Dynamic B0 shimming at 7 T

Dynamic slice-wise shimming improves B₀ field homogeneity by updating shim coil currents for every slice in a multislice acquisition, producing better field homogeneity over a volume than can be obtained by a single static global shim. The first aim of this work was to evaluate the performance of slice-wise field-map-based second-order dynamic shimming in a human high-field 7 T clinical scanner vis-à-vis image based second order static global shimming. Another goal was to characterize eddy currents induced by second and third order shim switching. A final aim was to compare global and dynamic shimming through shim orders to elucidate the relative benefits of going to higher orders and to dynamic shim updating from a static shimming regime. An external hardware module was used to store and dynamically update slice-optimized shim values during multislice data acquisition. High-bandwidth multislice gradient echo scans with B₀ field mapping and low-bandwidth single-shot echo planar scans were performed on phantoms and humans using second-order dynamic and static global shims. For the measurement of second and third order shim induced eddy currents, step response temporal phase changes of individual shims were measured and fit to shim harmonics spatially and to multiexponential decay functions temporally. Finally, an order-wise field-map-based comparison was performed with first, second and third order global static shimming, first and second order dynamic shimming, as well as combined second or third order global and first order dynamic shim. Dynamic shimming considerably improved B₀ homogeneity compared to static global shimming both in phantoms and in human subjects, reducing image distortion and signal dropout. The unshielded second and third order shims generated strong B₀ and self and cross-term eddy fields, with multiple time constants ranging from milliseconds to seconds. Field homogeneity improved with increasing order of shim, with dynamic shimming performing better than global shimming. Hybrid global and dynamic shimming approach yielded field homogeneity better than global static shims but worse than dynamic shims.     

基于边界元方法的超导核磁共振成像设备高阶轴向匀场线圈优化算法

在磁共振成像设备中,为了消除目标区域内的高阶谐波磁场分量,传统方法采用无源匀场,但该方法匀场精度较低,针对性较差,适用于全局匀场,而有源匀场则可以通过优化线圈分布来产生所需要的特定的磁场分布.但是,由于匀场线圈线型的复杂度会随着线圈阶数的增加而增加,难以满足设计需要,因此本文提出了一种用于磁共振成像超导匀场线圈系统的多变量非线性优化设计方法.该方法基于边界元方法,将匀场线圈所产生的磁场与目标磁场之间的偏差作为目标函数,线匝间距、线圈半径等作为约束条件,通过非线性优化算法,得到满足设计要求的线圈分布.通过一个中心磁场为0.5 T的开放式双平面磁共振成像超导轴向匀场线圈的设计案例,说明本方法具有计算效率高、灵活性好的特点.     

Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

The use of high magnetic fields increases the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS) of brain metabolites. Practical limitations arise, however, from susceptibility-induced field distortions, which are increased at higher magnetic field strengths. Solutions to this problem include optimized shimming, provided that active, i.e., electronic, shimming can operate over a sufficient range. To meet our shim requirements, which were an order of magnitude greater than the active shim capacity of our 7T MR system, we developed a combined passive and active shim approach. Simple geometries of ferromagnetic shim elements were derived and numerically optimized to generate a complete set of second-order spherical harmonic shim functions in a modular manner. The major goals of the shim design were maximization of shim field accuracy and ease of practical implementation. The theoretically optimized ferro-shim geometries were mounted on a cylindrical surface and placed inside the magnet bore, surrounding the subject's head and the RF coil. Passive shimming generated very strong shim fields and eliminated the worst of the field distortions, after which the field was further optimized by flexible and highly accurate active shimming. Here, the passive-shimming procedure was first evaluated theoretically, then applied in phantom studies and subsequently validated for in vivo 1H MRS in the macaque visual cortex. No artifacts due to the passive shim setup were observed; adjustments were reproducible between sessions. The modularity and the reduction to two pieces per shim term in this study is an important simplification that makes the method applicable also for passive shimming within single sessions. The feasibility of very strong, flexible and high-quality shimming via a combined approach of passive and active shimming is of great practical relevance for MR imaging and spectroscopy at high field strengths where shim power is limited or where shimming of specific anatomical regions inherently requires strong shim fields.     

Shim coil design for Halbach magnet by equivalent magnetic dipole method

Low-field nuclear magnetic resonance magnet(2 MHz) is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B_0 field only by using the passive shimming. Therefore, active shimming is necessarily used to further improve uniformity for Halbach magnet. In this work, an equivalent magnetic dipole method is presented for designing shim coils. The minimization of the coil power dissipation is considered as an optimal object to minimize coil heating effect, and the deviation from the target field is selected as a penalty function term. The lsqnonlin optimization toolbox of MATLAB is used to solve the optimization problem. Eight shim coils are obtained in accordance with the contour of the stream function. We simulate each shim coil by ANSYS Maxwell software to verify the validity of the designed coils. Measurement results of the field distribution of these coils are consistent with those of the target fields.The uniformity of the B_0 field is improved from 114.2 ppm to 26.9 ppm after using these shim coils.     

