Low-field (B0 < 0.2 T) magnetic resonance imaging (MRI) is emerging as a low cost, point-of-care alternative to provide access to diagnostic imaging technology even in resource scarce environments. MRI magnets can be constructed based on permanent neodymium-iron-boron (NdFeB) magnets in discretized arrangements, leading to substantially lower mass and costs. A challenge with these designs is, however, a good B0 field homogeneity, which is needed to produce high quality images free of distortions. In this work, we describe an iterative approach to build a low-field MR magnet based on a B0-shimming methodology using genetic algorithms. The methodology is tested by constructing a small bore (inner bore diameter = 130 mm) desktop MR magnet (<15 kg) at a field strength of B0 = 0.1 T and a target volume of 4 cm in diameter. The configuration consists of a base magnet and shim inserts, which can be placed iteratively without modifying the base magnet assembly and without changing the inner dimensions of the bore or the outer dimensions of the MR magnet. Applying the shims, B0 field inhomogeneity could be reduced by a factor 8 from 5,448 to 682 ppm in the target central slice of the magnet. Further improvements of these results can be achieved in a second or third iteration, using more sensitive magnetic field probes (e.g., nuclear magnetic resonance based magnetic field measurements). The presented methodology is scalable to bigger magnet designs. The MR magnet can be reproduced with off-the-shelf components and a 3D printer and no special tools are needed for construction. All design files and code to reproduce the results will be made available as open source hardware.

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Purpose To simultaneously acquire spectroscopic signals from two MRS voxels using a multi‐banded 2 spin‐echo, full‐intensity acquired localized (2SPECIAL) sequence, and to decompose the signal to their respective regions by a novel voxel‐GRAPPA (vGRAPPA) decomposition approach for in vivo brain applications at 7 T. Methods A wideband, uniform rate, smooth truncation (WURST) multi‐banded pulse was incorporated into SPECIAL to implement 2SPECIAL for simultaneous multi‐voxel spectroscopy (sMVS). To decompose the acquired data, the voxel‐GRAPPA decomposition algorithm is introduced, and its performance is compared to the SENSE‐based decomposition. Furthermore, the limitations of two‐voxel excitation concerning the multi‐banded adiabatic inversion pulse, as well as of the combined B0 shim and B1+ adjustments, are evaluated. Results It was successfully shown that the 2SPECIAL sequence enables sMVS without a significant loss in SNR while reducing the total scan time by 21.6% compared to two consecutive acquisitions. The proposed voxel‐GRAPPA algorithm properly reassigns the signal components to their respective origin region and shows no significant differences to the well‐established SENSE‐based algorithm in terms of leakage (both <10%) or Cramér‐Rao lower bounds (CRLB) for in vivo applications, while not requiring the acquisition of additional sensitivity maps and thus decreasing motion sensitivity. Conclusion The use of 2SPECIAL in combination with the novel voxel‐GRAPPA decomposition technique allows a substantial reduction of measurement time compared to the consecutive acquisition of two single voxels without a significant decrease in spectral quality or metabolite quantification accuracy and thus provides a new option for multiple‐voxel applications.

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