Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm

Bringing MRI to low- and middle-income countries: Directions, challenges and potential solutions

The global disparity of magnetic resonance imaging (MRI) is a major challenge, with many low- and middle-income countries (LMICs) experiencing limited access to MRI. The reasons for limited access are technological, economic and social. With the advancement of MRI technology, we explore why these challenges still prevail, highlighting the importance of MRI as the epidemiology of disease changes in LMICs. In this paper, we establish a framework to develop MRI with these challenges in mind and discuss the different aspects of MRI development, including maximising image quality using cost-effective components, integrating local technology and infrastructure and implementing sustainable practices. We also highlight the current solutions-including teleradiology, artificial intelligence and doctor and patient education strategies-and how these might be further improved to achieve greater access to MRI.

A genetic optimisation and iterative reconstruction framework for sparse multi-dimensional diffusion-relaxation correlation MRI

Multi-dimensional diffusion-relaxation correlation (DRC) magnetic resonance imaging (MRI) techniques have recently been developed to investigate tissue microstructures. Sub-voxel tissue heterogeneity is resolved from the local correlation distributions of relaxation time and molecular diffusivity. However, the implementation of these techniques considerably increases the total acquisition time, and simply reducing the scan time may be at the expense of detailed structural resolution. To overcome these limitations, an optimised framework was proposed for acquiring microstructural maps of the human brain on a clinically feasible timescale. First, the acquisition parameters of the multi-dimensional DRC MRI method were sparsely optimised using a genetic algorithm with a fitness function according to the spectral resolution of the correlation map, hardware requirements, and total scan time. Next, the acquired DRC MRI data were processed using a proposed numerical algorithm based on the dynamic inverse Laplace transform (ILT). Prior knowledge from one-dimensional data was then utilised in the iterative procedure to improve the spectral resolution. Finally, the proposed framework was validated using Monte Carlo simulations and experimental data acquired from healthy participants on an MRI scanner. The results demonstrated that the suggested approach is feasible for offering high-resolution DRC maps that correspond to distinct microstructures with a limited amount of optimised acquisition data from two-dimensional DRC sampling space. By significantly reducing scan time while retaining structural resolution, this approach may enable multi-dimensional DRC MRI to be more widely used for quantitative evaluation in biological and medical settings.

Characterization of concomitant gradient fields and their effects on image distortions using a low-field point-of-care Halbach-based MRI system

PURPOSE

Concomitant gradient fields have been extensively studied at clinical field strengths. However, their effects have not yet been modeled for low-field point-of-care (POC) systems. The purpose of this work is to characterize the effects associated with concomitant fields for POC Halbach-array-based systems.

METHODS

The concomitant fields associated with a cylindrical gradient coils designed for a transverseB 0 $$ {B}_0 $$ and a signal model including the tilting effect of the effective magnetic field are derived. The formalism is used to simulate and predict concomitant field related distortions. A 46-mT Halbach-array-based system with a maximum gradient strength of 15 mT/m is used to verify the model using two-dimensional spin-echo sequences.

RESULTS

The simulations and experimental results are in good agreement with the derived equations. The fundamental characteristics of the concomitant field equations are different to conventional MRI systems: Image distortions occur primarily in the transverse directions and a cross-term only exists when applying transverse gradient pulses simultaneously.

CONCLUSION

The level of image warping in the frequency encoding direction is insignificant for the POC systems discussed here. However, when trying to achieve short echo-times by using strong phase encoding and readout-dephasing gradients, the combination can result in image warping and blurring which should be accounted for in image interpretation.

Low-field MRI: A report on the 2022 ISMRM workshop

In March 2022, the first ISMRM Workshop on Low-Field MRI was held virtually. The goals of this workshop were to discuss recent low field MRI technology including hardware and software developments, novel methodology, new contrast mechanisms, as well as the clinical translation and dissemination of these systems. The virtual Workshop was attended by 368 registrants from 24 countries, and included 34 invited talks, 100 abstract presentations, 2 panel discussions, and 2 live scanner demonstrations. Here, we report on the scientific content of the Workshop and identify the key themes that emerged. The subject matter of the Workshop reflected the ongoing developments of low-field MRI as an accessible imaging modality that may expand the usage of MRI through cost reduction, portability, and ease of installation. Many talks in this Workshop addressed the use of computational power, efficient acquisitions, and contemporary hardware to overcome the SNR limitations associated with low field strength. Participants discussed the selection of appropriate clinical applications that leverage the unique capabilities of low-field MRI within traditional radiology practices, other point-of-care settings, and the broader community. The notion of "image quality" versus "information content" was also discussed, as images from low-field portable systems that are purpose-built for clinical decision-making may not replicate the current standard of clinical imaging. Speakers also described technical challenges and infrastructure challenges related to portability and widespread dissemination, and speculated about future directions for the field to improve the technology and establish clinical value.

Current role of portable MRI in diagnosis of acute neurological conditions

Neuroimaging is an inevitable component of the assessment of neurological emergencies. Magnetic resonance imaging (MRI) is the preferred imaging modality for detecting neurological pathologies and provides higher sensitivity than other modalities. However, difficulties such as intra-hospital transport, long exam times, and availability in strict access-controlled suites limit its utility in emergency departments and intensive care units (ICUs). The evolution of novel imaging technologies over the past decades has led to the development of portable MRI (pMRI) machines that can be deployed at point-of-care. This article reviews pMRI technologies and their clinical implications in acute neurological conditions. Benefits of pMRI include timely and accurate detection of major acute neurological pathologies such as stroke and intracranial hemorrhage. Additionally, pMRI can be potentially used to monitor the progression of neurological complications by facilitating serial measurements at the bedside.