Dynamic multi-coil shimming of the human brain at 7 T

High quality magnetic field homogenization of the human brain (i.e. shimming) for MR imaging and spectroscopy is a demanding task. The susceptibility differences between air and tissue are a longstanding problem as they induce complex field distortions in the prefrontal cortex and the temporal lobes. To date, the theoretical gains of high field MR have only been realized partially in the human brain due to limited magnetic field homogeneity.A novel shimming technique for the human brain is presented that is based on the combination of non-orthogonal basis fields from 48 individual, circular coils. Custom-built amplifier electronics enabled the dynamic application of the multi-coil shim fields in a slice-specific fashion. Dynamic multi-coil (DMC) shimming is shown to eliminate most of the magnetic field inhomogeneity apparent in the human brain at 7 T and provided improved performance compared to state-of-the-art dynamic shim updating with zero through third order spherical harmonic functions. The novel technique paves the way for high field MR applications of the human brain for which excellent magnetic field homogeneity is a prerequisite.     

Computer simulation studies of the effects of dynamic shimming on susceptibility artifacts in EPI at high field

Dynamic shimming in multi-slice imaging aims to achieve optimal magnetic field homogeneity by updating the shim coil currents for each slice in real time. Dynamic shimming may reduce the signal loss and geometric distortion caused by magnetic susceptibility variations between tissues and is likely to be valuable for fast T2*-sensitive imaging techniques like EPI. A computer simulation of dynamic shimming using real image data has been developed to demonstrate the effectiveness of higher order dynamic shimming for echo planar imaging at high magnetic field, and to investigate the potential benefits of different orders of shim coil. Geometric distortions and signal intensities for different degrees of dynamic shimming were simulated and the results are compared with the images obtained with a conventional shimming technique. These results demonstrate the effectiveness, necessity and difficulty of high order dynamic shimming.     

Local in vivo shimming using adaptive passive shim positioning

A subject-specific local in vivo passive shimming method, focusing on the prefrontal and temporal regions, is proposed. The aim of the investigation is to show that subject variability exists in optimal passive shimming and that the proposed method can be effectively used to overcome these differences. A shimming structure capable of adjusting the position of the passive shims to within a millimeter resolution is built. The optimal shim positions for each individual subject are computed from obtained field map using a convex optimization algorithm. Passive shim experiments at predetermined fixed shim positions vs. individually adjusted shim positions were performed and compared. The results show that intersubject variability exists in the optimal shim positions and that the location-sensitive method proposed can be useful for improving main field homogeneity in vivo.     

High-order MR shimming: a simulation study of the effectiveness of competing methods, using an established susceptibility model of the human head

The first step in the process of shimming a magnetic field is to characterize it by obtaining a field map and decomposing that map into a convenient set of basis functions. The strength of each member of the set is then calculated. Finally, a set of correction elements which generate fields corresponding to the same spatial distribution as the basis functions is energized so that the sum of their fields and the error fields is substantially zero. The basis functions used typically are solutions to Laplace’s equation and have been shown to be very effective when the region of interest is substantially free space. This paper addresses issues associated with shimming the magnetic field in a region in which there is a distribution of materials with different susceptibilities and which therefore is not free space. In such a region, Laplace’s equation is no longer valid and in principle cannot be used to describe the magnetic field there. It is demonstrated that in spite of this, the same set of basis functions suffices for analyzing the field and the same set of elements suffices for correcting the field. The motivation for this study stems from the need to improve the magnetic field homogeneity when biological specimens are being imaged by magnetic resonance. In particular, this paper describes a study carried out by various simulated shimming strategies to improve the uniformity of the magnetic field over a multitissue model of susceptibility of the human head. The topics of magnetic susceptibility, the effect of shimming on MR images, shimming hardware and shimming methods are briefly reviewed. Two slices of the human head model were selected for detailed study, both offset inferior to the origin and including the base of the brain and the anterior sinus. The results of the study include comparisons between the strategies of global shimming, local slice-selective shimming and combinations of the two; the effects of shimming to various orders of spherical harmonics; and the effects of rotation and displacement of the head with respect to the shim frame of reference.     

