Fig 8
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Portable low-cost magnetic resonance imaging (MRI) systems have the potential to enable “point-of-care” and timely MRI diagnosis, and to make this imaging modality available to routine scans and to people in underdeveloped countries and areas. With simplicity, no maintenance, no power consumption, and low cost, permanent magnets/magnet arrays/magne...
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... facilitate an implementation, the proposed design is segmented into 12 identical fan-shaped magnets with a uniform polarization. The 3D view of the magnet assembly is shown in Figure 18a. The segmented magnet assembly was simulated in COMSOL Multiphysics. ...
Similar publications
Low-field permanent magnet-based MRI systems are finding increasing use in portable, sustainable and point-of-care applications. In order to maximize performance while minimizing cost many components of such a system should ideally be designed specifically for low frequency operation. In this paper we describe recent developments in constructing an...
Citations
... Furthermore, the completed Halbach array includes a quadrupole layer comprising four ferromagnetic cubes, each with a side dimension of 40 mm, made of ferromagnetic steel and coated with nickel. This quadrupole layer is designed to manipulate magnetic field lines, concentrating and bending them to strengthen the magnetic field and minimize magnetic stray fields 29 (Fig. 3b). The focusers are simply an approach to design an anti-bend magnet based on a quadrupole 30 . ...
This study introduces a low-field NMR spectrometer (LF-NMR) featuring a multilayer Halbach magnet supported by a combined mechanical and electrical shimming system. This setup offers improved field homogeneity and sensitivity compared to spectrometers relying on typical Halbach and dipole magnets. The multilayer Halbach magnet was designed and assembled using three nested cylindrical magnets, with an additional inner Halbach layer that can be rotated for mechanical shimming. The coils and shim-kernel of the electrical shimming system were constructed and coated with layers of zirconia, thermal epoxy, and silver-paste resin to facilitate passive heat dissipation and ensure mechanical and thermal stability. Furthermore, the 7-channel shim coils were divided into two parts connected in parallel, resulting in a reduction of joule heating temperatures from 96.2 to 32.6 °C. Without the shimming system, the Halbach magnet exhibits a field inhomogeneity of approximately 140 ppm over the sample volume. The probehead was designed to incorporate a solenoidal mini coil, integrated into a single planar board. This design choice aimed to enhance sensitivity, minimize B1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{1}$$\end{document} inhomogeneity, and reduce impedance discrepancies, transmission loss, and signal reflections. Consequently, the resulting linewidth of water within a 3 mm length and 2.4 mm inner diameter sample volume was 4.5 Hz. To demonstrate the effectiveness of spectral editing in LF-NMR applications at 29.934 MHz, we selectively excited hydroxyl and/or methyl protons in neat acetic acid using optimal control pulses calculated through the Krotov algorithm.
... Accessibility can also be improved by designing these low-field systems such that they can be brought to the patient (point-of-care systems). Several recent reviews on the topic of low-field MRI have been published [4][5][6][7][8]. ...
Objective:
Low-cost low-field point-of-care MRI systems are used in many different applications. System design has correspondingly different requirements in terms of imaging field-of-view, spatial resolution and magnetic field strength. In this work an iterative framework has been created to design a cylindrical Halbach-based magnet along with integrated gradient and RF coils that most efficiently fulfil a set of user-specified imaging requirements.
Methods:
For efficient integration, target field methods are used for each of the main hardware components. These have not been used previously in magnet design, and a new mathematical model was derived accordingly. These methods result in a framework which can design an entire low-field MRI system within minutes using standard computing hardware.
Results:
Two distinct point-of-care systems are designed using the described framework, one for neuroimaging and the other for extremity imaging. Input parameters are taken from literature and the resulting systems are discussed in detail.
Discussion:
The framework allows the designer to optimize the different hardware components with respect to the desired imaging parameters taking into account the interdependencies between these components and thus give insight into the influence of the design choices.
... A PMA can be used to supply a homogeneous field, a gradient field, or a combination of the above in an MRI system. To generate a homogeneous field for MRI, in situ PMAs can be used [2]. Two main types of in situ PMAs are used to generate a homogeneous field for MRI. ...
... The other type is a cylindrical magnet array, such as a Halbach array [5] with dipolar transversal fields or an inward-outward (IO) ring pair with dipolar longitudinal fields [6][7][8][9], in which imaging is done in the bore. In the literature, comprehensive reviews on permanent magnets and PMAs for MRI are presented [2,10,11]. ...