Brain imaging with portable low-field MRI

The advent of portable, low-field MRI (LF-MRI) heralds new opportunities in neuroimaging. Low power requirements and transportability have enabled scanning outside the controlled environment of a conventional MRI suite, enhancing access to neuroimaging for indications that are not well suited to existing technologies. Maximizing the information extracted from the reduced signal-to-noise ratio of LF-MRI is crucial to developing clinically useful diagnostic images. Progress in electromagnetic noise cancellation and machine learning reconstruction algorithms from sparse k-space data as well as new approaches to image enhancement have now enabled these advancements. Coupling technological innovation with bedside imaging creates new prospects in visualizing the healthy brain and detecting acute and chronic pathological changes. Ongoing development of hardware, improvements in pulse sequences and image reconstruction, and validation of clinical utility will continue to accelerate this field. As further innovation occurs, portable LF-MRI will facilitate the democratization of MRI and create new applications not previously feasible with conventional systems.

Concept for gradient-free MRI on twin natural slices

OBJECTIVE

The design of an MRI for use in space requires that the hardware be kept to an absolute minimum in terms of mass, complexity, and power. In addition, NASA requirements are that the external stray field needs to be less than 3.2 Gauss, 7 cm from the MRI enclosure.

THEORY

RF encoding designs with Halbach magnets offer the best chance of meeting those requirements. Spatially non-uniform magnetic fields with foliations of isomagnetic surfaces, or natural slices, may be used to provide slice selection, and to reduce further the hardware complexity, for TRansmit Array Spatial Encoding (TRASE) Magnetic Resonance Imaging (MRI) or potentially for other radio frequency (RF) encoding methods. The design of such non-uniform magnetic fields in a Halbach configuration with built-in axial gradients leads to pairs of isomagnetic surfaces centered on either side of a central maximum field strength slice. If TRASE images from slices other than the central isomagnetic surface are desired, then the Nuclear Magnetic Resonance (NMR) signals originating from the twin natural slices must be separated during image reconstruction. Here, a design for simultaneously imaging on twin slices in such an inhomogeneous magnetic field using multiple receiver coils with spatially varying RF profiles is described mathematically and numerical simulation examples are given.

DESIGN APPROACH

To achieve RF encoding on the natural slices, at least three TRASE transmit coils are required. Here a solution with twisted solenoid coils is given. To achieve the twin slice separation at least two receive coils are required. Here a solution with two solenoids is given.

DISCUSSION

The MRI design presented here uses a combination of RF encoding (TRASE), a spatial encoding magnetic field (SEM, pairs of natural slices) and receive coil spatial profiles to encode enough information into the NMR signal for image slice reconstruction. The design presented here enables using Halbach magnets with a built-in axial gradient to be used for MRI.

CONCLUSION

The result is a new gradient-free TRASE MRI design capable of imaging pairs of electronically selectable axial slices.

An integrated target field framework for point-of-care halbach array low-field MRI system design

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.

MagTetris: A simulator for fast magnetic field and force calculation for permanent magnet array designs

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.

Off-resonance artifact correction for MRI: A review

In magnetic resonance imaging (MRI), inhomogeneity in the main magnetic field used for imaging, referred to as off-resonance, can lead to image artifacts ranging from mild to severe depending on the application. Off-resonance artifacts, such as signal loss, geometric distortions, and blurring, can compromise the clinical and scientific utility of MR images. In this review, we describe sources of off-resonance in MRI, how off-resonance affects images, and strategies to prevent and correct for off-resonance. Given recent advances and the great potential of low-field and/or portable MRI, we also highlight the advantages and challenges of imaging at low field with respect to off-resonance.

A General-Purpose Distributed Analytic Platform Based on Edge Computing and Computational Intelligence Applied on Smart Grids

Currently, in many data landscapes, the information is distributed across various sources and presented in diverse formats. This fragmentation can pose a significant challenge to the efficient application of analytical methods. In this sense, distributed data mining is mainly based on clustering or classification techniques, which are easier to implement in distributed environments. However, the solution to some problems is based on the usage of mathematical equations or stochastic models, which are more difficult to implement in distributed environments. Usually, these types of problems need to centralize the required information, and then a modelling technique is applied. In some environments, this centralization may cause an overloading of the communication channels due to massive data transmission and may also cause privacy issues when sending sensitive data. To mitigate this problem, this paper describes a general-purpose distributed analytic platform based on edge computing for distributed networks. Through the distributed analytical engine (DAE), the calculation process of the expressions (that requires data from diverse sources) is decomposed and distributed between the existing nodes, and this allows sending partial results without exchanging the original information. In this way, the master node ultimately obtains the result of the expressions. The proposed solution is examined using three different computational intelligence algorithms, i.e., genetic algorithm, genetic algorithm with evolution control, and particle swarm optimization, to decompose the expression to be calculated and to distribute the calculation tasks between the existing nodes. This engine has been successfully applied in a case study focused on the calculation of key performance indicators of a smart grid, achieving a reduction in the number of communication messages by more than 91% compared to the traditional approach.

Superconducting magnet designs and MRI accessibility: A review

Presently, magnetic resonance imaging (MRI) magnets must deliver excellent magnetic field (B₀ ) uniformity to achieve optimum image quality. Long magnets can satisfy the homogeneity requirements but require considerable superconducting material. These designs result in large, heavy, and costly systems that aggravate as field strength increases. Furthermore, the tight temperature tolerance of niobium titanium magnets adds instability to the system and requires operation at liquid helium temperature. These issues are crucial factors in the disparity of MR density and field strength use across the globe. Low-income settings show reduced access to MRI, especially to high field strengths. This article summarizes the proposed modifications to MRI superconducting magnet design and their impact on accessibility, including compact, reduced liquid helium, and specialty systems. Reducing the amount of superconductor inevitably entails shrinking the magnet size, resulting in higher field inhomogeneity. This work also reviews the state-of-the-art imaging and reconstruction methods to overcome this issue. Finally, we summarize the current and future challenges and opportunities in the design of accessible MRI.

The permanent magnet hypothesis: an intuitive approach to designing non-circular magnet arrays with high field homogeneity

Does the Halbach magnetization rotation rule that is used for designing circular magnet arrays for achieving the best homogeneity hold also for an elliptical or other non-circular cross-section? In this article, it is shown that a new numerically optimized magnetization rotation rule can provide more than three orders of magnitude improvement in field homogeneity as compared to a Halbach configuration for elliptical systems. Further it is demonstrated that such optimized magnetization rules can be easily derived in an intuitive way by studying virtual permanent magnets of a similar cross-section as the desired magnet array. This is coined as a permanent magnet hypothesis. Finally, it is shown that the applicability of this technique is not limited to circular or elliptical systems but can be applied to any arbitrarily shaped cross-section.

