![]() Their bi-planar gradient coils create a gradient field with a maximum strength of 26 mT 1m −1. The system operates at 64 mT, has a 30 cm vertical opening and uses an 8-channel receive only head coil. An H-type magnet system has been commercialized by Hyperfine creating a portable MRI for bedside imaging, which received FDA approval in February 2020. Commercial RF amplifiers (150 W), gradient amplifiers (10 A peak current) and spectrometer are used. This 200 kg weighing system with a vertical gap of only 16 cm was mounted inside a minivan. A similar system with a higher field strength of 0.2 T was built by. A custom-built (100 W) RF-amplifier and commercial gradient amplifier (150 A peak current) and spectrometer were used. Unshielded fingerprint planar gradient coils have efficiencies between 0.13 and 0.36 mTm −1A −1. In this region they obtain a field strength of 50.9 mT with a homogeneity of 120 ppm after passive shimming. ![]() The system has a clear bore of 26 cm and is designed for imaging within a 200 mm DSV. have designed an H-shaped dipolar magnet system for cerebral stroke imaging. In terms of systems which have been designed with portability and point-of-care in mind, He et al. Commercial RF amplifiers (2 kW), gradient amplifiers (107 A peak current) and multi-channel spectrometer are used. In addition, the system uses eleven shim coils resulting in 40 ppm B 0 homogeneity, measured over a 15 cm long cylindrical volume with a diameter of 30 cm. Reported gradient efficiencies are between 0.17 and 0.18 mTm −1A −1. Lurie’s group in Aberdeen have designed and built a fast field cycling system which uses a 50 cm diameter custom-built resistive magnet which can be ramped to create an axial magnetic field between 50 μT and 0.2 T. Commercial RF amplifiers (300 W), gradient amplifiers (200 A peak current) and spectrometer are used. The maximum gradient strength of the unshielded planar gradient coils is 0.7 mTm −1 using 140 A of peak current. The Walsworth group designed a Helmholtz coil based bi-planar electromagnet which, after passive shimming creates a magnetic field of 6.5 mT : the planes on which the conductors lie are separated by 79 cm, resulting in an open bore system which allows the subject to sit in an upright position. For a more detailed review, Sarracanie and Salameh have published an extensive review discussing general progress in low field MRI. The following paragraphs summarize the properties of systems which have been used for in vivo imaging in terms of their hardware design and performance. Several different magnet geometries have been used, and each system contains a unique combination of custom-built and commercial components. In the past 15 years several groups have shown significant progress in the design of low field (<100 mT) MRI systems. In addition, we report performance characterisation of the RF amplifier, the gradient amplifier, eddy currents from the gradient coils, and describe a quality control protocol for the overall system. These developments include the design of i) high-linearity gradient coils using a modified volume-based target field approach, ii) phased-array receive coils, and iii) a battery-operated three-axis gradient amplifier for improved portability and sustainability. ![]() In this paper we describe recent developments in constructing and characterising a low-field portable MRI system for in vivo imaging at 50 mT. In order to maximize performance while minimizing cost many components of such a system should ideally be designed specifically for low frequency operation. ![]() Low-field permanent magnet-based MRI systems are finding increasing use in portable, sustainable and point-of-care applications. 4Circuits and Systems, Delft University of Technology, Delft, Netherlands.3Dienst Elektronische en Mechanische Ontwikkeling (DEMO), Delft University of Technology, Delft, Netherlands.Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, Netherlands Bart de Vos 1, Javad Parsa 1,2, Zaynab Abdulrazaq 1, Wouter M.
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