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

Abstract

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 B 0 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 B 0 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. K E Y W O R D S accessible MRI, low-cost MRI, point of care, portable MRI | INTRODUCTIONMagnetic resonance imaging (MRI) has become a routine part of clinical medicine, especially for neuroimaging. However, expense, size, and siting requirements impose limitations on how conventional MRI scanners can be used within the health-care system. Installation of a whole-body MR scanner or even a head-only-type device using a conventional superconducting magnet entails a dedicated space, special infrastructure, and safety requirements, such as a 5-gauss exclusion area, high-power electrical supply, cooling system, and shielded room. These prerequisites preclude the use of MRI in many settings, such as rural or developingworld clinics that might not possess this infrastructure. F I G U R E 2 A, Illustration showing approximate desired magnet shape, ROI, and B 0 axis. B, Section of Halbach sphere magnet that approximates the desired magnet shape and B 0 direction. C, Discretized Halbach sphere section that approximates the continuous magnet design as an assembly of magnet blocks. D, Optimized discrete block magnet design. ROI, region of interest

Development and validation of 3D MP‐SSFP to enable MRI in inhomogeneous magnetic fields

“…With low-field permanent magnet systems, the flip angle limitation is nearly negligible based on a much lower SAR at low field strengths; therefore, FSE-based pulse sequences have instead been employed in these systems. 3,4 FSE is the SNR optimal pulse sequence compared with MP-SSFP for a given level of inhomogeneity; however, the higher flip angle needed for FSE means it is more SAR intensive, ruling it out at some point. The transition between use of FSE and MP-SSFP as the SNR optimal pulse sequence is, therefore, dependent on the SAR for these two sequences.…”

“…There is considerable research interest in creating MRI systems that are compact, cost-effective, and robust. [1][2][3][4][5][6][7][8] Most clinical MRI techniques presuppose a highly homogeneous magnetic field over the imaging volume, typically within ±10 ppm. Cost-effectively reducing magnet size can make MRI systems compact and lightweight; however, such size reduction will either reduce the magnetic field strength, magnetic field uniformity, or both compared with a clinical scanner.…”

En‐face optical coherence tomography for the detection of cancer in prostatectomy specimens: Quantitative analysis in 20 patients

“…OCT analysis would further help the pathologists by pointing out points of interests, just like the CAD systems support radiologists with mammograms. An alternative method to assess the prostatectomy specimen for free cutting edges would MRI—dedicated point‐of‐care MRI systems exist . However, the question is whether the MRI resolution is high enough, and the lack of blood flow makes the MRI assessment possible.…”

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

“…Details of the magnet design have been published previously 26 and are minimally described here. The Halbach magnet consists of 23 rings, with two layers of N48 neodymium boron iron magnets (12 × 12 × 12 mm 3 ) per ring.…”

“…More recently, such systems have eliminated the need for rotating the system by adding two coil-based phase-encoding gradients to achieve 3D imaging. 26,27 Arrays of permanent magnets can also be arranged in a so-called inward-outward pair, in which the magnets in two rings are arranged with their magnetization pointing radially inward and outward in respective rings, generating a B 0 field that is oriented along the bore, more closely resembling conventional MRI systems. 28,29 Recently, our group described the design, construction, and characterization of a larger 27-cm clear-bore Halbach array magnet operating at 50 mT.…”