A single-coil-based method for electromagnetic interference reduction in point-of-care low field MRI systems

One of the main challenges for point-of-care (POC) MRI systems is electromagnetic interference (EMI), since such systems are intended for use outside conventional Faraday-shielded rooms. Many methods have been proposed based on EMI detection via sensors external to the MRI system, followed by different types of signal processing to reduce artifacts in the image. Although these methods can be very effective, they do increase the complexity of the overall system, and introduce more potential failure points for systems designed for challenging environments. In this work we introduce a new method that does not require external sensors, but rather uses the "MR-silent" mode of an RF coil to detect the EMI, followed by simple subtraction from the signal from the "MR-active" mode. This method can be performed post-acquisition if there are two receive channels available, or as demonstrated here can operate with a single-channel receive detection system with the addition of a simple passive 180° power splitter/combiner into the receive chain. Proof-of-concept in vivo results show that a reduction in the standard deviation of the EMI up to ∼ 97 % is possible, with average values ∼ 90 %.

High-performance permanent magnet array design by a fast genetic algorithm (GA)-based optimization for low-field portable MRI

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 (151kg) and compact (inner bore diameter: 270mm, outer diameter: 616mm, length: 480mm, 5-Gauss range: 1840×1840×2340mm³). 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 0.76mT 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 200mm and a length of 125mm. It has a magnetic field generation efficiency of 0.67mT/kg, which is high among the sparse PMAs that were designed for an FoV with a diameter of 200mm. 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.

Emerging Techniques and Future Directions: Fast and Portable Magnetic Resonance Imaging

Fast MRI and portable MRI are emerging as promising technologies to improve the speed, efficiency, and availability of MR imaging. Fast MRI methods are increasingly being adopted to create screening protocols for the diagnosis and management of acute pathology in the emergency department. Faster imaging can facilitate timely diagnosis, reduce motion artifacts, and improve departmental MR operations. Point-of-care and portable MRI are emerging technologies that require radiologists to reenvision the role of MRI as a tool with greater accessibility, fewer siting constraints, and the ability to provide valuable diagnostic information at the bedside. Recently introduced commercially available pulse sequences and new MRI scanners are bringing these technologies closer to the patient's clinical setting, and we expect their use to only increase over the coming decade. This article provides an overview of these emerging technologies for emergency radiologists.

Additively Manufactured NdFeB Polyphenylene Sulfide Halbach Magnets to Generate Variable Magnetic Fields for Neutron Reflectometry

Halbach arrays are the most efficient closed structures for generating directed magnetic fields and gradients, and are widely used in various electric machines. We utilized fused deposition modeling-based Big Area Additive Manufacturing technology to print customized, compensated concentric Halbach array rings, using polyphenylene sulfide-bonded NdFeB permanent magnets for polarized neutron reflectometry. The Halbach rings could generate a 0 ≤ B ≤ 0.30 T field, while preserving 90% polarization of an axial neutron beam. Polarized neutron beams are used to study a wide range of structural and magnetic phenomena spanning physics, chemistry, and biology. In this study, we demonstrate the effectiveness of additive manufacturing for producing prototype Halbach arrays, characterize their magnetic properties, and generated magnetic fields, and discuss the conservation of neutron beam polarization as a function of magnetic field.

Portable magnetic resonance imaging of patients indoors, outdoors and at home

Mobile medical imaging devices are invaluable for clinical diagnostic purposes both in and outside healthcare institutions. Among the various imaging modalities, only a few are readily portable. Magnetic resonance imaging (MRI), the gold standard for numerous healthcare conditions, does not traditionally belong to this group. Recently, low-field MRI technology companies have demonstrated the first decisive steps towards portability within medical facilities and vehicles. However, these scanners' weight and dimensions are incompatible with more demanding use cases such as in remote and developing regions, sports facilities and events, medical and military camps, or home healthcare. Here we present in vivo images taken with a light, small footprint, low-field extremity MRI scanner outside the controlled environment provided by medical facilities. To demonstrate the true portability of the system and benchmark its performance in various relevant scenarios, we have acquired images of a volunteer's knee in: (i) an MRI physics laboratory; (ii) an office room; (iii) outside a campus building, connected to a nearby power outlet; (iv) in open air, powered from a small fuel-based generator; and (v) at the volunteer's home. All images have been acquired within clinically viable times, and signal-to-noise ratios and tissue contrast suffice for 2D and 3D reconstructions with diagnostic value. Furthermore, the volunteer carries a fixation metallic implant screwed to the femur, which leads to strong artifacts in standard clinical systems but appears sharp in our low-field acquisitions. Altogether, this work opens a path towards highly accessible MRI under circumstances previously unrealistic.

A passive shimming method for Halbach magnet based on magnetic sheet arrays

Halbach magnet has great potential in nuclear magnetic resonance device, where the field homogeneity requirement puts heavy demands on high efficiencypassive shimming (PS) technique. Conventional PS involves a tedious iteration process of the magnetic block/sheet number and positions optimization. In this paper, we propose a PS method based on magnetic sheet arrays (MSAs) targeting at spherical harmonic basis up to the 3rd order including dedicated composition of Y(4Z2-X2-Y2), Z3 and X(4Z2-X2-Y2) (n = 3, m = -1,0,1) with cross terms to implement structural field compensation. With this approach, the homogeneity of a 0.5 T Halbach magnet was improved from the original 811 ppm to 4.7 ppm in a L15 mm × R2.5 mm water sample. Rough shimming in another 0.93 T Halbach magnet also improved the homogeneity from 1103 ppm to 125 ppm in R2.5 mm sphere. This work provides a flexible, convenient PS method based on MSAs for compact Halbach magnet, which can be applied in NMR spectrometers and other high homogeneity application circumstances.

Sustainable low-field cardiovascular magnetic resonance in changing healthcare systems

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.