In this paper, a simulator named "MagTetris" is proposed for fast magnetic field (B-field) and force calculation for permanent magnet arrays (PMAs) designs consisting of cuboid and arc-shaped magnets (approximated by cuboids) with arbitrary configurations. The proposed simulator can compute the B-field of a PMA on arbitrary observation planes and the magnetic force acting on any magnet/group of magnets. An accelerated calculation method for B-fields of PMAs is developed based on the current model of permanent magnet, which is further extended to magnetic force calculation. The proposed method and the associated codes were validated with numerical simulation and experimental results. The calculation speed of "MagTetris" is at least 500 times higher than that using finite-element method (FEM)-based software with uncompromised accuracy. Compared with a freeware in Python, Magpylib, "MagTetris" has a calculation acceleration of greater than 50% using the same language. "MagTetris" has a simple data structure, which can be easily migrated to other programming languages maintaining similar performances. This proposed simulator can accelerate a PMA design and/or allow designs with high flexibility considering the B-field and force simultaneously. It can facilitate and accelerate innovations of magnet designs to advance dedicated portable MRI in terms of compactness, weight, and performance.
... The Halbach cylinder was utilized for Nuclear Magnetic Resonance and Magnetic resonance imaging applications. The Halbach array, in which flux is directed on one end and can be zero on the other end, allows an MRI system to fit into a tiny area, like an ambulance, without de-metaling in the area beyond the imaging volume [15]. The MRI system has been developed using an optimized Halbach array of permanent rare-earth magnet made of neodymium of around 120 kg including the gradient coils and radio frequency (RF) coils. ...
... An IO ring-pair aggregate made up of two identical annular magnets, based on an Aubert ring pair, was suggested to supply a spatial encoding magnetic field for head imaging [15] [18]. Patrick McDaniel et.al deployed a 2D generalized projection imaging with a lightweight Halbach magnet designed for brain imaging, and then improved the magnetic field pattern for better rotational encoding [19]. ...
... Constructing an organ-specific imaging system either by permanent magnets or superconducting magnets will effectively minimize the size of the system [15]. Here, to build a compact scanner, the magnet and associated gear like gradient coils, radio frequency coils, and detecting coils must be constructed around the organ being scanned. ...
... This makes them ideal for measuring large or awkward shaped samples, as well as enabling them to be made extremely portable if necessary. At the moment, there are no suitable commercial single sided sensors, but there exist a variety of research one-sided MRI sensors that could be adapted to measure on seabirds (Huang et al., 2019;Wald et al., 2019). The results here support the development of specialized custom MRI sensors for the measurement of plastic in the digestive system of seabirds and other marine wildlife. ...
Monitoring plastic ingestion by marine wildlife is important for both characterizing the extent of plastic pollution in the environment and understanding its effect on species and ecosystems. Current methods to detect plastic in the digestive system of animals are slow and invasive, such that the number of animals that can be screened is limited. In this article, magnetic resonance imaging (MRI) is investigated as a possible technology to perform rapid, non-invasive detection of plastic ingestion. Standard MRI methods were able to directly measure one type of plastic in a fulmar stomach and another type was able to be indirectly detected. In addition to MRI, other standard nuclear magnetic resonance (NMR) measurements were made. Different types of plastic were tested, and distinctive NMR signal characteristics were found in common for each type, allowing them to be distinguished from one another. The NMR results indicate specialized MRI sequences could be used to directly image several types of plastic. Although current commercial MRI technology is not suitable for field use, existing single-sided MRI research systems could be adapted for use outside the laboratory and become an important tool for future monitoring of wild animals.
... A PMA can be used to supply a homogeneous field, a gradient field, or a combination of the above in an MRI system. To generate a homogeneous field for MRI, in situ PMAs can be used [2]. Two main types of in situ PMAs are used to generate a homogeneous field for MRI. ...
... The other type is a cylindrical magnet array, such as a Halbach array [5] with dipolar transversal fields or an inward-outward (IO) ring pair with dipolar longitudinal fields [6][7][8][9], in which imaging is done in the bore. In the literature, comprehensive reviews on permanent magnets and PMAs for MRI are presented [2,10,11]. ...
