Introduction: Modern clinical MRI systems are able to offer sub-millimetre imaging of the human body. However, the high up-front and maintenance cost of these systems means that much of the world lacks access to this technology. For clinical conditions such as hydrocephalus where very high resolution images are not required, low-field MRI systems can offer a low-cost approach towards providing clinically useful MR images in low-resource settings. In this work we designed and constructed a large-bore, low-cost, low-field, Halbach-based MRI scanner intended for neuroimaging in young children.
Methods: The Halbach array is constructed from 23 layers of 12 mm cuboid N48 neodymium magnets, each layer consisting of two rings of magnets with a total magnet length of 50.6 cm with a bore size of 27 cm. The homogeneity of the magnet is improved by tapering the diameter of the Halbach array at the ends. Shimming was performed by placing additional 3 mm cuboid magnets inside the bore of the magnet. Gradient coils were constructed using 1.5 mm diameter copper wire wound on plastic cylindrical formers. A solenoidal RF coil was used for transmit/receive. Total hardware costs were less than 30 000 euros.
Results & discussion: The field strength was 50.45 mT with homogeneity 2500 ppm over a 20 cm diameter spherical volume (DSV). Two dimensional phantom images have been acquired with a resolution of 1x1x35 mm in 16 minutes using a spin echo sequence. Image distortion due to gradient non-linearity was corrected by including simulated gradient fields in a model-based reconstruction.
Conclusion: The homogeneity of a Halbach array can be improved by tapering the diameter of the magnet away from the center. Initial 2D images have been acquired demonstrating the potential for a low-cost MR system.