Design and experimental validation of a unilateral magnet for MRI-guided small animal radiation experiments

Small animal radiation experiments are of paramount importance for the advancement of human radiation therapy. These experiments use a dedicated radiation platform to deliver radiation to small animals, such as mice and rats, similar to how human radiation therapy is performed. By acquiring images immediately before radiation delivery to guide positioning of the animals, image guidance plays a critical role to ensure accuracy of the experiments. Recently, MR-based image guidance has been enabled in human radiation therapy. This paper proposes a new concept using a unilateral magnet-based MRI scanner to realize image guidance for small animal radiation experiments. We reported our design, optimization, construction, and characterization of the magnet. The magnet was designed using eight 2-inch neodymium magnet cubes arranged in a modified Halbach ring configuration. The ring has an opening to allow for animal positioning. We considered a spherical region of interest (ROI) located outside of the ring's plane to allow radiation delivery to the ROI without obstruction of the magnet. An optimization problem was formulated and solved to determine the positions and orientations of the magnet cubes to generate a magnetic field with desired properties in the ROI. The optimization improved the average magnetic flux density from 55 mT to 72 mT and reduced variation from 1.2 T/m to 1.0 T/m. We constructed the magnet using 3D-printed templates to hold the neodymium magnet cubes with the optimized positions and orientations. We measured the spatial distribution of the magnetic flux density. The measurement results and computed results agreed with an average difference of 0.35% through the ROI.

In vivo T1 and T2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

PURPOSE

Low-field (B₀ < 0.1 T) MRI has generated much interest as a means of increased accessibility via reduced cost and improved portability compared to conventional clinical systems (B₀ ≥ 1.5 Tesla). Here we measure MR relaxation times at 50 mT and compare results with commonly used models based on both in vivo and ex vivo measurements.

METHODS

Using 3D turbo spin echo readouts, T₁ and T₂ maps of the human brain and lower leg were acquired on a custom-built 50 mT MRI scanner using inversion-recovery and multi-echo-based sequences, respectively. Image segmentation was performed based on a histogram analysis of the relaxation times.

RESULTS

The average T₁ times of gray matter, white matter, and cerebrospinal fluid (CSF) were 327 ± 10 ms, 275 ± 5 ms, and 3695 ± 287 ms, respectively. Corresponding values of T₂ were 102 ± 6 ms, 102 ± 6 ms, and 1584 ± 124 ms. T₁ times in the calf muscle were measured to be 171 ± 11 ms and were 130 ± 5 ms in subcutaneous and bone marrow lipid. Corresponding T₂ times were 39 ± 2 ms in muscle and 90 ± 13 ms in lipid.

CONCLUSIONS

For tissues except for CSF, the measured T₁ times are much shorter than reported at higher fields and generally lie within the range of different models in the literature. As expected, T₂ times are similar to those seen at typical clinical field strengths. Analysis of the relaxation maps indicates that segmentation of white and gray matter based purely on T₁ or T₂ will be quite challenging at low field given the relatively small difference in relaxation times.

Emerging ethical issues raised by highly portable MRI research in remote and resource-limited international settings

Smaller, more affordable, and more portable MRI brain scanners offer exciting opportunities to address unmet research needs and long-standing health inequities in remote and resource-limited international settings. Field-based neuroimaging research in low- and middle-income countries (LMICs) can improve local capacity to conduct both structural and functional neuroscience studies, expand knowledge of brain injury and neuropsychiatric and neurodevelopmental disorders, and ultimately improve the timeliness and quality of clinical diagnosis and treatment around the globe. Facilitating MRI research in remote settings can also diversify reference databases in neuroscience, improve understanding of brain development and degeneration across the lifespan in diverse populations, and help to create reliable measurements of infant and child development. These deeper understandings can lead to new strategies for collaborating with communities to mitigate and hopefully overcome challenges that negatively impact brain development and quality of life. Despite the potential importance of research using highly portable MRI in remote and resource-limited settings, there is little analysis of the attendant ethical, legal, and social issues (ELSI). To begin addressing this gap, this paper presents findings from the first phase of an envisioned multi-staged and iterative approach for creating ethical and legal guidance in a complex global landscape. Section 1 provides a brief introduction to the emerging technology for field-based MRI research. Section 2 presents our methodology for generating plausible use cases for MRI research in remote and resource-limited settings and identifying associated ELSI issues. Section 3 analyzes core ELSI issues in designing and conducting field-based MRI research in remote, resource-limited settings and offers recommendations. We argue that a guiding principle for field-based MRI research in these contexts should be including local communities and research participants throughout the research process in order to create sustained local value. Section 4 presents a recommended path for the next phase of work that could further adapt these use cases, address ethical and legal issues, and co-develop guidance in partnership with local communities.

Magnetostatic reciprocity for MR magnet design

Electromagnetic reciprocity has long been a staple in magnetic resonance (MR) radio-frequency development, offering geometrical insights and a figure of merit for various resonator designs. In a similar manner, we use magnetostatic reciprocity to compute manufacturable solutions of complex magnet geometries, by establishing a quantitative metric for the placement and subsequent orientation of discrete pieces of permanent magnetic material. Based on magnetostatic theory and non-linear finite element modelling (FEM) simulations, it is shown how assembled permanent magnet setups perform in the embodiment of a variety of designs and how magnetostatic reciprocity is leveraged in the presence of difficulties associated with self-interactions, to fulfil various design objectives, including self-assembled micro-magnets, adjustable magnetic arrays, and an unbounded magnetic field intensity in a small volume, despite realistic saturation field strengths.

Characterization of displacement forces and image artifacts in the presence of passive medical implants in low-field (<100 mT) permanent magnet-based MRI systems, and comparisons with clinical MRI systems

PURPOSE

To investigate the displacement forces and image artifacts associated with passive medical implants for recently-developed low-field (<100 mT) MRI systems, and to compare these with values from higher field strengths used for clinical diagnosis.