Lightweight and compact permanent magnet arrays (PMAs) are suitable for portable dedicated magnetic resonance imaging (MRI). It is worth exploring different PMA design possibilities and optimization methods with an adequate balance between weight, size, and performance, in addition to Halbach arrays and C-shaped/H-shaped magnets which are widely used. In this paper, the design and optimization of a sparse high-performance inward-outward ring-pair PMA consisting of magnet cuboids is presented for portable imaging of the brain. The design is lightweight (151 kg) and compact (inner bore diameter: 270 mm, outer diameter: 616 mm, length: 480 mm, 5-Gauss range: 1840 × 1840 × 2340 mm³). The optimization framework is based on the genetic algorithm with a consideration of both field properties and simulated image quality. The resulting PMA design has an average field strength of 101.5 mT and a field pattern with a built-in linear readout gradient. Subtracting the best fit to the linear gradient target resulted in a residual deviation from the target field of 0.76 mT and an average linear regression coefficient of 0.85 to the linear gradient. The required radiofrequency bandwidth is 6.9% within a field of view (FoV) with a diameter of 200 mm and a length of 125 mm. It has a magnetic field generation efficiency of 0.67 mT/kg, which is high among the sparse PMAs that were designed for an FoV with a diameter of 200 mm. The field can be used to supply gradients in one direction working with gradient coils in the other two directions, or can be rotated to encode signals for imaging with axial slice selection. The encoding capability of the designed PMA was examined through the simulated reconstructed images. The force experienced by each magnet in the design was calculated, and the feasibility of a physical implementation was confirmed. The design can offer an increased field strength, and thus, an increased signal-to-noise ratio. It has a longitudinal field direction that allows the application of technologies developed for solenoidal magnets. This proposed design can be a promising alternative to supplying the main and gradient fields in combination for dedicated portable MRI. Lastly, the design is resulted from a fast genetic algorithm-based optimization in which fast magnetic field calculation was applied and high design flexibility was feasible. Within optimization iterations, image quality metrics were used for the encoding field of a magnet configuration to guide the design of the magnet array.
... In bioelectromagnetic applications, current-fed electromagnets [12] or permanent magnets [13] could be preferred as static magnetic field sources. Coils require power supplies and, in some cases, coolers. ...
One of the challenges with magnetic fluid hyperthermia (MFH) is the limited control of magnetic nanoparticle (MNP) oscillations. To overcome this problem new approaches such as localization of MNP oscillations are being explored. In this study, we investigated the manipulation of field free region form by dual Halbach array displacements. We used finite element method simulation to examine gradient patterns in the workspace. Then, we created an experiment platform and took point probe measurements. As a result of the research, it was found that the field free region form can be manipulated by parametric distance changes of dual Halbach array. According to the findings, the field free region can expand and its shape can change from a point-like form to an ellipse-like surface by varying the distance between the arrays. The mapping of dual Halbach array generated gradient patterns for focused MFH was investigated for the first time in this study.
... 32,34,87 Modern cryocoolers employing Gifford-Mahon pulse tubes use direct conduction cooling, which allows for a dry or nearly dry system, reducing operational costs; the drawbacks are the need for regular maintenance and potential field disruption by mechanical vibrations. 34 Another key strategy to reduce MRI cost and footprint includes reducing the bore diameter and configuring RF coils only around the organ system under investigation 88 ; for instance, Panther et al.'s 89 design of a head-only, conduction-cooled, 0.5 T scanner weighing just over 1100 kg. ...
... Permanent magnets have minimal energy requirements and absolve the need for a cooling system. 88 Though traditional permanent magnet array set-ups are inherently heavy to maintain field homogeneity, recent work on Halbach arrays in neuroimaging has been shown to significantly reduce weight and lower costs. 32,90,91 These strategies may find use in cardiac imaging, though optimization of gradient performance is essential to sustain the high demands of [90][91][92] It has also been proven that point-of-care MRI scanners that utilize superconductive magnets are feasible clinical imaging solutions. ...
Cardiovascular disease continues to be a major burden facing healthcare systems worldwide. In the developed world, cardiovascular magnetic resonance (CMR) is a well-established non-invasive imaging modality in the diagnosis of cardiovascular disease. However, there is significant global inequality in availability and access to CMR due to its high cost, technical demands as well as existing disparities in healthcare and technical infrastructures across high-income and low-income countries. Recent renewed interest in low-field CMR has been spurred by the clinical need to provide sustainable imaging technology capable of yielding diagnosticquality images whilst also being tailored to the local populations and healthcare ecosystems. This review aims to evaluate the technical, practical and cost considerations of low field CMR whilst also exploring the key barriers to implementing sustainable MRI in both the developing and developed world.
... Moreover, Hömmen et al. [10] revealed that existing ultra low-eld MRI systems can produce a su cient signal-to-noise ratio (SNR) required for clinical imaging. Huang et al. [11] revealed that portable low-cost MRI systems can provide a point of care and timely MRI diagnosis especially to low-income countries where there are less than 0.1 MRI scans per 1,000,000 people [12] [13]. Several studies [8], [14] have indicated that low-eld MRI scanners have a low signal-to-noise ratio (SNR), resulting in noisy images. ...
