Clinical Use of Bedside Portable Low-field Brain Magnetic Resonance Imaging in Patients on ECMO: The Results from Multicenter SAFE MRI ECMO Study

Purpose

Early detection of acute brain injury (ABI) is critical for improving survival for patients with extracorporeal membrane oxygenation (ECMO) support. We aimed to evaluate the safety of ultra-low-field portable MRI (ULF-pMRI) and the frequency and types of ABI observed during ECMO support.

Methods

We conducted a multicenter prospective observational study (NCT05469139) at two academic tertiary centers (August 2022-November 2023). Primary outcomes were safety and validation of ULF-pMRI in ECMO, defined as exam completion without adverse events (AEs); secondary outcomes were ABI frequency and type.

Results

ULF-pMRI was performed in 50 patients with 34 (68%) on venoarterial (VA)-ECMO (11 central; 23 peripheral) and 16 (32%) with venovenous (VV)-ECMO (9 single lumen; 7 double lumen). All patients were imaged successfully with ULF-pMRI, demonstrating discernible intracranial pathologies with good quality. AEs occurred in 3 (6%) patients (2 minor; 1 serious) without causing significant clinical issues.ABI was observed in ULF-pMRI scans for 22 patients (44%): ischemic stroke (36%), intracranial hemorrhage (6%), and hypoxic-ischemic brain injury (4%). Of 18 patients with both ULF-pMRI and head CT (HCT) within 24 hours, ABI was observed in 9 patients with 10 events: 8 ischemic (8 observed on ULF-oMRI, 4 on HCT) and 2 hemorrhagic (1 observed on ULF-pMRI, 2 on HCT).

Conclusions

ULF-pMRI was shown to be safe and valid in ECMO patients across different ECMO cannulation strategies. The incidence of ABI was high, and ULF-pMRI may more sensitive to ischemic ABI than HCT. ULF-pMRI may benefit both clinical care and future studies of ECMO-associated ABI.

Conceptual Design of a Portable, Solid-Nitrogen-Cooled 0.5-T/560-mm Point-of-Care MRI Magnet

We describe the conceptual design of a portable, liquid-helium-free, all-REBCO, 0.5-T/560-mm point-of-care magnetic resonance imaging (MRI) magnet. It is free from an external power supply and a refrigeration system during operation. In our portable MRI magnet, we use a detachable "cryocirculator" that circulates, in a closed circuit, cold working fluid, and most importantly for portability, it can be readily coupled to or decoupled from the magnet, in contrast, a conventional cryocooler is mechanically attached to the magnet. Another unique feature of our system is a volume of solid nitrogen (SN2) in the cold chamber that adds enough thermal mass to the magnet in the 30-36-K operating temperature range, enabling it to maintain its field over a period of, for this system, ≥10 hours, plenty enough for this portable MRI system, uncoupled from its cryocirculator, to perform its mission before it needs recooling.

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.

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.

Sensitivity of portable low-field magnetic resonance imaging for multiple sclerosis lesions

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Paired, same-day, 3T and 64mT MRI studies were analyzed in 33 MS patients.

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64mT MRI showed 94% sensitivity for detecting any lesions in 3T confirmed cases.

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The diameter of the smallest detected lesion was larger at 64mT compared to 3T.

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Total lesion volume estimates were strongly correlated between 3T and 64mT scans.

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Portable low-field MRI detects white matter lesions, but smaller lesions may be missed.