METHODS

Setups were constructed to measure displacement forces in a permanent magnet-based Halbach array used for in vivo MRI at 50 mT, and results compared with measurements at 7 T. Image artifacts were assessed using turbo (fast) spin echo imaging sequences for four different passive medical implants: a septal occluder, iliac stent, pedicle screw and (ferromagnetic) endoscopic clip. Comparisons were made with artifacts produced at 1.5, 3 and 7 T. Finally, specific absorption rate (SAR) simulations were performed to determine under what operating conditions the limits might be approached at low-field.

RESULTS

Displacement forces at 50 mT on all but the ferromagnetic implant were between 1 and 10 mN. Image artifacts at 50 mT were much less than at clinical field strengths for all passive devices, and with the exception of the ferromagnetic clip. SAR simulations show that very long echo train (>128) turbo spin echo sequences can be run with short inter-pulse times (5-10 ms) within SAR limits.

CONCLUSIONS

This work presents the first evaluation of the effects of passive implants at field strengths less than 100 mT in terms of displacement forces, image artifacts and SAR. The results support previous claims that such systems can be used safely and usefully in challenging enviroments such as the intensive care unit.

Medical imaging and nuclear medicine: a Lancet Oncology Commission

The diagnosis and treatment of patients with cancer requires access to imaging to ensure accurate management decisions and optimal outcomes. Our global assessment of imaging and nuclear medicine resources identified substantial shortages in equipment and workforce, particularly in low-income and middle-income countries (LMICs). A microsimulation model of 11 cancers showed that the scale-up of imaging would avert 3·2% (2·46 million) of all 76·0 million deaths caused by the modelled cancers worldwide between 2020 and 2030, saving 54·92 million life-years. A comprehensive scale-up of imaging, treatment, and care quality would avert 9·55 million (12·5%) of all cancer deaths caused by the modelled cancers worldwide, saving 232·30 million life-years. Scale-up of imaging would cost US$6·84 billion in 2020-30 but yield lifetime productivity gains of $1·23 trillion worldwide, a net return of $179·19 per $1 invested. Combining the scale-up of imaging, treatment, and quality of care would provide a net benefit of $2·66 trillion and a net return of $12·43 per $1 invested. With the use of a conservative approach regarding human capital, the scale-up of imaging alone would provide a net benefit of $209·46 billion and net return of $31·61 per $1 invested. With comprehensive scale-up, the worldwide net benefit using the human capital approach is $340·42 billion and the return per dollar invested is $2·46. These improved health and economic outcomes hold true across all geographical regions. We propose actions and investments that would enhance access to imaging equipment, workforce capacity, digital technology, radiopharmaceuticals, and research and training programmes in LMICs, to produce massive health and economic benefits and reduce the burden of cancer globally.

Dual polarity encoded MRI using high bandwidth radiofrequency pulses for robust imaging with large field inhomogeneity

PURPOSE

The ability to use dual polarity encoded MRI with the missing pulse steady-state free precession (MP-SSFP) sequence is demonstrated to perform robust MRI with low radiofrequency (RF) amplitude, where the field is distorted by embedding metallic screws in an agar phantom. Image-based estimation of the 3D ΔB₀ map and image distortion correction is shown to require ~1 minute to perform.

THEORY AND METHODS

Dual polarity encoded MP-SSFP was implemented at 1.5T and used to image agar phantoms with one stainless steel and one titanium screw embedded inside. A multispectral fast spin-echo acquisition was performed for comparison. Self-consistent ΔB₀ estimation is performed iteratively using a 3D B-spline basis, which is compared to the ΔB₀ estimate generated by the multispectral sequence.

RESULTS

Dual polarity encoded MP-SSFP yields image quality similar to the multispectral sequence used with substantially less imaging time, provided the MP-SSFP experimental parameters are chosen well. The multispectral sequence appears to visualize modestly closer in proximity to the metallic screws used, despite the spectral bins covering the same bandwidth as the pulses used in MP-SSFP. However, MP-SSFP avoids ripple artifacts characteristic of the multispectral sequence. The ΔB₀ estimate generated by MP-SSFP is qualitatively similar to that generated by the multispectral sequence but larger in magnitude.

CONCLUSION

Despite longer processing time compared to multispectral imaging, MP-SSFP yields similar image quality with significantly lower acquisition times in the absence of parallel imaging. The work herein demonstrates the ability to perform 3D ΔB₀ estimation and image correction within a reasonable amount of time, ~1 minute.

Improving the field homogeneity of fixed- and variable-diameter discrete Halbach magnet arrays for MRI via optimization of the angular magnetization distribution

The aim of this work was to maximize the homogeneity of fixed- or variable-diameter Halbach array of discrete magnets by optimizing the angular rotation of individual magnets within each ring of the array. Numerical simulations have been performed for magnet arrays with various length:radius ratios (L/R) using a dipole-approximation model. These simulations used an uninformed random-search algorithm, with the initial state corresponding to the classical Halbach dipole configuration. Two different classes of systems were studied, one with magnet rings of constant radius, and the other in which the radius of the rings was allowed to vary to increase the homogeneity. Simulation results showed that for a fixed-diameter array optimization of the angular orientation of individual magnets increased the homogeneity by ~17% for very short magnets, with the improvement dropping to ~5% for L/R values greater than ~3:1, where the homogeneity was measured over a region-of-interest equal to one-half the diameter of the magnet array. An empirical formula was derived which allows easy estimation of the required magnetization angles for any L/R. For a 23-ring variable diameter magnet with L/R of ~4:1 the optimization procedure produces an increase in homogeneity of ~18%.

Image distortion correction for MRI in low field permanent magnet systems with strong B0 inhomogeneity and gradient field nonlinearities

Objective

To correct for image distortions produced by standard Fourier reconstruction techniques on low field permanent magnet MRI systems with strong \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document}B0 inhomogeneity and gradient field nonlinearities.