... Hömmen et al. [10] also discovered that existing extreme low-eld MRI systems can generate the needed signal-to-noise ratio (SNR) for clinical imaging. According to Huang et al. [11], portable low-cost MRI equipment can provide a point of care and fast MRI diagnosis, especially in low-income countries where 0.1 MRI scans per 1,000,000 persons are common [12] [13]. Several studies [8], [14] have found that low-eld MRI scanners have a low signal-to-noise ratio (SNR), resulting in noisy images. ...
... Mbarara University of Science and Technology (MUST) in Uganda is working on a low-eld MRI system for hydrocephalus diagnosis with Leiden University Medical Center (LUMC) in the Netherlands, Pennsylvania State University (PSU) in the United States, and the Delft University of Technology (TU Delft) in the Netherlands (Figure 1shows the low eld MRI systems under development). Due to their low cost, portability, and compatibility with patients who have metallic implants, low-eld portable MRI scanners, as opposed to conventional MRI scanners based on superconducting magnets, may provide a supplementary medical imaging solution in a moving environment (e.g., the ambulance, the eld hospital), rural areas, or developing countries[11]. ...
Background: Magnetic Resonance Imaging (MRI) and spectroscopic techniques are frequently employed for clinical diagnostics as well as basic research in areas like cognitive neuroimaging. MRI is a widely used imaging modality for intracranial diseases. However, conventional MRI is expensive to purchase, maintain and sustain, limiting their use in low-income countries. Low field MRI can provide an economical, long-term, and safe imaging option to high-field MRI and computed tomography (CT) for brain imaging. This paper offers a review of the image reconstruction techniques used in low field magnetic resonance imaging (MRI). It is aimed at familiarizing the readers with the relevant knowledge, literature, and the latest updates on the state-of-art image reconstruction techniques that have been used in low field MRI citing their strengths, and areas for improvement.
Methods: An in-depth keyword-based search was undertaken for publications on image reconstruction approaches in low-field MRI in the top scientific databases such as Google Scholar, Wiley, Science Direct, Springer, IEEE, Scopus, Nature, Elsevier, and PubMed throughout this study. This research also contained relevant postgraduate theses. For the selection of relevant research publications, the PRISMA flow diagram and protocol were also used.
Results: Studies revealed that Inhomogeneities are present in low field MRI, implying that the traditional method of acquiring the image, using the inverse Fourier Transform, is no longer viable. The image reconstruction techniques reviewed include iterative methods, dictionary learning methods, and deep learning methods. Experimental results from the literature revealed improved image quality of the reconstructed images using data driven and learning based methods (deep learning and dictionary learning methods).
Conclusion: The study revealed that there is limited literature on the image reconstruction approaches in low field MRI even if though there are sufficient studies on the subject in high field MRI. Data driven and learning based methods improves image reconstruction quality when compared to analytic and iterative approaches.
... This can be more common with portable and compact MRI systems. 14,15 In the third case, permanently magnetized materials outside the bore may alter B 0 in the imaging region. If the main magnetic field is low, field perturbation perpendicular to B 0 becomes significant, 16,17 so all the components of �⃗ B obj should be considered. ...
... Here, the full �⃗ B app variation was again utilized but B obj was calculated by the KD(2,1) method, following Equation (14). A KD method operates on a single computational grid including both the source and the target. ...
Purpose
Most previous work on the calculation of susceptibility‐induced static magnetic field (B0) inhomogeneity has considered strictly unidirectional magnetic fields. Here, we present the theory and implementation of a computational method to rapidly calculate static magnetic field vectors produced by an arbitrary distribution of voxelated magnetization vectors.
Theory and Methods
Two existing B0 calculation methods were systematically extended to include arbitrary orientations of the magnetization and the magnetic field; they are (1) Fourier‐domain convolution with k‐space‐discretized (KD) dipolar field, and (2) generalized susceptibility voxel convolution (gSVC). The methods were tested on an analytical ellipsoid model and a tilted human head model, as well as against experimentally measured B0 fields induced by a stainless‐steel implant located in an inhomogeneous region of a clinical 3T MRI magnet.
Results
Both methods were capable of correctly calculating B0 fields inside a magnetized ellipsoid in all tested orientations. The KD method generally required a larger grid and longer computation time to achieve accuracy comparable to gSVC. Measured B0 fields due to the implant showed a good match with the gSVC‐calculated fields that accounted for the spatial variation of the applied magnetic field including the radial components.
Conclusion
Our method can provide a reliable and efficient computational tool to calculate B0 perturbation by magnetized objects under a variety of circumstances, including those with inhomogeneous magnetizing fields, anisotropic susceptibility, and a rotated coordinate system.
