J Bause

Quantitative and functional pulsed arterial spin labeling in the human brain at 9.4 t

The feasibility of multislice pulsed arterial spin labeling (PASL) of the human brain at 9.4 T was investigated. To demonstrate the potential of arterial spin labeling (ASL) at this field strength, quantitative, functional, and high‐resolution (1.05 × 1.05 × 2 mm3) ASL experiments were performed.

Whole brain MP2RAGE-based mapping of the longitudinal relaxation time at 9.4T.

ABSTRACT Mapping of the longitudinal relaxation time (T1) with high accuracy and precision is central for neuroscientific and clinical research, since it opens up the possibility to obtain accurate brain tissue segmentation and gain myelin‐related information. An ideal, quantitative method should enable whole brain coverage within a limited scan time yet allow for detailed sampling with sub‐millimeter voxel sizes. The use of ultra‐high magnetic fields is well suited for this purpose, however the inhomogeneous transmit field potentially hampers its use. In the present work, we conducted whole brain T1 mapping based on the MP2RAGE sequence at 9.4 T and explored potential pitfalls for automated tissue classification compared with 3 T. Data accuracy and T2‐dependent variation of the adiabatic inversion efficiency were investigated by single slice T1 mapping with inversion recovery EPI measurements, quantitative T2 mapping using multi‐echo techniques and simulations of the Bloch equations. We found that the prominent spatial variation of the transmit field at 9.4 T (yielding flip angles between 20% and 180% of nominal values) profoundly affected the result of image segmentation and T1 mapping. These effects could be mitigated by correcting for both flip angle and inversion efficiency deviations. Based on the corrected T1 maps, new, ‘flattened’, MP2RAGE contrast images were generated, that were no longer affected by variations of the transmit field. Unlike the uncorrected MP2RAGE contrast images acquired at 9.4 T, these flattened images yielded image segmentations comparable to 3 T, making bias‐field correction prior to image segmentation and tissue classification unnecessary. In terms of the T1 estimates at high field, the proposed correction methods resulted in an improved precision, with test‐retest variability below 1% and a coefficient‐of‐variation across 25 subjects below 3%. HIGHLIGHTST1 maps convey accurate brain tissue segmentation and myelin‐related information.B1+ inhomogeneity and short tissue T2 limit T1 accuracy at high fields.A correction method is proposed and validated against 3T data.Improved T1 precision and image segmentations are demonstrated at 9.4T.

Fast and efficient free induction decay MR spectroscopic imaging of the human brain at 9.4 Tesla

The purpose of this work was to develop a fast and efficient MRSI‐FID acquisition scheme and test its performance in vivo. The aim was to find a trade‐off between the minimal total acquisition time and signal‐to‐noise ratio of the acquired spectra.

Efficient generation of T2*-weighted contrast by interslice echo-shifting for human functional and anatomical imaging at 9.4 Tesla

Standard gradient‐echo sequences are often prohibitively slow for T2* ‐weighted imaging as long echo times prolong the repetition time of the sequence. Echo‐shifting offers a way out of this dilemma by allowing an echo time that exceeds the repetition time. The purpose of this work is to present a gradient‐echo sequence that is optimized for multislice T2* ‐weighted imaging applications by combining echo‐shifting with an interleaved slice excitation order.

Depth‐dependence of visual signals in the human superior colliculus at 9.4 T

The superior colliculus (SC) is a layered structure located in the midbrain. We exploited the improved spatial resolution and BOLD signal strength available at 9.4 T to investigate the depth profile of visual BOLD responses in the human SC based on distortion‐corrected EPI data with a 1 mm isotropic resolution. We used high resolution (350 µm in‐plane) anatomical images to determine regions‐of‐interest of the SC and applied a semi‐automated method to segment it into superficial, intermediate, and deep zones. A greater than linear increase in sensitivity of the functional signal at 9.4 T allowed us to detect a statistically significant depth pattern in a group analysis with a 20 min stimulation paradigm. Descriptive data showed consistent depth profiles also in single individuals. The highest signals were localized to the superficial layers of the right and left SC during contralateral stimulation, which was in good agreement with its functional architecture known from non‐human primates. This study thus demonstrates the potential of 9.4 T MRI for functional neuroimaging even in deeply located, particularly challenging brain structures such as the SC. Hum Brain Mapp 38:574–587, 2017.

In-vivo quantitative structural imaging of the human midbrain and the superior colliculus at 9.4T

&NA; We explored anatomical details of the superior colliculus (SC) by in vivo magnetic resonance imaging (MRI) at 9.4T. The high signal‐to‐noise ratio allowed the acquisition of high resolution, multi‐modal images with voxel sizes ranging between 176 × 132 × 600 &mgr;m and (800)3&mgr;m. Quantitative mapping of the longitudinal relaxation rate R1, the effective transverse relaxation rate R2*, and the magnetic susceptibility QSM was performed in 14 healthy volunteers. The images were analyzed in native space as well as after normalization to a common brain space (MNI). The coefficient‐of‐variation (CoV) across subjects was evaluated in prominent regions of the midbrain, reaching the best reproducibility (CoV of 5%) in the R2* maps of the SC in MNI space, while the CoV in the QSM maps remained high regardless of brain‐space. To investigate whether more complex neurobiological architectural features could be detected, depth profiles through the SC layers towards the red nucleus (RN) were evaluated at different levels of the SC along the rostro‐caudal axis. This analysis revealed alterations of the quantitative MRI parameters concordant with previous post mortem histology studies of the cyto‐ and myeloarchitecture of the SC. In general, the R1 maps were hyperintense in areas characterized by the presence of abundant myelinated fibers, and likely enabled detection of the deep white layer VII of the SC adjacent to the periaqueductal gray. While R1 maps failed to reveal finer details, possibly due to the relatively coarse spatial sampling used for this modality, these could be recovered in R2* maps and in QSM. In the central part of the SC along its rostro‐caudal axis, increased R2* values and decreased susceptibility values were observed 2 mm below the SC surface, likely reflecting the myelinated fibers in the superficial optic layer (layer III). Towards the deeper layers, a second increase in R2* was paralleled by a paramagnetic shift in QSM suggesting the presence of an iron‐rich layer about 3 mm below the surface of the SC, attributed to the intermediate gray layer (IV) composed of multipolar neurons. These results dovetail observations in histological specimens and animal studies and demonstrate that high‐resolution multi‐modal MRI at 9.4T can reveal several microstructural features of the SC in vivo. HighlightsWe show quantitative MRI at 9.4T of the human brain stem, with smallest voxel size of 132x176x600&mgr;m (13.9nanoliters) from 14 healthy subjects.MRI shows microstructure and is concordant with histology studies of the cyto‐ and myeloarchitecture of the brain stem.At least two of the seven layers of the Superior Colliculus can reliably be identified in vivo.

Comparison of prospective head motion correction with NMR field probes and an optical tracking system

The aim of this study was to compare prospective head motion correction and motion tracking abilities of two tracking systems: Active NMR field probes and a Moiré phase tracking camera system using an optical marker.

Dynamic B0 shimming of the human brain at 9.4 T with a 16-channel multi-coil shim setup

A 16‐channel multi‐coil shimming setup was developed to mitigate severe B0 field perturbations at ultrahigh field and improve data quality for human brain imaging and spectroscopy.