Materials and methods

Conventional image distortion correction algorithms require accurate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Delta B}_{0}$$\end{document}ΔB0 maps which are not possible to acquire directly when the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document}B0 inhomogeneities also produce significant image distortions. Here we use a readout gradient time-shift in a TSE sequence to encode the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document}B0 field inhomogeneities in the k-space signals. Using a non-shifted and a shifted acquisition as input, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {B}_{0}$$\end{document}ΔB0 maps and images were reconstructed in an iterative manner. In each iteration, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {B}_{0}$$\end{document}ΔB0 maps were reconstructed from the phase difference using Tikhonov regularization, while images were reconstructed using either conjugate phase reconstruction (CPR) or model-based (MB) image reconstruction, taking the reconstructed field map into account. MB reconstructions were, furthermore, combined with compressed sensing (CS) to show the flexibility of this approach towards undersampling. These methods were compared to the standard fast Fourier transform (FFT) image reconstruction approach in simulations and measurements. Distortions due to gradient nonlinearities were corrected in CPR and MB using simulated gradient maps.

Results

Simulation results show that for moderate field inhomogeneities and gradient nonlinearities, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {B}_{0}$$\end{document}ΔB0 maps and images reconstructed using iterative CPR result in comparable quality to that for iterative MB reconstructions. However, for stronger inhomogeneities, iterative MB reconstruction outperforms iterative CPR in terms of signal intensity correction. Combining MB with CS, similar image and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {B}_{0}$$\end{document}ΔB0 map quality can be obtained without a scan time penalty. These findings were confirmed by experimental results.

Discussion

In case of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${B}_{0}$$\end{document}B0 inhomogeneities in the order of kHz, iterative MB reconstructions can help to improve both image quality and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {B}_{0}$$\end{document}ΔB0 map estimation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-021-00907-2.

NMR spectroscopy goes mobile: Using NMR as process analytical technology at the fume hood

Nuclear magnetic resonance (NMR) is potentially a very powerful process analytical technology (PAT) tool as it gives an atomic resolution picture of the reaction mixture without the need for chromatography. NMR is well suited for interrogating transient intermediates, providing kinetic information via NMR active nuclei, and most importantly provides universally quantitative information for all species in solution. This contrasts with commonly used PAT instruments, such as Raman or Flow-infrared (IR), which requires a separate calibration curve for every component of the reaction mixture. To date, the large footprint of high-field (≥400 MHz) NMR spectrometers and the immobility of superconducting magnets, coupled with strict requirements for the architecture for the room it is to be installed, have been a major obstacle to using this technology right next to fume hoods where chemists perform synthetic work. Here, we describe the use of a small, lightweight 60 MHz Benchtop NMR system (Nanalysis Pro-60) located on a mobile platform, that was used to monitor both small and intermediate scale Grignard formation and coupling reactions. We also show how low field NMR can provide a deceptively simple yes/no answer (for a system that would otherwise require laborious off-line testing) in the enrichment of one component versus another in a kilogram scale distillation. Benchtop NMR was also used to derive molecule specific information from Flow-IR, a technology found in most manufacturing sites, and compare the ease at which the concentrations of the reaction mixtures can be derived by NMR versus IR.

A portable scanner for magnetic resonance imaging of the brain

Access to scanners for magnetic resonance imaging (MRI) is typically limited by cost and by infrastructure requirements. Here, we report the design and testing of a portable prototype scanner for brain MRI that uses a compact and lightweight permanent rare-earth magnet with a built-in readout field gradient. The 122-kg low-field (80 mT) magnet has a Halbach cylinder design that results in a minimal stray field and requires neither cryogenics nor external power. The built-in magnetic field gradient reduces the reliance on high-power gradient drivers, lowering the overall requirements for power and cooling, and reducing acoustic noise. Imperfections in the encoding fields are mitigated with a generalized iterative image reconstruction technique that leverages previous characterization of the field patterns. In healthy adult volunteers, the scanner can generate T1-weighted, T2-weighted and proton density-weighted brain images with a spatial resolution of 2.2 × 1.3 × 6.8 mm³. Future versions of the scanner could improve the accessibility of brain MRI at the point of care, particularly for critically ill patients.

Proton nuclear magnetic resonance J-spectroscopy of phantoms containing brain metabolites on a portable 0.05 T MRI scanner

We examined approaches for obtaining ¹H NMR spectra of brain metabolites on a low-field (B₀ = 0.05 T) portable MRI scanner, which was developed in our laboratory with the aim of bringing cost-effective radiological services to populations in underserved, remote regions. The low static magnetic field B₀ dictates low signal to noise ratio for metabolites in the mM concentration range, and results in an overall spectral region for the ¹H resonances of these metabolites narrower than the linewidth obtainable in our scanner. The narrow spectral range also precludes the possibility of suppressing the large contribution of the water resonance at the acquisition stage. We used a spectroscopic Carr-Purcell-Meiboom-Gill (CPMG) sequence to acquire multiecho data from solutions of J-coupled brain metabolites, focusing on lactic acid, a metabolite whose concentration is negligible in the healthy brain and increases significantly in several disease conditions. The J spectra we obtained for lactate from the Fourier transformation of the multiecho data are spectrally well-resolved for a range of echo spacing values. We show that the J spectra at different echo spacings fit well with simulations of the evolution of echo train signal of the lactate under the same conditions. Applying a J-refocused variant of the CPMG sequence, the J modulation of the echo decay is removed, providing a way for subtracting the large contribution of the non-modulated component in the J spectrum in conditions where notching it using post-processing methods is impossible. We also demonstrate by means of experimental data and simulations that in our experimental conditions, J-spectra of other prominent brain metabolites, such as the neurotransmitter glutamate, do not yield discernible peaks and only contribute to a broad peak at zero frequency.

