MRI at higher fields gives a higher signal-to-noise ratio. The mouse-brain images in Figure 1 are taken at 21.1 T (left) and 9.4 T (right) using the same spin echo pulse sequence and imaging parameters. Presently human head imaging is limited to less than 12 T (≤ 500 MHz). Going to 14.1 T (600 MHz) will require using Nb3Sn superconductors in human head magnets for the first time. While this technology has been used for decades in high field magnets such as small animal MRI (Fig. 1) and large magnets for fusion and condensed matter physics (Fig. 2), this conductor is approximately ten times the cost of the NbTi superconductor traditionally used for human MRI magnets and the clinical environment is not conducive to the protection technologies traditionally used for large magnets.
High resolution NMR (~10 ppb) is now possible at much higher fields due to the advent of magnets based on the High Temperature Superconductors (HTS). In the past few years fields available have jumped from 23.4 T (1.0 GHz) to 2.8.2 T (1.2 GHz) with five systems delivered as of October 2021 and at least five more on order. This technology shows promise of enabling NMR at still higher fields with two groups worldwide pursuing 30.5 T (1.3 GHz) NMR and the MagLab developing a 40 T superconducting magnet for condensed matter physics.
Human head MRI magnets at the 20 T level will likely become possible in coming years by scaling up these high resolution HTS magnets to a scale similar to that of the present Iseult magnet. Importantly, tHigh resolution NMR (~10 ppb) is now possible at much higher fields due to the advent of magnets based on the High Temperature Superconductors (HTS). In the past few years fields available have jumped from 23.4 T (1.0 GHz) to 28.2 T (1.2 GHz) with five systems delivered as of October 2021 and at least five more on order. This technology shows promise of enabling NMR at still higher fields with two groups worldwide pursuing 30.5 T (1.3 GHz) NMR and the MagLab developing a 40 T superconducting magnet for condensed matter physicshe HTS materials not only function at higher field than the traditional NbTi and Nb3Sn, they also operate at much higher current density. Hence, the size of a 20 T MRI magnet might not be larger than that of the present 11.7 T Iseult magnet. While a 20 T heam MRI magnet will require ~1,000-fold larger HTS coils than those presently used in NMR coils, the fusion community is hard at work building large-scale HTS coils for tokamaks. 20 T test coils of size similar to that required for MRI have already been built, although lots of work remains to meet all the requirements of a human MRI system.
NMR is now also possible at still higher fields using resistive magnet technology. The National High Magnetic Field Laboratory has developed and is operating a resistive/superconducting hybrid NMR magnet at 35.2 T (1.5 GHz) with ~0.1 ppm homogeneity and stability. NMR has also been done in pulsed resistive magnets with field strengths up to 55 T. Such systems are only able to maintain field for ~50 ms and have limited uniformity and stability.

Acknowledgements

This work was performed at the National High Magnetic Field Laboratory which is funded by the US National Science Foundation (DMR-1839796) and the State of Florida.

References

[1] Schepkin, et al., MRI, 28 (2010) 400 – 407.