Very-low-field MRI scanners: from the ideal to the real permanent magnet array

Very-low-field MRIs are becoming increasingly popular due to their portability and adaptability to different environments. They are being successfully used for various clinical applications, leading to a paradigm shift in the way imaging care is typically performed. The development of low-cost MRI scanner prototypes began a few years ago, with some interesting and promising open-source projects emerging in both hardware and software design. Using permanent magnets (PMs) to generate the static magnetic field B0 can substantially reduce the manufacturing cost while achieving satisfactory homogeneity. This article aims to explore the reasons behind discrepancies between magnet design and prototype performance in terms of magnetic field homogeneity. Understanding the impact of the practical implementation of magnet design could inform the development of more tolerant designs in future, simplifying subsequent B0 shimming procedures or even making them unnecessary. This work also evidences the impact of using different numerical model approximations in the modelling phase, proving how they also impact the quality of the design outcomes.