Contemporary approaches to high-field magnetic resonance imaging with large field inhomogeneity

Despite its importance as a clinical imaging modality, magnetic resonance imaging remains inaccessible to most of the world's population due to its high cost and infrastructure requirements. Substantial effort is underway to develop portable, low-cost systems able to address MRI access inequality and to enable new uses of MRI such as bedside imaging. A key barrier to development of portable MRI systems is increased magnetic field inhomogeneity when using small polarizing magnets, which degrades image quality through distortions and signal dropout. Many approaches address field inhomogeneity by using a low polarizing field, approximately ten to hundreds of milli-Tesla. At low-field, even a large relative field inhomogeneity of several thousand parts-per-million (ppm) results in resonance frequency dispersion of only 1-2 kHz. Under these conditions, with necessarily wide pulse bandwidths, fast spin-echo sequences may be used at low field with negligible subject heating, and a broad range of other available imaging sequences can be implemented. However, high-field MRI, 1.5 T or greater, can provide substantially improved signal-to-noise ratio and image contrast, so that higher spatial resolution, clinical quality images may be acquired in significantly less time than is necessary at low-field. The challenge posed by small, high-field systems is that the relative field inhomogeneity, still thousands of ppm, becomes tens of kilohertz over the imaging volume. This article describes the physical consequences of field inhomogeneity on established gradient- and spin-echo MRI sequences, and suggests ways to reduce signal dropout and image distortion from field inhomogeneity. Finally, the practicality of currently available image contrasts is reviewed when imaging with a high magnetic field with large inhomogeneity.

Accounting for material imperfections in the design and optimization of low cost Halbach magnets

We demonstrate an experimental method for the improvement of the magnetic field homogeneity in Halbach magnets by taking magnet material imperfection into account. This method relies on the determination of the magnetization magnitude only for individual magnet blocks based on nuclear magnetic resonance field measurements in a simplified system, which, in our case, consists of four blocks. Then, a set of configurations with highest homogeneities can be found from simplified field map simulations of all possible configurations or by applying sophisticated optimum search algorithms if the number of blocks is large. Finally, the residual effect of angular magnetization deviations can be reduced by the experimental selection of the best configuration from the set found on the simulation step. This selection strategy is based on the conclusions made from statistical analysis of simulated field maps. By applying the described method to our eight-element magnet, we experimentally achieved tenfold field homogeneity improvement. Thus, in the best configuration, we obtained an average value of the magnetic field of 598.0 mT and a half-width of 226.9 ppm for a sample with a diameter of 4 mm and a height of 10 mm. These parameters along with the compact magnet size (40 × 40 × 102 mm³) and weight (0.6 kg) provide reasonable magnet quality compared with analogous systems having more complex magnet arrangements and significantly higher costs.

Low-cost and portable MRI

Research in MRI technology has traditionally expanded diagnostic benefit by developing acquisition techniques and instrumentation to enable MRI scanners to "see more." This typically focuses on improving MRI's sensitivity and spatiotemporal resolution, or expanding its range of biological contrasts and targets. In complement to the clear benefits achieved in this direction, extending the reach of MRI by reducing its cost, siting, and operational burdens also directly benefits healthcare by increasing the number of patients with access to MRI examinations and tilting its cost-benefit equation to allow more frequent and varied use. The introduction of low-cost, and/or truly portable scanners, could also enable new point-of-care and monitoring applications not feasible for today's scanners in centralized settings. While cost and accessibility have always been considered, we have seen tremendous advances in the speed and spatial-temporal capabilities of general-purpose MRI scanners and quantum leaps in patient comfort (such as magnet length and bore diameter), but only modest success in the reduction of cost and siting constraints. The introduction of specialty scanners (eg, extremity, brain-only, or breast-only scanners) have not been commercially successful enough to tilt the balance away from the prevailing model: a general-purpose scanner in a centralized healthcare location. Portable MRI scanners equivalent to their counterparts in ultrasound or even computed tomography have not emerged and MR monitoring devices exist only in research laboratories. Nonetheless, recent advances in hardware and computational technology as well as burgeoning markets for MRI in the developing world has created a resurgence of interest in the topic of low-cost and accessible MRI. This review examines the technical forces and trade-offs that might facilitate a large step forward in the push to "jail-break" MRI from its centralized location in healthcare and allow it to reach larger patient populations and achieve new uses. Level of Evidence: 5 Technical Efficacy Stage: 6 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2020;52:686-696.

Selective magnetic resonance signal suppression by colored Frank excitation

Inspired by the growing interest in miniaturized NMR devices and their applications in material science as well as in chemical and biological research, low power rf excitation is explored. ¹H NMR spectra have been measured with low power Frank excitation and are compared to spectra obtained by single-pulse excitation. Frank excitation consists of a large number of phase-modulated, constant-amplitude rf-pulses. A Frank sequence is divided into packages of discrete phase wavelets that correspond to a scan across a spectral frequency range. The largely coherent excitation is found experimentally to require less power than white noise excitation. The package structure suggests that individual wavelets can be omitted to skip individual frequency regions in the excitation, converting the white Frank excitation into colored Frank excitation. This work explores different approaches of colored, selective Frank excitation for spectroscopy and imaging. It is motivated by the aim to eliminate the rf amplifier from the NMR spectrometer so as to enable further miniaturization of NMR instruments. Colored Frank excitation bears promise as a low-power modality for solvent signal suppression in spectroscopy and motion tagging in magnetic resonance imaging.

In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array

Purpose

To design a low‐cost, portable permanent magnet‐based MRI system capable of obtaining in vivo MR images within a reasonable scan time.

Methods

A discretized Halbach permanent magnet array with a clear bore diameter of 27 cm was designed for operation at 50 mT. Custom‐built gradient coils, RF coil, gradient amplifiers, and RF amplifier were integrated and tested on both phantoms and in vivo.

Results

Phantom results showed that the gradient nonlinearity in the y‐direction and z‐direction was less than 5% over a 15‐cm FOV and did not need correcting. For the x‐direction, it was significantly greater, but could be partially corrected in postprocessing. Three‐dimensional in vivo scans of the brain of a healthy volunteer using a turbo spin‐echo sequence were acquired at a spatial resolution of 4 × 4 × 4 mm in a time of about 2 minutes. The T₁‐weighted and T₂‐weighted scans showed a good degree of tissue contrast. In addition, in vivo scans of the knee of a healthy volunteer were acquired at a spatial resolution of about 3 × 2 × 2 mm within 12 minutes to show the applicability of the system to extremity imaging.

