No 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.

High-resolution blood volume-sensitive fMRI VASO can capture functional signal changes with high localization specivity compared to conventional BOLD sequences. However, it suffers from reduced sampling efficiencies and lower detection sensitivity. Here, we overcome these limitations by combining VASO contrasts with the high efficiency of multi-echo enabled spiral k-space sampling and CAIPI-optimized SMS acceleration. We find that spiral sampling enables faster VASO acquisitions than possible with Cartesian EPI sampling. For high spatial resolutions (1.25mm), we confirm that the proposed method is insensitive to large draining veins while having more stable fMRI signals (higher tSNR).

We developed several ASL sequences at 7T with a FAIR-QUIPSS II labeling scheme and various spiral readout strategies using Pulseq. Iterative algebraic image reconstruction was performed with CG-SENSE, using the field evolution data measured with external NMR probes. Robust performance in detecting brain’s perfusion signal was observed in 2D single- and multi-band spiral acquisitions especially at relatively high spatial resolution, without the requirement for a longer scan time. 3D spiral acquisition showed reduced contrast level in perfusion maps and requires further investigation and optimization.

VASO fMRI can provide beneficial localization specificity and quantifiability compared to the commonly used BOLD contrast. Previous work has also shown the benefits of using spiral readouts compared to Cartesian. In this work, we explore the benefits of 3D stack of spirals readouts and compare it with the current state of the art 3D EPI readouts for VASO fMRI. The sequence implementation is done using Pulseq, images were reconstructed using gpuNUFFT; functional analysis with an openly available pipeline. We find that a tSNR efficiency improvement of a factor of 2.5 over EPI is achieved using the proposed spiral implementation.