An Optimized Passive Shimming Method for Bi-planar Permanent MRI Magnets

An optimized passive shimming method with iron shims is presented in this paper. First, the influence value of a single iron or magnetized shim is fast calculated and determined by analytic solution with a single practical measurement. Then, the correlation between the influence value and parameters of a single shim is analyzed, and the proper parameters, including the position, polarity, and size (radius and thickness), of the shimming pieces are well selected. Finally, the numbers and locations of the passive shims are optimized by mixed-integer linear programming method based on a modified central magnetic field. The optimized method is applied to a 0.5 T Bi-planar permanent magnet magnetic resonance imaging system, and the presented results prove the efficacy of this optimized passive shimming methodology.     

上海光源可变椭圆极化波荡器积分场垫补

详细介绍了上海光源波荡器EPU100积分场多极分量和一、二次积分的垫补及优化计算方法,其中的积分场多极分量采用“Magic Fingers”技术,即：在插入件两端部中平面上下使用一组适当的磁柱组合来抵消积分场误差;一、二次积分采用两端部的8个调补线圈垫补。通过垫补,EPU100在全磁隙,各种极化模式下四极分量小于9×10-3 T,六极分量小于1.6 T/m,八极分量小于30 T/m2,达到了设计指标。     

一种永磁磁共振中磁体涡流场的测量方法

磁共振成像（Magntic Resonance Imaging,MRI）技术是一种先进的医疗影像技术.在MRI系统中,通过梯度线圈电流快速切换方向,对待测区域施加梯度磁场,产生的梯度磁场会在其周围的金属体内激发出变化的涡旋电场,进而导致金属体内闭合的回路中产生对原来的梯度电流起抑制作用的感生电流,也就是我们所说的涡流.本文介绍了一种测量磁体涡流场的方法,结合电磁感应定律,设计了一种磁体涡流场测量装置,通过硬件采集以及软件处理的方法,将理想梯度场与实际磁场进行相减并将波形实时呈现,实验结果表明该方法可实现对磁体涡流场的测量.     

强流回旋加速器综合试验装置的磁场测量与垫补

强流回旋加速器综合试验装置的主体是一台紧凑型回旋加速器, 加速器主磁铁材料的不均匀性, 磁铁加工和安装的非理想因素将引起中心平面的磁场的非理想分布, 因此有必要对其中心平面的磁场进行测量和垫补. 本文主要讲述该综合试验装置的霍尔感应磁场测量系统的设计和使用;通过磁场测量数据分析进行镶条的再加工, 最终实现对等时性磁场和束流的纵向聚焦的垫补;研究与实践了一种对磁场一次谐波进行垫补的方法, 垫补的结果满足了设计的要求.     

Methodology of1H NMR spectroscopy of the human brain at very high magnetic fields

An ultrashort-echo-time stimulated echo-acquisition mode (STEAM) pulse sequence with interleaved outer volume suppression and VAPOR (variable power and optimized relaxation delays) water suppression was redesigned and optimized for human applications at 4 and 7 T, taking into account the specific requirements for spectroscopy at high magnetic fields and limitations of currently available hardware. In combination with automatic shimming, automated parameter adjustments and data processing, this method provided a user-friendly tool for routine¹H nuclear magnetic resonance (NMR) spectroscopy of the human brain at very high magnetic fields. Effects of first- and second-order shimming, single-scan averaging, frequency and phase corrections, and eddy currents were described. LCModel analysis of an in vivo¹H NMR spectrum measured from the human brain at 7 T allowed reliable quantification of more than fifteen metabolites noninvasively, illustrating the potential of high-field NMR spectroscopy. Examples of spectroscopic studies performed at 4 and 7 T demonstrated the high reproducibility of acquired spectra quality.     

Coherent cross-polarization theory for a spin-12 coupled to a general object

This work describes a segmented radial turbo-spin-echo technique (DW-rTSE) for high-resolution multislice diffusion-weighted imaging and quantitative ADC mapping. Diffusion-weighted images with an in-plane resolution of 700 microm and almost free of bulk motion can be obtained in vivo without cardiac gating. However, eddy currents and pulsatile brain motion cause severe artifacts when strong diffusion weighting is applied. This work explains in detail the artifacts in projection reconstruction (PR) imaging arising from eddy currents and describes an effective eddy current compensation based on the adjustment of gradient timing. Application of the diffusion gradients in all three orthogonal directions is possible without degradation of the images due to eddy current artifacts, allowing studies of the diffusional anisotropy. Finally, a self-navigation approach is proposed to reduce residual nonrigid body motion artifacts. Five healthy volunteers were examined to show the feasibility of this method.     

