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.

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.

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.

Assessing the effects of subject motion on T2 relaxation under spin tagging (TRUST) cerebral oxygenation measurements using volume navigators.

PURPOSE: Subject motion may cause errors in estimates of blood T2 when using the T2 -relaxation under spin tagging (TRUST) technique on noncompliant subjects like neonates. By incorporating 3D volume navigators (vNavs) into the TRUST pulse sequence, independent measurements of motion during scanning permit evaluation of these errors. METHODS: The effects of integrated vNavs on TRUST-based T2 estimates were evaluated using simulations and in vivo subject data. Two subjects were scanned with the TRUST+vNav sequence during prescribed movements. Mean motion scores were derived from vNavs and TRUST images, along with a metric of exponential fit quality. Regression analysis was performed between T2 estimates and mean motion scores. Also, motion scores were determined from independent neonatal scans. RESULTS: vNavs negligibly affected venous blood T2 estimates and better detected subject motion than fit quality metrics. Regression analysis showed that T2 is biased upward by 4.1 ms per 1 mm of mean motion score. During neonatal scans, mean motion scores of 0.6 to 2.0 mm were detected. CONCLUSION: Motion during TRUST causes an overestimate of T2 , which suggests a cautious approach when comparing TRUST-based cerebral oxygenation measurements of noncompliant subjects. Magn Reson Med 78:2283-2289, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Mitigation of partial volume effects in susceptibility-based oxygenation measurements by joint utilization of magnitude and phase (JUMP).

PURPOSE: Susceptibility-based blood oxygenation measurements in small vessels of the brain derive from gradient echo (GRE) phase and can provide localized assessment of brain function and pathology. However, when vessel diameter becomes smaller than the acquisition voxel size, partial volume effects compromise these measurements. The purpose of this study was to develop a technique to improve the reliability of vessel oxygenation estimates in the presence of partial volume effects. METHODS: Intravoxel susceptibility variations are present when a vessel and parenchyma experience partial volume effects, modifying the voxel's GRE phase signal and attenuating the GRE magnitude signal. Using joint utilization of magnitude and phase (JUMP), both vessel susceptibility and voxel partial volume fraction can be estimated, providing measurements of venous oxygen saturation ( Yv) in straight, nearly vertical vessels that have improved robustness to partial volume effects. RESULTS: JUMP was demonstrated by estimating vessel Yv in numerical and in vivo experiments. Deviations from ground truth of Yv measurements in vessels tilted up to 30° from B0 were reduced by over 50% when using JUMP compared with phase-only techniques. CONCLUSION: JUMP exploits both magnitude and phase data in GRE imaging to mitigate partial volume effects in estimation of vessel oxygenation. Magn Reson Med 77:1713-1727, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Single-step quantitative susceptibility mapping with variational penalties.

Quantitative susceptibility mapping (QSM) estimates the underlying tissue magnetic susceptibility from the gradient echo (GRE) phase signal through background phase removal and dipole inversion steps. Each of these steps typically requires the solution of an ill-posed inverse problem and thus necessitates additional regularization. Recently developed single-step QSM algorithms directly relate the unprocessed GRE phase to the unknown susceptibility distribution, thereby requiring the solution of a single inverse problem. In this work, we show that such a holistic approach provides susceptibility estimation with artifact mitigation and develop efficient algorithms that involve simple analytical solutions for all of the optimization steps. Our methods employ total variation (TV) and total generalized variation (TGV) to jointly perform the background removal and dipole inversion in a single step. Using multiple spherical mean value (SMV) kernels of varying radii permits high-fidelity background removal whilst retaining the phase information in the cortex. Using numerical simulations, we demonstrate that the proposed single-step methods reduce the reconstruction error by up to 66% relative to the multi-step methods that involve SMV background filtering with the same number of SMV kernels, followed by TV- or TGV-regularized dipole inversion. In vivo single-step experiments demonstrate a dramatic reduction in dipole streaking artifacts and improved homogeneity of image contrast. These acquisitions employ the rapid three-dimensional echo planar imaging (3D EPI) and Wave-CAIPI (controlled aliasing in parallel imaging) trajectories for signal-to-noise ratio-efficient whole-brain imaging. Herein, we also demonstrate the multi-echo capability of the Wave-CAIPI sequence for the first time, and introduce an automated, phase-sensitive coil sensitivity estimation scheme based on a 4-s calibration acquisition. Copyright © 2016 John Wiley & Sons, Ltd.

General design approach and practical realization of decoupling matrices for parallel transmission coils.

PURPOSE: In a coupled parallel transmit (pTx) array, the power delivered to a channel is partially distributed to other channels because of coupling. This power is dissipated in circulators resulting in a significant reduction in power efficiency. In this study, a technique for designing robust decoupling matrices interfaced between the RF amplifiers and the coils is proposed. The decoupling matrices ensure that most forward power is delivered to the load without loss of encoding capabilities of the pTx array. THEORY AND METHODS: The decoupling condition requires that the impedance matrix seen by the power amplifiers is a diagonal matrix whose entries match the characteristic impedance of the power amplifiers. In this work, the impedance matrix of the coupled coils is diagonalized by a successive multiplication by its eigenvectors. A general design procedure and software are developed to generate automatically the hardware that implements diagonalization using passive components. RESULTS: The general design method is demonstrated by decoupling two example parallel transmit arrays. Our decoupling matrices achieve better than -20 db decoupling in both cases. CONCLUSION: A robust framework for designing decoupling matrices for pTx arrays is presented and validated. The proposed decoupling strategy theoretically scales to any arbitrary number of channels. Magn Reson Med 76:329-339, 2016. © 2015 Wiley Periodicals, Inc.

Identification of oocyte-selective NLRP genes in rhesus macaque monkeys (Macaca mulatta).

Oocyte-selective genes control multiple aspects of female gamete development and preimplantation embryogenesis. Several key oocyte-selective factors have been identified in mice recently; however, these factors are not well documented in more advanced species such as nonhuman primates. One of such oocyte-selective factors is NLRP5 (NLR family, Pyrin domain containing 5), also known as Maternal Antigen That Embryos Require (MATER), which is required for preimplantation embryo development beyond the 2-cell stage in mice. Human NLRP family contains 14 members. We identified 14 NLRP gene homologues and examined their spatial and temporal expression in rhesus macaque monkeys (Macaca mulatta). While all 14 NLRP genes are detectable in the macaque gonad, eight of them (NLRP2, 4, 5, 8, 9, 11, 13, and 14) are specifically or preferentially expressed in the ovary. In situ hybridization elucidated a specific oocyte expression pattern of the eight NLRP genes within the ovary. During the oocyte-to-embryo transition, seven of these oocyte-selective NLRP transcripts (excluding NLPR2) are enriched in maturing oocytes and early preimplantation embryos but diminish upon embryo genome activation, indicating an exclusive maternal origin of these transcripts. Though functionally unknown, the spatial and temporal distribution of these oocyte-selective NLRP genes implies important roles of the NLRP family in oogenesis and early embryo development in nonhuman primates.