Analysis of coil element distribution and dimension for matrix gradient coils

OBJECTIVE

The goal of this work is to analyze the influence of the distributions and dimensions of the coil elements and to present a method for improving the performance of the matrix gradient coil.

METHODS

Three typical models (five structures in total) are presented, and a double-layer biplanar matrix gradient coil is used to install coil elements. Two metrics, namely, the role of coil elements and mutual inductance between coil elements, are proposed to assess the performance of coil systems. An optimization approach to design matrix gradient coils is introduced based on analyzing the distributions and dimensions of coil elements. The flexibility of the magnetic field generation of the designed coil structure is demonstrated by generating full third-order spherical harmonic fields and different oblique gradient fields.

RESULTS

Matrix gradient coils with suitable distributions are capable of generating target magnetic fields. The role of coil elements quantitatively illustrates that the coil elements have different impacts on generating magnetic fields. Increasing the coil element dimension within a certain range can reduce the mutual inductance between coil elements and improve the performance of the coil system. The designed novel double-layer biplanar matrix gradient coil achieves an acceptable performance in generating different magnetic fields.

CONCLUSIONS

The proposed metrics can provide theoretical support for designing matrix gradient coils and evaluating their performance. The role of coil elements contributes to analyzing the distributions of coil elements to decrease the number of coil elements and power amplifiers. The mutual inductance between coil elements can be a reference for designing the dimensions of coil elements.

"Shimming a Plate" Technique to Correct the Coronal Malalignment in Metadiaphyseal Distal Femur Fracture: Case Report

We present a case of 19-year-old male patient with right sided comminuted metadiaphyseal distal femur fracture. Fixation was done using distal femur condylar locking plate. Washers were used with the screws to shimming the plate to achieve the coronal alignment and prevent stress over the screws. 2-year follow-up shows good alignment, radiological union and functional outcome without any complications. "Shimming" a locking plate with washers helps to achieve the better coronal alignment in comminuted metadiaphyseal distal femur fractures with coronal malalignment. It reduces the stress over screws and prevents implant-related complications.

NMR magnets for portable applications using 3D printed materials

In this paper, we introduce 3D printing as a possibility for realizing lightweight, yet high-precision NMR magnets. Using a commercially available filament containing steel particles allows for the realization of critical components of NMR magnets such as pole pieces and even the flux-conducting yoke. In contrast to shimming structures made of iron, 3D printed structures made of the lightweight filament allow for a robust and inexpensive way of realizing high-performance NMR magnets for future portable NMR applications. We demonstrate the versatility and achievable high performance of the proposed solution with two different H-shaped NMR magnets. In the first magnet, the 3D-printed filament is used to realize the yoke that guides the magnetic flux inside the magnet, providing the potential for a substantial weight reduction compared to a conventional iron yoke. In the second magnet, we use the 3D-printed material to realize arbitrarily shaped passive shim structures. Numerical size and shape optimizations using non-uniform rational basis splines (NURBS) have been applied to obtain the optimal geometry. The two manufactured magnets achieve measured NMR spectral line widths of 54 ppm and 250 ppm, respectively. Our results clearly demonstrate the efficiency and versatility of the proposed design and optimization approach.

Essential Practical Steps for MRI of the Kidney in Experimental Research

Magnetic resonance imaging (MRI) is an emerging method to obtain valuable functional and structural information about the kidney noninvasively. Before performing specialized MR measurements for probing tissue structure and function, some essential practical steps are needed, which are common for most applications. Here we describe in a step-by-step manner how to (1) achieve the double-oblique slice orientation coronal-to-the-kidney, (2) adapt the scan protocol for avoiding aortic flow artifacts and covering both kidneys, (3) perform localized shimming on the kidney, and (4) check perfusion in the large renal blood vessels using time-of-flight (TOF) angiography. The procedures are tailored to preclinical MRI but conceptionally are also applicable to human MRI.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This experimental protocol chapter explains the initial and essential MRI steps that precede specific functional and structural MR imaging techniques (T₁- and T₂*-mapping, DWI , ASL , etc.), which are described in separate chapters.

