In Vivo Measurement of Rat Brain Water Content at 9.4 T MR Using Super-Resolution Reconstruction: Validation With Ex Vivo Experiments.

BACKGROUND: Given that changes in brain water content are often correlated with disease, investigating water content non-invasively and in vivo could lead to a better understanding of the pathogenesis of several neurologic diseases. PURPOSE: To adapt a super-resolution-based technique, previously developed for humans, to the rat brain and report in vivo high-resolution (HR) water content maps in comparison with ex vivo wet/dry methods. STUDY TYPE: Prospective. ANIMAL MODEL: Eight healthy male Wistar rats. FIELD STRENGTH/SEQUENCE: 9.4-T, multi-echo gradient-echo (mGRE) sequence. ASSESSMENT: Using super-resolution reconstruction (SRR), a HR mGRE image (200 µm isotropic) was reconstructed from three low-resolution (LR) orthogonal whole-brain images in each animal, which was followed by water content mapping in vivo. The animals were subsequently sacrificed, the brains excised and divided into five regions (front left, front right, middle left, middle right, and cerebellum-brainstem regions), and the water content was measured ex vivo using wet/dry measurements as the reference standard. The water content values of the in vivo and ex vivo methods were then compared for the whole brain and also for the different regions separately. STATISTICAL TESTS: Friedman's non-parametric test was used to test difference between the five regions, and Pearson's correlation coefficient was used for correlation between in vivo and ex vivo measurements. A P-value <0.05 was considered statistically significant. RESULTS: Water content values derived from in vivo MR measurements showed strong correlations with water content measured ex vivo at a regional level (r = 0.902). Different brain regions showed significantly different water content values. Water content values were highest in the frontal brain, followed by the midbrain, and lowest in the cerebellum and brainstem regions. DATA CONCLUSION: An in vivo technique to achieve HR isotropic water content maps in the rat brain using SRR was adopted in this study. The MRI-derived water content values obtained using the technique showed strong correlations with water content values obtained using ex vivo wet/dry methods. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 1.

Automatic Segmentation of Human Cortical Layer-Complexes and Architectural Areas Using Ex vivo Diffusion MRI and Its Validation.

Recently, several magnetic resonance imaging contrast mechanisms have been shown to distinguish cortical substructure corresponding to selected cortical layers. Here, we investigate cortical layer and area differentiation by automatized unsupervised clustering of high-resolution diffusion MRI data. Several groups of adjacent layers could be distinguished in human primary motor and premotor cortex. We then used the signature of diffusion MRI signals along cortical depth as a criterion to detect area boundaries and find borders at which the signature changes abruptly. We validate our clustering results by histological analysis of the same tissue. These results confirm earlier studies which show that diffusion MRI can probe layer-specific intracortical fiber organization and, moreover, suggests that it contains enough information to automatically classify architecturally distinct cortical areas. We discuss the strengths and weaknesses of the automatic clustering approach and its appeal for MR-based cortical histology.

Mapping tissue sodium concentration in the human brain: a comparison of MR sequences at 9.4Tesla.

Sodium is the second most abundant MR-active nucleus in the human body and is of fundamental importance for the function of cells. Previous studies have shown that many pathophysiological conditions induce an increase of the average tissue sodium concentration. To date, several MR sequences have been used to measure sodium. The aim of this study was to evaluate the performance and suitability of five different MR sequences for quantitative sodium imaging on a whole-body 9.4Tesla MR scanner. Numerical simulations, phantom experiments and in vivo imaging on healthy subjects were carried out. The results demonstrate that, of these five sequences, the Twisted Projection Imaging sequence is optimal for quantitative sodium imaging, as it combines a number of features which are particularly relevant in order to obtain high quality quantitative images of sodium. These include: ultra-short echo times, efficient k-space sampling, and robustness against off-resonance effects. Mapping of sodium in the human brain is a technique not yet fully explored in neuroscience. Ultra-high field sodium MRI may provide new insights into the pathogenesis of neurological disorders, and may help to develop new and disease-specific biomarkers for the early diagnosis and therapeutic intervention before irreversible brain damage has taken place.

B0 insensitive multiple-quantum resolved sodium imaging using a phase-rotation scheme.

Triple-quantum filtering has been suggested as a mechanism to differentiate signals from different physiological compartments. However, the filtering method is sensitive to static field inhomogeneities because different coherence pathways may interfere destructively. Previously suggested methods employed additional phase-cycles to separately acquire pathways. Whilst this removes the signal dropouts, it reduces the signal-to-noise per unit time. In this work we suggest the use of a phase-rotation scheme to simultaneously acquire all coherence pathways and then separate them via Fourier transform. Hence the method yields single-, double- and triple-quantum filtered images. The phase-rotation requires a minimum of 36 instead of six cycling steps. However, destructive interference is circumvented whilst maintaining full signal-to-noise efficiency for all coherences.

Advances in multimodal neuroimaging: hybrid MR-PET and MR-PET-EEG at 3 T and 9.4 T.

Multi-modal MR-PET-EEG data acquisition in simultaneous mode confers a number of advantages at 3 T and 9.4 T. The three modalities complement each other well; structural-functional imaging being the domain of MRI, molecular imaging with specific tracers is the strength of PET, and EEG provides a temporal dimension where the other two modalities are weak. The utility of hybrid MR-PET at 3 T in a clinical setting is presented and critically discussed. The potential problems and the putative gains to be accrued from hybrid imaging at 9.4 T, with examples from the human brain, are outlined. Steps on the road to 9.4 T multi-modal MR-PET-EEG are also illustrated. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320 µm are presented, setting the stage for hybrid imaging at ultra-high field. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are presented. Examples of tumour imaging on a 3 T MR-PET system are presented and discussed. Finally, the perspectives for multi-modal imaging are discussed based on two on-going studies, the first comparing MR and PET methods for the measurement of perfusion and the second which looks at tumour delineation based on MRI contrasts but the knowledge of tumour extent is based on simultaneously acquired PET data.

EEG acquisition in ultra-high static magnetic fields up to 9.4 T.

The simultaneous acquisition of electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) data has gained momentum in recent years due to the synergistic effects of the two modalities with regard to temporal and spatial resolution. Currently, only EEG-data recorded in fields of up to 7 T have been reported. We investigated the feasibility of recording EEG inside a 9.4 T static magnetic field, specifically to determine whether meaningful EEG information could be recovered from the data after removal of the cardiac-related artefact. EEG-data were recorded reliably and reproducibly at 9.4 T and the cardiac-related artefact increased in amplitude with increasing B0, as expected. Furthermore, we were able to correct for the cardiac-related artefact and identify auditory event related responses at 9.4 T in 75% of subjects using independent component analysis (ICA). Also by means of ICA we detected event related spectral perturbations (ERSP) in subjects at 9.4 T in response to opening/closing the eyes comparable with the response at 0 T. Overall our results suggest that it is possible to record meaningful EEG data at ultra-high magnetic fields. The simultaneous EEG-fMRI approach at ultra-high-fields opens up the horizon for investigating brain dynamics at a superb spatial resolution and a temporal resolution in the millisecond domain.