Shimming Effect Research Articles

An Improved Passive Shimming Design Method

Passive shimming technology can make the uniformity of DSV region of MRI system meet the imaging requirements. In this paper, the dichotomy method is introduced to improve the OTMF algorithm, so that it can quickly get the minimum shimming sheet laying scheme under the lowest magnetic field inhomogeneity, and greatly reduce the iteration time. Based on Improved OTMF algorithm by Dichotomy, this paper continues to study the influence of the number of shimming trays, the number of pockets in each shimming tray, the size of the shimming pieces, and the spacing between the shimming pieces on the shimming effect through DOE test design method and Monte Carlo simulation test method, and gives the best shimming design scheme for a 1.5T MRI superconducting magnet. This scheme has been proved to reduce the inhomogeneity of the magnetic field at 20cm DSV from 1199.2373ppm to 0.57ppm. The shimming design method introduced in this paper has certain guiding significance in exploring the factors affecting the passive shimming effect and shimming design.

Finite Element Simulation and Statistical Analysis of Laminated Composite Shims for Gaps in Aircraft Structures Assembly

Aerospace grade composite laminated shims are frequently used in aircraft structure assembly of parts made of advance composite materials with High Modulus of AS4 grade. The assembly of composite components requires extensive use of shims compared to metal assembly. The shim use at assembly stage is very time consuming process and poses challenges to maintain the product quality. This paper aims to bring out the statistical and finite element analysis of shims using gaps data taken out of prototype samples. Finite element analysis is carried out without shims and with shims of thickness of 0.8, 1.2 and 2.0 mm. The nature of variation of measured gap and the effect of shims on stress in the wing skin and spar due to static loading are assessed.

Development of Precise Magnetic Field Shimming Algorithm for Inclined 45° Continuous Cutting on Spiral Pole of SC Cyclotron

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.

Simulation and Analysis of Irregular Multicoil B0 Shimming in C-type Permanent Magnets Using Genetic Algorithm and Simulated Annealing

Multicoil (MC) B0 shimming, a new shimming technology developed in the recent years, exhibits unique advantages. Irregularly shaped coils can further improve the shimming effect of the MC, but they increase the difficulty and complexity of MC design. In this paper, the shimming characteristics of MC in C-type permanent magnets are studied. Aiming at improving the efficiency of finding the optimal shape for irregular MC, this paper presents a method that uses genetic algorithm and simulated annealing (GASA) to optimize irregular MC shapes for shimming. By simulating the shimming of the irregular MC of the flat-plate model and shoulder-joint model in C-type permanent magnets, the effects of parameters such as complexity, size, and number of groups of coils are analyzed. The shimming of MC in superconducting magnets and permanent magnets is compared and the optimization efficiencies of the genetic algorithms and genetic–annealing algorithms are compared. The results show that an irregular MC can achieve a better shimming effect for a locally nonuniform B0 field in a sample boundary region of a C-type permanent magnet. GASA is found to be a more stable and highly efficient optimization method.

Experimental and numerical analyses of the shimming effect on bolted joints with nonuniform gaps

In order to investigate the effect of shim compensation for nonuniform gaps in aircraft assembly, the influence of the shims with different material and parameters on bolted joints is studied in this paper. According to the real material and assembly conditions of the aircraft joint structures, the specimen and experiment are designed to obtain the tensile performance of the joint structures with different shims. A three-dimensional finite element model, which incorporates the Johnson–Cook material property of the alloys, traction-separation law of liquid shims, contact relationships between the joint elements, and boundary conditions of the tensile process, is established with the specimen configurations. After validating through comparing with the experimental results, the modeling method is adopted to simulate the tensile response of the bolted joints with shims. Furthermore, both the influence of the shim material and thickness on the mechanical behaviors of bolted joints is investigated in detail. Shims can considerably reduce the assembly stress of joint structures and improve the joint stiffness and load capacity, and this effect is more remarkable with the increase of gap values. Liquid shims improve the joint stiffness due to its cohesive ability, while solid shims improve the joint load capacity. Hybrid shims possess a composite shimming effect of liquid and solid shims. Whatever the shim material is applied, the joint stiffness and strength drop with the growth of shim thickness, so strict deviation control method should be taken to ensure the assembly gaps as small as possible. The research results enhance the knowledge of shimming effect on joint structures, and thus offer positive guidance for practical application in aircraft assembly.

