Magnetic resonance imaging at 9.4 T: the Maastricht journey

Abstract

The 9.4 T scanner in Maastricht is a whole-body magnet with head gradients and parallel RF transmit capability. At the time of the design, it was conceptualized to be one of the best fMRI scanners in the world, but it has also been used for anatomical and diffusion imaging. 9.4 T offers increases in sensitivity and contrast, but the technical ultra-high field (UHF) challenges, such as field inhomogeneities and constraints set by RF power deposition, are exacerbated compared to 7 T. This article reviews some of the 9.4 T work done in Maastricht. Functional imaging experiments included blood oxygenation level-dependent (BOLD) and blood-volume weighted (VASO) fMRI using different readouts. BOLD benefits from shorter T2* at 9.4 T while VASO from longer T1. We show examples of both ex vivo and in vivo anatomical imaging. For many applications, pTx and optimized coils are essential to harness the full potential of 9.4 T. Our experience shows that, while considerable effort was required compared to our 7 T scanner, we could obtain high-quality anatomical and functional data, which illustrates the potential of MR acquisitions at even higher field strengths. The practical challenges of working with a relatively unique system are also discussed.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…Novel RF array designs include optimal combinations of loop-dipole building blocks, as well as dielectric resonators, with numerous studies demonstrating their added value at 7 T [32] and 10.5 T. This Special Issue highlights the merits of transmit RF arrays. Configurations using 2, 8, 16, or 32 transmit channels were successfully utilised for a spectrum of applications, ranging from functional brain mapping at 9.4 T with 1 mm spatial resolution [33] to high spatial and temporal resolution spin-echo line scanning that ensures microvascular specificity of functional responses [34], as well as high spatiotemporal resolution quantification of near-wall haemodynamic parameters in in vitro intracranial aneurysms [35], resolving wall shear stress patterns, and cardiac and body imaging in humans and large experimental models at 7 T [36] and 10.5 T [8].…”

“…As we move up the mountain face, we should pause for a moment to acknowledge and applaud the pioneers who climbed the very steep mountains to 9.4 T [33], 10.5 T [8,37,38] and 11.7 T [39]. These explorers were essentially free climbing, reaching the summit without vendor-provided RF coils and without push-button software, yet nevertheless navigating through the enormous challenges towards beautiful images of the brain and the body enabled by parallel transmission.…”

“…One example is provided by Raimondo et al, who paved the way for spin-echo line scanning, which can achieve spatial and temporal resolutions of 0.25 mm and 200 ms, respectively as pointed out in [34] of this Special Issue. Alternatively, highly anisotropic FLASH acquisitions can provide spatial resolutions of 0.1 mm along one axis, to match the columnar or laminar structure of the cortex as demonstrated in [33] of this Special Issue. When leaving the well-trodden path of 'functional' MRI, ultrahigh-field MR scanners also offer an increase in spatial and spectral resolution to investigate brain metabolism and neuro-metabolites such as GABA or glutamine and glutamate [42], and can provide incredibly detailed post-mortem brain images [33].…”

“…Alternatively, highly anisotropic FLASH acquisitions can provide spatial resolutions of 0.1 mm along one axis, to match the columnar or laminar structure of the cortex as demonstrated in [33] of this Special Issue. When leaving the well-trodden path of 'functional' MRI, ultrahigh-field MR scanners also offer an increase in spatial and spectral resolution to investigate brain metabolism and neuro-metabolites such as GABA or glutamine and glutamate [42], and can provide incredibly detailed post-mortem brain images [33]. Given how busy Everest is nowadays, we hope that these first experiments are similarly just the beginning of a thriving and innovative neuroscience community exploring the human brain using the vast potential of ultrahigh-field MRI.…”

Detection of sub-nmol amounts of the antiviral drug favipiravir in 19F MRI using photo-chemically induced dynamic nuclear polarization

“…The SNR is approximately a quadratic function of the magnetic field strength 3 although other factors such as coil characteristics also play an important role. For clinical applications 7 T whole body MRI was recently introduced, and even higher magnetic fields are already available for human research MRI and experimental MRS 4 . However, in addition to being very expensive, ultra high field MRI also poses unique technical problems and altered relaxation times, all of which affect image contrast.…”