Conclusion

This work has shown that it is possible to construct a low‐field MRI unit with hardware components costing less than 10 000 Euros, which is able to acquire human images in vivo within a reasonable data‐acquisition time. The system has a high degree of portability with magnet weight of approximately 75 kg, gradient and RF amplifiers each 15 kg, gradient coils 10 kg, and spectrometer 5 kg.

Controlling two-dimensional collective formation and cooperative behavior of magnetic microrobot swarms

Magnetically actuated mobile microrobots can access distant, enclosed, and small spaces, such as inside microfluidic channels and the human body, making them appealing for minimally invasive tasks. Despite their simplicity when scaling down, creating collective microrobots that can work closely and cooperatively, as well as reconfigure their formations for different tasks, would significantly enhance their capabilities such as manipulation of objects. However, a challenge of realizing such cooperative magnetic microrobots is to program and reconfigure their formations and collective motions with under-actuated control signals. This article presents a method of controlling 2D static and time-varying formations among collective self-repelling ferromagnetic microrobots (100 [Formula: see text]m to 350 [Formula: see text]m in diameter, up to 260 in number) by spatially and temporally programming an external magnetic potential energy distribution at the air–water interface or on solid surfaces. A general design method is introduced to program external magnetic potential energy using ferromagnets. A predictive model of the collective system is also presented to predict the formation and guide the design procedure. With the proposed method, versatile complex static formations are experimentally demonstrated and the programmability and scaling effects of formations are analyzed. We also demonstrate the collective mobility of these magnetic microrobots by controlling them to exhibit bio-inspired collective behaviors such as aggregation, directional motion with arbitrary swarm headings, and rotational swarming motion. Finally, the functions of the produced microrobotic swarm are demonstrated by controlling them to navigate through cluttered environments and complete reconfigurable cooperative manipulation tasks.

Design and implementation of a low-cost, tabletop MRI scanner for education and research prototyping

While access to a laboratory MRI system is ideal for teaching MR physics as well as many aspects of signal processing, providing multiple MRI scanners can be prohibitively expensive for educational settings. To address this need, we developed a small, low-cost, open-interface tabletop MRI scanner for academic use. We constructed and tested 20 of these scanners for parallel use by teams of 2-3 students in a teaching laboratory. With simplification and down-scaling to a 1 cm FOV, fully-functional scanners were achieved within a budget of $10,000 USD each. The design was successful for teaching MR principles and basic signal processing skills and serves as an accessible testbed for more advanced MR research projects. Customizable GUIs, pulse sequences, and reconstruction code accessible to the students facilitated tailoring the scanner to the needs of laboratory exercise. The scanners have been used by >800 students in 6 different courses and all designs, schematics, sequences, GUIs, and reconstruction code is open-source.

Three-dimensional MRI in a homogenous 27 cm diameter bore Halbach array magnet

Modern clinical MRI systems utilise very high magnetic fields strengths to produce high resolution images of the human body. The high up-front and maintenance cost of these systems means that much of the world lacks access to this technology. In this paper we propose a low cost, head-only, homogenous Halbach magnet array with the potential for paediatric neuroimaging in low-resource settings. The homogeneity of the Halbach array is improved by allowing the diameter of the Halbach array to vary along its length, and also adding smaller internal shim magnets. The constructed magnet has a bore diameter of 27 cm, mean B₀ field strength of 50.4 mT and a homogeneity of 2400 ppm over a 20 cm diameter spherical volume. The level of homogeneity of the system means that coil-based gradients can be used for spatial encoding which greatly increases the flexibility in image acquisition. 3D images of a "brain phantom" were acquired over a 22 × 22 × 22 cm field of view with a 3.5 mm isotropic resolution using a spin-echo sequence. Future development of a low-cost gradient amplifier and an open-source spectrometer has the potential of offering a fully open-source, low-cost MRI system for paediatric neuroimaging in low-resource settings.

Deconstructing and reconstructing MRI hardware

Three topics concerning future hardware development for clinical MRI are discussed, with the general theme of operational simplification.

The MR Cap: A single-sided MRI system designed for potential point-of-care limited field-of-view brain imaging

PURPOSE

The size, cost, and siting requirements of conventional MRI systems limit their availability and preclude usage as monitoring or point-of-care devices. To address this, we developed a lightweight MRI for point-of-care brain imaging over a reduced field of view (FOV).

METHODS

The B₀ magnet was designed with a genetic algorithm optimizing homogeneity over a 3 × 8 × 8 cm FOV and a built-in gradient for slice selection or readout encoding. An external pair of gradient coils enables phase encoding in the other two directions and a radiofrequency (RF) coil provides excitation and detection. The system was demonstrated with high-resolution 1D "depth profiling" and 3D phantom imaging.

RESULTS

The lightweight B₀ magnet achieved a 64-mT average field over the imaging region at a materials cost of <$450 USD. The weight of the magnet, gradient, and RF coil was 8.3 kg. Depth profiles were obtained at high resolution (0.89 mm) and multislice rapid acquisition with refocused echoes (RARE) images were obtained with a resolution ~2 mm in-plane and ~6-mm slice thickness, each in an imaging time of 11 min.

CONCLUSION

The system demonstrates the feasibility of a lightweight brain MRI system capable of 1D to 3D imaging within a reduced FOV. The proposed system is low-cost and small enough to be used in point-of-care applications.

Wireless resonant circuits for the minimally invasive sensing of biophysical processes in magnetic resonance imaging

Biological electromagnetic fields arise throughout all tissue depths and types, and correlate with physiological processes and signalling in organs of the body. Most of the methods for monitoring these fields are either highly invasive or spatially coarse. Here, we show that implantable active coil-based transducers that are detectable via magnetic resonance imaging enable the remote sensing of biological fields. These devices consist of inductively coupled resonant circuits that change their properties in response to electrical or photonic cues, thereby modulating the local magnetic resonance imaging signal without the need for onboard power or wired connectivity. We discuss design parameters relevant to the construction of the transducers on millimetre and submillimetre scales, and demonstrate their in vivo functionality for measuring time-resolved bioluminescence in rodent brains. Biophysical sensing via microcircuits that leverage the capabilities of magnetic resonance imaging may enable a wide range of biological and biomedical applications.