Magnetic resonance imaging (MRI) is a fundamental tool in the diagnosis and management of neurological diseases such as multiple sclerosis (MS). New portable, low-field strength, MRI scanners could potentially lower financial and technical barriers to neuroimaging and reach underserved or disabled populations, but the sensitivity of these devices for MS lesions is unknown. We sought to determine if white matter lesions can be detected on a portable 64mT scanner, compare automated lesion segmentations and total lesion volume between paired 3T and 64mT scans, identify features that contribute to lesion detection accuracy, and explore super-resolution imaging at low-field. In this prospective, cross-sectional study, same-day brain MRI (FLAIR, T1w, and T2w) scans were collected from 36 adults (32 women; mean age, 50 ± 14 years) with known or suspected MS using Siemens 3T (FLAIR: 1 mm isotropic, T1w: 1 mm isotropic, and T2w: 0.34–0.5 × 0.34–0.5 × 3–5 mm) and Hyperfine 64mT (FLAIR: 1.6 × 1.6 × 5 mm, T1w: 1.5 × 1.5 × 5 mm, and T2w: 1.5 × 1.5 × 5 mm) scanners at two centers. Images were reviewed by neuroradiologists. MS lesions were measured manually and segmented using an automated algorithm. Statistical analyses assessed accuracy and variability of segmentations across scanners and systematic scanner biases in automated volumetric measurements. Lesions were identified on 64mT scans in 94% (31/33) of patients with confirmed MS. The average smallest lesions manually detected were 5.7 ± 1.3 mm in maximum diameter at 64mT vs 2.1 ± 0.6 mm at 3T, approaching the spatial resolution of the respective scanner sequences (3T: 1 mm, 64mT: 5 mm slice thickness). Automated lesion volume estimates were highly correlated between 3T and 64mT scans (r = 0.89, p < 0.001). Bland-Altman analysis identified bias in 64mT segmentations (mean = 1.6 ml, standard error = 5.2 ml, limits of agreement = –19.0–15.9 ml), which over-estimated low lesion volume and under-estimated high volume (r = 0.74, p < 0.001). Visual inspection revealed over-segmentation was driven venous hyperintensities on 64mT T2-FLAIR. Lesion size drove segmentation accuracy, with 93% of lesions > 1.0 ml and all lesions > 1.5 ml being detected. Using multi-acquisition volume averaging, we were able to generate 1.6 mm isotropic images on the 64mT device. Overall, our results demonstrate that in established MS, a portable 64mT MRI scanner can identify white matter lesions, and that automated estimates of total lesion volume correlate with measurements from 3T scans.

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.

Assessing the SAfety and FEasibility of bedside portable low-field brain Magnetic Resonance Imaging in patients on ECMO (SAFE-MRI ECMO study): study protocol and first case series experience

Background

To assess the safety and feasibility of imaging of the brain with a point-of-care (POC) magnetic resonance imaging (MRI) system in patients on extracorporeal membrane oxygenation (ECMO). Early detection of acute brain injury (ABI) is critical in improving survival for patients with ECMO support.

Methods

Patients from a single tertiary academic ECMO center who underwent head CT (HCT), followed by POC brain MRI examinations within 24 h following HCT while on ECMO. Primary outcomes were safety and feasibility, defined as completion of MRI examination without serious adverse events (SAEs). Secondary outcome was the quality of MR images in assessing ABIs.

Results

We report 3 consecutive adult patients (median age 47 years; 67% male) with veno-arterial (n = 1) and veno-venous ECMO (n = 2) (VA- and VV-ECMO) support. All patients were imaged successfully without SAEs. Times to complete POC brain MRI examinations were 34, 40, and 43 min. Two patients had ECMO suction events, resolved with fluid and repositioning. Two patients were found to have an unsuspected acute stroke, well visualized with MRI.

Conclusions

Adult patients with VA- or VV-ECMO support can be safely imaged with low-field POC brain MRI in the intensive care unit, allowing for the assessment of presence and timing of ABI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03990-6.

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.

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.

An easily reproducible, hand-held, single-sided, MRI sensor

Single-sided MRI sensors allow the imaging of samples that are larger than the magnet. Thus, they enable truly portable imagers with potential applications in medicine, quality assurance (QA), agriculture, material science, and other fields. However, despite recent advancements, single-sided MRI systems are relatively uncommon. This is partially due to the limited number of commercial products. Also, current implementations often require large and/or complex magnet arrays which require machining techniques such as milling or drilling. These techniques must be performed to tight tolerances to ensure accuracy of the B₀ field. Furthermore, these systems generally have hand-wound RF or gradient coils that are not trivial to construct. The main goals of this work are to reduce the size of single-sided MRI sensors while simultaneously making them more accessible for others to build. To this end, we present a hand-held, single-sided, MRI sensor that is constructed using an easy-to-assemble magnet array, a 3D-printed housing, and printed circuit boards (PCBs) that contain the RF coil, gradient coils, and matching network. By implementing all coils directly on PCBs, the geometry can be easily optimized and then manufactured at low cost. Both spin density-weighted and T₁-weighted images of various samples are presented to demonstrate the capabilities of the proposed sensor.

