NeuroImage
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Comparative Study
Investigating the benefits of multi-echo EPI for fMRI at 7 T.
Functional MRI studies on humans with BOLD contrast are increasingly performed at high static magnetic field in order to exploit the increased sensitivity. The downside of high-field fMRI using the gradient-echo echo-planar imaging (GE-EPI) method is that images are typically very strongly affected by image distortion and signal loss. It has been demonstrated at 1.5 T and 3 T that image artifacts can be reduced and functional sensitivity simultaneously increased by the use of parallel-accelerated multi-echo EPI. ⋯ The consequence is that (d) in practice the performance of simple echo summation at very high field is comparable to that based on a CNR filter. Finally, (e) temporal noise observed in the different echo time courses is not strongly correlated, thus explaining why echo summation is advantageous. The results at typical spatial resolution show that multi-echo EPI acquisition leads to considerable artifact reduction and sensitivity gains, making it superior to conventional GE-EPI for fMRI at 7 T.
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T2-weighted gradient echo (GE) images yield good contrast of iron-rich structures like the subthalamic nuclei due to microscopic susceptibility induced field gradients, providing landmarks for the exact placement of deep brain stimulation electrodes in Parkinson's disease treatment. An additional advantage is the low radio frequency (RF) exposure of GE sequences. However, T2-weighted images are also sensitive to macroscopic field inhomogeneities, resulting in signal losses, in particular in orbitofrontal and temporal brain areas, limiting anatomical information from these areas. ⋯ In a second step, intensity corrected images acquired at different echo times TE are combined using optimized weighting factors: in areas not affected by macroscopic field inhomogeneities, data acquired at long TE are weighted more strongly to achieve the contrast required. For large field gradients, data acquired at short TE are favored to avoid signal losses. When compared to the original data sets acquired at different TE and the respective intensity corrected data sets, the resulting combined data sets feature reduced signal losses in areas with major field gradients, while intensity profiles and a contrast-to-noise (CNR) analysis between subthalamic nucleus, red nucleus and the surrounding white matter demonstrate good contrast in deep brain areas.
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A visual task for semantic access involves a number of brain regions. However, previous studies either examined the role of each region separately using univariate approach, or analyzed a single brain network using covariance connectivity analysis. ⋯ Our results demonstrated that there were three task-related independent components (ICs), corresponding to various cognitive components involved in the visual task. Furthermore, ICA separation on the auditory task showed consistency of the results with our hypothesis, regardless of the input modalities.
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Recently, there has been strong interest in the development of imaging techniques to quantify axonal and myelin pathology in patients with multiple sclerosis (MS). Optic neuritis, a condition characterised by inflammatory demyelination of the optic nerve, is one of the commonest sites of MS relapse, and exhibits similar pathological alterations to MS lesions elsewhere in the central nervous system (CNS). Unlike other regions of the CNS, however, the function of the optic nerve can be accurately assessed using clinical measures, as well as electrophysiological techniques such as visual evoked potential recordings. ⋯ To further investigate this disassociation, we used linear regression analysis with optic nerve atrophy and optic nerve FA as independent variables and mfVEP amplitude as the dependent variable. The resulting linear regression model was highly significant (R=0.819, p=0.001). These results show that, 4 years after unilateral optic neuritis, MRI-based measures of optic nerve structural abnormalities (decreased anisotropy and volume) independently predict visual dysfunction.