane acceleration (Rinplane=4) achieved with virtual-coil GRAPPA were also incorporated into a 1 mm isotropic resolution gSlider acquisition/reconstruction framework to achieve an 8-11 fold reduction in geometric distortion compared to single-shot EPI. Results: The slab-boundary artifacts were alleviated by the proposed B1+ and T1 corrections compared to the standard gSlider reconstruction pipeline for short-TR acquisitions. Dynamic shimming provided >50% reduction in geometric distortion compared to conventional global 2nd order shimming. 1 mm isotropic resolution diffusion data show that the typically problematic temporal and frontal lobes of the brain can be imaged with high geometric fidelity using dynamic shimming. Conclusions: The proposed B1+ and T1 corrections and local-field control substantially improved the fidelity of high isotropic resolution diffusion imaging, with reduced slab-boundary artifacts and geometric distortion compared to conventional gSlider acquisition and reconstruction. This enabled high-fidelity whole-brain 1 mm isotropic diffusion imaging with 64 diffusion-directions in 20 minutes using a 3T clinical scanner.

Purpose Multi-phase PCASL has been proposed as a means to achieve accurate perfusion quantification that is robust to imperfect shim in the labelling plane. However, there exists a bias in the estimation process that is a function of noise in the data. In this work, this bias is characterised and then addressed in animal and human data. Methods The proposed algorithm to overcome bias uses the initial biased voxel-wise estimate of phase tracking error to cluster regions with different off-resonance phase shifts, from which a high-SNR estimate of regional phase offset is derived. Simulations were used to predict the bias expected at typical SNR. Multi-phase PCASL in three rat strains (n = 21) at 9.4 T was considered, along with 20 human subjects previously imaged using ASL at 3 T. The algorithm was extended to include estimation of arterial blood flow velocity. Results Based on simulations, a perfusion estimation bias of 6–8% was expected using 8-phase data at typical SNR. This bias was eliminated when a high-precision estimate of phase error was available. In the preclinical data, the bias-corrected measure of perfusion (107±14 mL/100g/min) was lower than the standard analysis (116±14 mL/100g/min), corresponding to a mean observed bias across strains of 8.0%. In the human data, bias correction resulted in a 15% decrease in the estimate of perfusion. Conclusion Using a retrospective algorithmic approach, it was possible to exploit common information found in multiple voxels within a whole region of the brain, offering superior SNR and thus overcoming the bias in perfusion quantification from multi-phase PCASL.

Purpose To demonstrate feasibility and performance of prospective motion and B0 shim correction for MRS in human brain at 7T. Methods Prospective motion correction using an optical camera and linear B0 shim correction using FASTMAP-like navigators were implemented into a semi-LASER sequence. The effect of motion on spectral quality was assessed without and with prospective correction in prefrontal cortex in 11 subjects. Results Without prospective motion and shim correction, motion resulted in considerable degradation of MR spectra (broader linewidth, lower signal-to-noise ratio, degraded water suppression). With prospective motion and shim correction, spectral quality remained excellent despite motion. Prospective motion correction alone was not sufficient to prevent degradation of spectral quality. Conclusion Prospective motion and B0 shim correction is feasible at 7T and should help improve the robustness of MRS, particularly in motion-prone populations.

The purpose of this paper is to study dynamic characteristics of the AP1000 reactor during control rod sequence exchange (CRSE) operations under the Mechanical Shim (MSHIM) control strategy so as to assess the feasibility of and further optimize the CRSE logic. For this purpose, a dynamic simulation platform was developed in MATLAB/Simulink environment first, with implementation of a one-dimensional nodal core model and the reactor power control system integrating the basic MSHIM control logic and the CRSE logic. Based on the platform, simulation studies of typical CRSE transients of the AP1000 reactor at the beginning of cycle life (BOC), middle of cycle life (MOC) and end of cycle life (EOL) have been performed to determine the performance of the current CRSE design. From the simulation results, impacts of the CRSE operations on the reactor power and axial power difference have been analyzed. It shows that the AP1000 reactor can accomplish the CRSE operations smoothly and safely implementing the CRSE logic. To further improve the control performance of the reactor during CRSE operations, the trigger rod position, a key factor in the CRSE design, has been optimized with the optimization results assessed through dynamic simulations against the above CRSE transients. The simulation results have demonstrated the effectiveness of the optimization.

To describe a platform for 4-channel parallel transmission (pTx) integrated on a 3 T MRI system; to demonstrate utility of the platform with a pTx method to reduce localized radiofrequency (RF) heating during MRI of a test object implanted with an electrically conductive wire; and to compare the pTx results with those from a standard birdcage (BC) head coil. A 4-channel pTx system with independent amplitude and phase control was added to a Siemens Prisma MRI system. Turbo spin echo pTx MRI of a homogenous tissue-mimicking head phantom, including an implanted wire to simulate a deep brain stimulation (DBS) electrode, was performed in (a) circular-polarized “quadrature” transmission mode to demonstrate localized RF heating effects, and (b) an optimized RF shim “suppression” mode to counteract RF heating at the tip of the implanted wire. Signal-to-noise ratio (SNR) and temperature results from the pTx experiment were compared with imaging using a Siemens BC head coil. The pTx platform provided effective control of RF amplitude and phase to minimize localized RF heating at the wire tip. No temperature elevation was detected for pTx in suppression mode, whereas maximum increases of 6.9 °C and 2.7 °C were observed for pTx quadrature mode and imaging using the BC head coil, respectively. The platform has been adapted to a Siemens MRI system without impacting the standard RF transmission pathway, while providing accurate RF signal modulation. Demonstration of the platform showed substantial reduction of localized RF heating in a simulated DBS implant without significant SNR loss.