Achieving 1 ppm field homogeneity above 24 T: Application of differential mapping for shimming Keck and the Series Connected Hybrid magnets at the NHMFL

Powered resistive and resistive-superconductive hybrid magnets can reach fields higher than superconducting NMR magnets but lack the field homogeneity and temporal stability needed for high resolution NMR. Due to field fluctuations in powered magnets, commercially available mapping systems fail to produce maps of these magnets with sufficient reproducibility, thus hampering attempts to improve homogeneity of the field they generate. Starting with a commercial mapper, we built a mapping system which uses a two-channel (measurement + reference) mapper probe. We used this system to map and then to shim two magnets of Florida Bitter type at the National High Magnetic Field Laboratory in Tallahassee, FL. With a combination of passive (ferromagnetic) and active shims we achieved 2.3 ppm homogeneity in 1 cm diameter spherical volume (dsv) at 25.0 T in the Keck resistive magnet, and 0.9 ppm homogeneity in 1 cm dsv at 23.5, 28.2, and 35.2 T in the series-connected resistive-superconductive hybrid (SCH) magnet.

Simple and low-cost tabletop NMR system for chemical-shift-resolution spectra measurements

We have been working on developing a low-cost tabletop NMR system. We reported that a field homogeneity as high as 50 ppm was achieved with a simple NMR magnet by employing two facing ferrite magnets with iron disks in between (Chonlathep et al., 2017). In this paper, we report two improvements added to our previous system: (1) an FPGA based signal processing unit to improve the S/N ratio and (2) a simple shimming mechanism to improve the field homogeneity. We obtained as high as 1 ppm field homogeneity in the best case. The signals from hydrogen nuclear spins in a methyl and carboxy group in acetic acid were resolved in NMR spectra.

N,N-Diethylmethylamine as lineshape standard for NMR above 130 K

We demonstrate that N,N-Diethylmethylamine (DEMA) is a useful compound for shimming the magnetic field when doing NMR experiments at room temperature and 130 K, near the temperature used in many dynamic nuclear polarization (DNP) experiments. The resonance assigned to the N-methyl carbon in DEMA at 14.7 T and 140 K has a full-width-half-max linewidth of <4 Hz and has a spin-lattice relaxation time of 0.17 ± 0.03 s.

Magnetic Resonance Spectroscopy Studies of Mouse Models of Cancer

Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) enables the detection of metabolites, amino acids, and lipids, among other biomolecules, in tumors of live mouse models of cancer. Tumor-bearing mice are anesthetized by breathing isoflurane in a magnetic resonance (MR) scanner dedicated to small animal MR. Here we describe the overall setup and steps for measuring ¹H and ³¹P MRS and ¹H MRSI of orthotopic breast tumor models in mice with surface coils. This protocol can be adapted to the use of volume coils to measure ¹H and ³¹P MRS(I) of tumor models that grow inside the body. We address issues of animal handling, setting up the measurement, measurement options, and data analysis.

Shimming Halbach magnets utilizing genetic algorithms to profit from material imperfections

In recent years, permanent magnet-based NMR spectrometers have resurfaced as low-cost portable alternatives to superconducting instruments. While the development of these devices as well as clever shimming methods have yielded impressive advancements, scaling the size of these magnets to miniature lengths remains a problem to be addressed. Here we present the results of a study of a discrete shimming scheme for NMR Mandhalas constructed from a set of individual magnet blocks. While our calculations predict a modest reduction in field deviation by a factor of 9.3 in the case of the shimmed ideal Mandhala, a factor of 28 is obtained in the case of the shimmed imperfect Mandhala. This indicates that imperfections of magnet blocks can lead to improved field homogeneity. We also present a new algorithm to improve the homogeneity of a permanent magnet assembly. Strategies for future magnet construction can improve the agreement between simulation and practical implementation by using data from real magnets in these assemblies as the input to such an algorithm to optimize the homogeneity of a given design.

Still shimming or already measuring?--Quantitative reaction monitoring for small molecules on the sub minute timescale by NMR