Shimming effect on the mechanical behaviors of composite assembly structures of aircraft

The mechanical behaviors of composite assembly structures of aircraft might change due to filling the fit-up gap between two components with liquid shim or laminated shim. In order to investigate the change qualitatively and quantitatively, a kind of implicit nonlinear finite element models are developed, and mixed failure criteria considering the nonlinear shear relationship between composite matrix and fiber, out-of-plane stresses, and buckling of fiber are incorporated into finite element models. And tensile experimental research is also performed to validate the finite element models. Good agreement between the numerical and experimental results builds confidence that the finite element models are effective in predicting the shimming effect. And it is also observed that the mechanical behaviors of composite assembly structures with different filling present obvious differences.

Active shimming method for a 21.3 MHz small-animal MRI magnet

In a 21.3 MHz animal nuclear magnetic resonance (NMR) analyzer and imaging system, high uniformity of the static field B0 is a crucial prerequisite for obtaining high-quality images and detecting weak signals with short relaxation time. Using only passive shimming and gradient shimming, it is impossible to achieve perfect B0 uniformity. This paper presents an approach involving high-order active shimming for an NMR analyzer and an imaging magnet. The diameter spherical volume (DSV) of the magnet is only 60 mm. The spherical harmonic function is introduced to analyze the magnetic field and provide a foundation for active shimming. Eleven pairs of bi-planar active shim coils (X, Y, Z, XZ, YZ, Z2, X2 + Y2, X2 − Y2, Z3, Z4, Z5) were designed using the target field-and stream-function methods, and coils were manufactured with printed circuit board technology in this research. The magnetic field created by these electrified shim coils compensates effectively and accurately for the static magnetic field B0. The full width at half maximum of the resonance spectrum was used to measure the shimming effect. The results of practical shimming experiments in a 21.3 MHz animal NMR analyzer and imaging device showed that this new approach helps to reduce inhomogeneity from 24 ppm (24 × 10−6) to 0.39 ppm (0.39 × 10−6) over a 60 mm DSV, which meets the requirements of imaging and analytical practice.

Quantitative study of liver magnetic resonance spectroscopy quality at 3T using body and phased array coils with physical analysis and clinical evaluation.

This study aims to investigate the quality difference of short echo time (TE) breathhold 1H magnetic resonance spectroscopy (MRS) of the liver at 3.0T using the body and phased array coils, respectively. In total, 20 pairs of single-voxel proton spectra of the liver were acquired at 3.0T using the phased array and body coils as receivers. Consecutive stacks of breathhold spectra were acquired using the point resolved spectroscopy (PRESS) technique at a short TE of 30 ms and a repetition time (TR) of 1500 ms. The first spectroscopy sequence was “copied” for the second acquisition to ensure identical voxel positioning. The MRS prescan adjustments of shimming and water suppression, signal-to noise ratio (SNR), and major liver quantitative information were compared between paired spectra. Theoretical calculation of the SNR and homogeneity of the region of interest (ROI, 2 cm×2 cm×2 cm) using different coils loaded with 3D liver electromagnetic model of real human body was implemented in the theoretical analysis. The theoretical analysis showed that, inside the ROI, the SNR of the phase array coil was 2.8387 times larger than that of body coil and the homogeneity of the phase array coil and body coil was 80.10% and 93.86%, respectively. The experimental results showed excellent correlations between the paired data (all r > 0.86). Compared with the body coil group, the phased array group had slightly worse shimming effect and better SNR (all P values < .01). The discrepancy of the line width because of the different coils was approximately 0.8 Hz (0.00625 ppm). No significant differences of the major liver quantitative information of Cho/Lip2 height, Cho/Lip2 area, and lipid content were observed (all P values >0.05). The theoretical analysis and clinical experiment showed that the phased array coil was superior to the body coil with respect to 3.0T breathhold hepatic proton MRS.

Precise isochronous field shimming using correlation matrix for compact cyclotrons

An isochronous field shimming method using pre-calculated correlation matrix was applied to the magnet shaping of a 10MeV compact proton cyclotron. Measurement results demonstrate this method effective with high accuracy. Only after performing two iterative pole shimming procedures, ±4° for history phase slip was achieved. As well as the shimming effect from measurement has a good match to the predicted value. Detailed method and experimental procedures are described.