Practical aspects involved in the design and set up of a 0.15 T, 6-coil resistive magnet, whole body NMR imaging facility

Many technical and logistical questions must be addressed when planning the installation of an NMR imaging system. These considerations become particularly significant when the facility is being established within an existing medical center complex. This paper presents a report on the practical aspects and experience obtained in siting a 6-coil 0.15 T resistive magnet system. The topics discussed include: floor loading; ferromagnetic environment; the effect of iron on the magnet field strength and homogeneity characteristics; shimming procedures; temperature stability requirements; rf shielding; and effects of the magnetic field on common medical instrumentation and magnetic media. It was found that the field shift as a function of the distance of a steel mass from the center of the magnet exhibited an (1/r)^5.2±0.5 to (1/r)^4.2±0.3 dependence for axial and radial positions respectively which, as expected, is somewhat weaker than the (1/r)⁶ dependence expected by point dipole approximations. Field distortions caused by the presence of ferromagnetic material in radial positions may be essentially fully compensated with first order transverse shim coils (most conveniently, the x and y imaging gradient coils could be used). Axially distributed material requires, in addition to first order z-gradient correction, higher order axial shim compensation. The temperature stability of the magnet system over the scan period must be better than 0.2°C to insure that temperature-induced field fluctuations are less than the intrinsic static inhomogeneity: and, ideally, below 0.01°C to reduce these fluctuations to less than those caused by power supply instability.     

Projection-Reconstruction Spectroscopic Imaging forB0Field Plotting and Shimming without Pulsed Gradients

Shimming is important. Noniterative methods are desirable. Such methods exist for shimming a spectrometer with pulsed field gradients, generally based on field maps made by spin-warp Fourier imaging. For spectrometers with no pulsed gradients (or for cases whereT₂is too short to permit echo imaging), an alternative method is presented: projection-reconstruction spectroscopic imaging, which can be accomplished using only the shim coils of a conventional spectrometer. Images so acquired can be used to map the field, even in the presence of multiple spectral components. Noniterative optimization of the axial shims of a GN-300 spectrometer is demonstrated using 1D + 1D spectroscopic images. Prospects for extending the technique to include the radial shims using 3D + 1D spectroscopic images are discussed.     

Software and hardware integration of a microprogrammable state machine for NMR imaging.

We have integrated a commercially available microprogrammable state machine (Tecmag PULSkit) for use as a magnetic resonance pulse programmer. Providing the capability for active research environment imaging protocols, it features timing resolution of 100 nsec, ten 16-bit loop counters, and individually addressable look-up tables. This integration involved hardware and software integration with a VAX 11/750 at several levels. Hardware: Each of the three gradient channels employs three digital-to-analog converters (DACs). An 8-bit, 4-quadrant, multiplying DAC generates the gradient waveform shape. A 12-bit DAC generates the multiplying DAC scaling voltage, controlling gradient amplitude and sign. A third 12-bit DAC produces a gradient offset (shim) voltage. An eddy current compensation network is present for each gradient channel. Software: The software design philosophy was to create a flexible interface (interactive window environment), while not constraining complex manipulation of the hardware (direct use of the pulse-sequence compiler primitives and microprogramming). The software levels include (a) pulse-sequence microprogramming, (b) pulse-sequence compiler, (c) interactive parameter specification, and (d) canned pulse-sequence microcode library.     

Spectral resolution enhancement by chemical shift imaging

Three-dimensional chemical shift imaging (3D CSI) with appropriate data postprocessing can be used as a tool to improve spectral resolution in samples where large susceptibility differences and limited shim capabilities prevent good sample shimming. Data postprocessing is reduced to the realignment of individual 3D voxel spectra. As a result, the line broadening due to the field inhomogeneity over the sample's volume is reduced to the broadening by inhomogeneity within individual voxels. We compared this method with the resolution enhancement by window multiplication. We demonstrated, theoretically and experimentally, that in the presence of large, lower-order gradients, 3D CSI achieves better resolution enhancement with smaller sensitivity losses. An application of the method to a simple biological system is presented as well.     

2+1维U(1)改进格点规范理论中0++胶球波函数的三阶结果

用改进的格点规范场哈密顿量和截断本征方程法计算2+1维U(1)规范场的0++胶球波函数，三阶结果比二阶结果的标度行为有了较大的改善.     

Development of a temperature-variable magnetic resonance imaging system using a 1.0T yokeless permanent magnet

A temperature variable magnetic resonance imaging (MRI) system has been developed using a 1.0 T permanent magnet. A permanent magnet, gradient coils, radiofrequency coil, and shim coil were installed in a temperature variable thermostatic bath. First, the variation in the magnetic field inhomogeneity with temperature was measured. The inhomogeneity has a specific spatial symmetry, which scales linearly with temperature, and a single-channel shim coil was designed to compensate for the inhomogeneity. The inhomogeneity was drastically reduced by shimming over a wide range of temperature from −5 °C to 45 °C. MR images of an okra pod acquired at different temperatures demonstrated the high potential of the system for visualizing thermally sensitive properties.