Low-field magnetic resonance imaging has recently received renewed attention as a result of advances in permanent-magnet-based systems. These platforms offer several advantages, including portability, reduced costs for acquisition and maintenance, improved environmental sustainability, and increased flexibility for the development of innovative, application-specific designs. This flexibility is particularly valuable when MRI is combined with other imaging modalities such as PET or CT, or with alternative detection and hyperpolarization techniques.
The GAMMA-MRI project [1] investigates the feasibility of using magnetic resonance methods to spatially encode gamma photons emitted by hyperpolarized xenon isomers. In this approach, gamma detectors are positioned around the subject to record the emitted photons [2]. This strategy has the potential to significantly extend sensitivity limits — reaching approximately 10¹³ spins as demonstrated in [2] — while preserving the adaptability and spatial resolution typically associated with conventional high-field MRI. To support these objectives, a dedicated low-field MRI system has been developed that satisfies multiple technical constraints without compromising image quality.
In this work, we present fundamental principles and representative examples of custom-built MR instrumentation [3,4], with particular emphasis on a low-field preclinical scanner for small-animal imaging developed within the GAMMA-MRI framework. To enable efficient detection of photons emitted by xenon nuclei, an unconventional magnet geometry was adopted: a yokeless, C-shaped permanent magnet featuring a central bore and a wide (160 mm) separation between its two pole faces. This design results in a compact and portable system with a total mass of approximately 40 kg. We report on the performance of the magnet assembly, including the achieved magnetic field strength (50 mT) and its homogeneity and temporal stability.
We further introduce the system’s custom-designed gradient coils and second-order shim elements, both engineered to preserve access through the central bore. Preliminary imaging results are presented, including proton MRI of various phantom samples, acquired using standard inductive Faraday detection and reconstructed using both conventional pulsed-field-gradient Fourier methods and alternative techniques such as inverse Laplace and Radon transforms. Measurements of sensitivity and spatial resolution in the system’s standard operating mode are also reported [5]. Finally, we outline ongoing developments toward full system integration and the implementation of in vivo imaging capabilities.
 
Acknowledgement:  The GAMMA-MRI project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 964644.

References:
[1] https://gamma-mri.eu
[2] Y. Zheng, G. Miller, W. Tobias, et al. A method for imaging and spectroscopy using γ-rays and magnetic resonance. Nature 537, 652 (2016). https://doi.org/10.1038/nature19775
[3] R. de Oliveira-Silva, A. Bélime, C. Le Coeur, et al. Coupling NMR to SANS: addressing at once structure and dynamics in soft-matter, Journal of Neutron Research, 21, 155 (2019).
[4] R. de Oliveira-Silva, E. Lucas-Oliveira, A. G. Araujo-Ferreira, W. A. Trevizan, E. L. G, Vidoto, D. Sakellariou, T. J. Bonagamba.  A benchtop single-sided magnet with NMR well-logging tool specifications - Examples of application, J. Magn. Reson. 322, 106871 (2021).
[5] R. de Oliveira-Silva et al. manuscript in preparation (2026).