Development and application of a new method to shim first harmonic in compact cyclotron

In order to adequately eliminate 1st harmonic in the magnetic field of the compact cyclotron, CIAE has developed a 1st harmonic shimming method that can be applied to the four-sector compact cyclotron. The amplitude of the 1st harmonic can be reduced by adjusting the azimuthal width of the shimming bars at both sides of the sector in this method. Based on the beam dynamics requirement on the amplitude of 1st harmonic, the principle of 1st harmonic shimming method is illustrated through numerical analysis, and meanwhile, the shimming has been implemented on the magnet of CYCIAE-CRM by processing the azimuthal width using NC milling machining. The shimming result shows that it satisfies the requirement of beam dynamics. The shimming effect using this method is also analyzed and expounded in detail. The extension of this method is conducted so that it can be used to shim β θ at the median plane induced by the installation error of the upper and lower sector poles.

Exploration of factors influencing shimming and water suppression on hepatic 1 H-MR spectroscopy in vivo on 3.0 T

：Objective Tocharacterize the clinical factors which influence water suppression and auto-shimming linewidth for liver 3.0 T 1H-MRS.Methods Fifty-seven cases with liver 1H-MRspectroscopy(1H-MRS) were retrospectively studied, including chronic type B hepatitis(n=5), fatty liver (n=14), chronic type B hepatitis combining fatty liver (n=3) and normalsituation (n=35).Independent t test was used to characterize the difference of generalcondition (height, weight, body mass index etc.) between different water suppressioneffect groups and between different shimming effect groups.Using Chi-square test toanalyze whether water suppression rate and auto-shimming line width between fatty livergroups and non-fatty liver exist significance difference .Results By comparing WS ≥90%(n=47)group with WS < 90% (n=10) group, the former showed smaller average height[(164.7±8.0)and (170.5±3.7) cm respectively], weight[(61.8±10.4) and (73.4±5.4) kg respectively],BMI [(22.7±3.2) and (25.2±1.3)kg/m2 respectively]and LW[(17.7±3.7)and(24.6±6.3) Hzrespectively]than the latter (t=-3.488,-3.415,-4.002 and-3.327, P < 0.05).By comparingLW 20 Hz (n=41) group with LW 20 Hz (n=16) group, the former showed better watersuppression rate [(93.0±2.7)% and (86.1±8.5)% respectively]than the latter(t=3.213,P<0.05), whereas larger weight[(62.2±11.6) and (68.1±6.1)kg respectively]and BMI[(22.8±3.4)and (24.2±1.9)kg/m2 respectively](t=-2.516,-2.024,P <0.05).LW≤20 Hz in the fatty liver andnon-fatty liver group was 7/17 and 34/40, respectively (X2=11.347, P < 0.05).WS <90% in the fatty liver and non-fatty liver group was 10/17 and 0/44, respectively(X2=28.536, P < 0.05) .Conclusion Hepatic steatosis exerts an adverse effect in watersuppression and shimming. Key words: Liver; Fatty liver; Magnetic resonance spectroscopy

Mechanical Characteristics of Center Bevelled Double Structure Blade

This paper reports on deformation characteristics and mechanical features of a tapered bottom tip of a cutting blade subjected to pushing loads. Whereas a 16° high carbon steel bevel blade of 0.16 mm tip thickness was experimentally investigated, various tapered bottom tips were numerically analyzed for effect of tip thickness and apex angle using elasto-plastic Finite Element Method. From this research, the transition point at which a large increase of crushed height also known as the shimming effect starts has been revealed. Major findings of this research have been (i) tip thickness moves the transition point without changing cutting direction stiffness, and (ii) up to a certain range the apex angle controls the lateral deflection mode of the blade.

Electric field in cylinder condensers of finite length

The distribution of potential in a cylinder condenser is calculated by computer for various electrode structures. The results of the calculation show that the potential applied to the Matsuda plates, φ m, is very important if a good electric field is to be obtained. The effect of shims is also investigated and in this case it is also very important to choose a suitable value for φ m as well as the size of shims. Graphs are given to determine suitable dimensions of electrodes and the potential φ m.

The Construction and Operation of a Cyclotron to Produce One Million Volt Deuterons

A cyclotron for producing one million volt deuterons is described. The general magnetic field characteristics, and the effect of shims on the magnetic field have been examined. The production of ions for acceleration with and without filaments is discussed. The operation with a continuous glow discharge in the vacuum chamber is possible only because a transmission line is used to feed power to the dees. Various construction and operation details are given. The cyclotron has been found very reliable in operation and a maximum ion beam of 11 microamperes has been maintained for long periods of time. The beam has been removed in vacuum to a point about four feet from the vacuum chamber and partially focused. This allows bombardment work to be done with little interference from background radiation and high intensity fields.