In order to enable monitoring of rapidly occurring reactions Wagner et al. recently presented a simple scheme for 1D NMR experiments with continuous data acquisition, without inter-scan delays, using a spatially-selective and frequency-shifted excitation approach (Wagner et al., 2013). This scheme allows acquisition of proton spectra with temporal resolutions on the millisecond timescale. Such high temporal resolutions are desired in the case of reaction monitoring using stopped flow setups. In regular (1)H NMR-spectra without spatial selection the line width increases for a given shim setting with changes in sample volume, susceptibility, convection and temperature or concentration gradients due to the disturbance of magnetic field homogeneity. Concerning reaction monitoring this is unfortunate as shimming prior to acquisition becomes necessary to obtain narrow signals after injection of a reactant into an NMR sample. Even automatic shim routines may last up to minutes. Thus fast reactions can hardly be monitored online without large hardware dead times in a single stopped flow experiment. This problem is reduced in the spatially-selective and frequency-shifted continuous NMR experiment as magnetic field inhomogeneties are less pronounced and negative effects on the obtained line shapes are reduced as pointed out by Bax and Freeman (1980) [2] and demonstrated by Wagner et al. (2013). Here we present the utilization of this technique for observation of reactions in small molecule systems in which chemical conversion and longitudinal relaxation occur on the same timescale. By means of the alkaline ethyl acetate hydrolysis, a stoichiometric reaction, we show advantages of spatially-selective excitation on both temporal resolution and line shapes in stopped flow experiments. Results are compared to data obtained by non-selective small angle excitation experiments.

Dynamic multi-coil technique (DYNAMITE) shimming for echo-planar imaging of the human brain at 7 Tesla

Gradient-echo echo-planar imaging (EPI) is the primary method of choice in functional MRI and other methods relying on fast MRI to image brain activation and connectivity. However, the high susceptibility of EPI towards B0 magnetic field inhomogeneity poses serious challenges. Conventional magnetic field shimming with low-order spherical harmonic (SH) functions is capable of compensating shallow field distortions, but performs poorly for global brain shimming or on specific areas with strong susceptibility-induced B0 distortions such as the prefrontal cortex (PFC). Excellent B0 homogeneity has been demonstrated recently in the human brain at 7 Tesla with the DYNAmic Multi-coIl TEchnique (DYNAMITE) for magnetic field shimming (J Magn Reson (2011) 212:280-288). Here, we report the benefits of DYNAMITE shimming for multi-slice EPI and T2* mapping. A standard deviation of 13Hz was achieved for the residual B0 distribution in the human brain at 7 Tesla with DYNAMITE shimming and was 60% lower compared to conventional shimming that employs static zero through third order SH shapes. The residual field inhomogeneity with SH shimming led to an average 8mm shift at acquisition parameters commonly used for fMRI and was reduced to 1.5-3mm with DYNAMITE shimming. T2* values obtained from the prefrontal and temporal cortices with DYNAMITE shimming were 10-50% longer than those measured with SH shimming. The reduction of the confounding macroscopic B0 field gradients with DYNAMITE shimming thereby promises improved access to the relevant microscopic T2* effects. The combination of high spatial resolution and DYNAMITE shimming allows largely artifact-free EPI and T2* mapping throughout the brain, including prefrontal and temporal lobe areas. DYNAMITE shimming is expected to critically benefit a wide range of MRI applications that rely on excellent B0 magnetic field conditions including EPI-based fMRI to study various cognitive processes and assessing large-scale brain connectivity in vivo. As such, DYNAMITE shimming has the potential to replace conventional SH shim systems in human MR scanners.

Paramagnetic shimming for wide-range variable-field NMR

We propose a new passive shimming strategy for variable-field NMR experiments, in which the magnetic field produced by paramagnetic shim pieces placed inside the magnet bore compensates the inhomogeneity of a variable-field magnet for a wide range of magnet currents. Paramagnetic shimming is demonstrated in (7)Li, (87)Rb, and (45)Sc NMR of a liquid solution sample in magnetic fields of 3.4, 4.0, and 5.4T at a fixed carrier frequency of 56.0MHz. Since both the main-field inhomogeneity and the paramagnetic magnetization are proportional to the main-magnet current, the resonance lines are equally narrowed by the improved field homogeneity with an identical configuration of the paramagnetic shim pieces. Paramagnetic shimming presented in this work opens the possibility of high-resolution variable-field NMR experiments.

Proton magnetic resonance spectroscopy: technique for the neuroradiologist

Magnetic resonance spectroscopy (MRS) provides information on neuronal and axonal viability, energetics of cellular structures, and status of cellular membranes. Proton MRS appeals to clinicians and scientists because its application in the clinical setting can increase the specificity of MR imaging. The objective of this article is to provide descriptive concepts of the technique and its application in vivo for a variety of patient populations. When appropriately incorporating MRS into the neuroradiologic evaluation, this technique produces relevant information to radiologists and clinicians for their understanding of adult and pediatric neurologically based disease processes.