Development of a small car-mounted magnetic resonance imaging system for human elbows using a 0.2 T permanent magnet

Portable magnetic resonance imaging (MRI) scanners can provide opportunities for mobile operation in many environments including disease screening and primary care suites. Here, we develop a new, compact transportable MRI system for imaging small joints of the extremities using a 0.2 T, 200 kg permanent magnet. The whole system, including the magnet, gradient coils, RF probes, and MRI consoles (80 kg in weight) was installed in a standard-size minivan-style vehicle. The use of the open-geometry magnet enables easy patient positioning within the limited space of the vehicle. We show that our portable MRI system provides clinically relevant images of screening for elbow injuries induced by overuse of overhand throwing. This transportable system is deployable during sport events or in environments with poor access to MRI systems, and could be applicable for mass screening, early diagnosis, and case finding.

Magnetic resonance imaging with RF encoding on curved natural slices

While the idea of using spatial encoding fields (SEM) for image formation has been proven, conventional wisdom still holds that a magnetic resonance imaging (MRI) system begins with a highly uniform magnetic field. In particular, radio frequency (RF) encoding MRIs designed and tested to date have largely used uniform magnetic fields. Here we demonstrate magnetic resonance imaging in a magnetic field with a built-in gradient that gives non-planar slices - curved surfaces - when the nuclear spins are excited with narrow band RF pulses. Image encoding on these naturally occurring non-planar slices was accomplished with RF encoding using a non-linear spatially varying B₁ phase gradient. The imaging methods were demonstrated on a small prototype MRI instrument. The MRI has no switched magnetic field gradients - it is "gradient-free". A low field gradient-free MRI with low mass permanent magnets and simple, low power, RF encoding hardware is ideal for deployment on the International Space Station for the study of astronaut muscle and bone mass loss. Gradient-free natural slice encoding MRI designs would also be portable enough for application in remote terrestrial locations, in emergency rooms and in operating rooms where they can be used with minimally invasive and robotic surgery.

Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields

Transmit Array Spatial Encoding (TRASE) is a promising new MR encoding method that uses transmit RF (B1(+)) phase gradients over the field-of-view to perform Fourier spatial encoding. Acquisitions use a spin echo train in which the transmit coil phase ramp is modulated to jump from one k-space point to the next. This work extends the capability of TRASE by using swept radiofrequency (RF) pulses and a quadratic phase removal method to enable TRASE where it is arguably most needed: portable imaging systems with inhomogeneous B0 fields. The approach is particularly well-suited for portable MR scanners where (a) inhomogeneous B0 fields are a byproduct of lightweight magnet design, (b) heavy, high power-consumption gradient coil systems are a limitation to siting the system in non-conventional locations and (c) synergy with the use of spin echo trains is required to overcome intra-voxel dephasing (short T2(∗)) in the inhomogeneous field. TRASE does not use a modulation of the B0 field to encode, but it does suffer from secondary effects of the inhomogeneous field. Severe artifacts arise in TRASE images due to off-resonance effects when the RF pulse does not cover the full bandwidth of spin resonances in the imaging FOV. Thus, for highly inhomogeneous B0 fields, the peak RF power needed for high-bandwidth refocusing hard pulses becomes very expensive, in addition to requiring RF coils that can withstand thousands of volts. In this work, we use swept WURST RF pulse echo trains to achieve TRASE imaging in a highly inhomogeneous magnetic field (ΔB0/B0∼0.33% over the sample). By accurately exciting and refocusing the full bandwidth of spins, the WURST pulses eliminate artifacts caused by the limited bandwidth of the hard pulses used in previous realizations of TRASE imaging. We introduce a correction scheme to remove the unwanted quadratic phase modulation caused by the swept pulses. Also, a phase alternation scheme is employed to mitigate artifacts caused by mixture of the even and odd-echo coherence pathways due to defects in the refocusing pulse. In this paper, we describe this needed methodology and demonstrate the ability of TRASE to Fourier encode in an inhomogeneous field (ΔB0/B0∼1% over the full FOV).