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    ABSTRACT: Purpose: To examine the effects of the reconstruction algorithm of magnitude images from multichannel diffusion MRI on fiber orientation estimation. Theory and methods: It is well established that the method used to combine signals from different coil elements in multichannel MRI can have an impact on the properties of the reconstructed magnitude image. Using a root-sum-of-squares approach results in a magnitude signal that follows an effective noncentral-χ distribution. As a result, the noise floor, the minimum measurable in the absence of any true signal, is elevated. This is particularly relevant for diffusion-weighted MRI, where the signal attenuation is of interest. Results: In this study, we illustrate problems that such image reconstruction characteristics may cause in the estimation of fiber orientations, both for model-based and model-free approaches, when modern 32-channel coils are used. We further propose an alternative image reconstruction method that is based on sensitivity encoding (SENSE) and preserves the Rician nature of the single-channel, magnitude MR signal. We show that for the same k-space data, root-sum-of-squares can cause excessive overfitting and reduced precision in orientation estimation compared with the SENSE-based approach. Conclusion: These results highlight the importance of choosing the appropriate image reconstruction method for tractography studies that use multichannel receiver coils for diffusion MRI acquisition.
    Full-text · Article · Dec 2013 · Magnetic Resonance in Medicine
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    ABSTRACT: Transcranial direct current stimulation (tDCS) has been used to modify motor performance in healthy and patient populations. However, our understanding of the large-scale neuroplastic changes that support such behavioural effects is limited. Here, we used both seed-based and independent component analyses (ICA) approaches to probe tDCS-induced modifications in resting state activity with the aim of establishing the effects of tDCS applied to the primary motor cortex (M1) on both motor and non-motor networks within the brain. Subjects participated in three separate sessions, during which resting fMRI scans were acquired before and after ten minutes of 1 mA anodal, cathodal, or sham tDCS. Cathodal tDCS increased the inter-hemispheric coherence of resting fMRI signal between the left and right supplementary motor area (SMA), and between the left and right hand areas of M1. A similar trend was documented for the premotor cortex (PMC). Increased functional connectivity following cathodal tDCS was apparent within the ICA-generated motor and default mode networks. Additionally, the overall strength of the default mode network was increased. Neither anodal nor sham tDCS produced significant changes in resting state connectivity. This work indicates that cathodal tDCS to M1 affects the motor network at rest. In addition, the effects of cathodal tDCS on the default mode network support the hypothesis that diminished top-down control may contribute to the impaired motor performance induced by cathodal tDCS.
    Full-text · Article · Nov 2013 · NeuroImage
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    ABSTRACT: Spontaneous fluctuations in activity in different parts of the brain can be used to study functional brain networks. We review the use of resting-state functional MRI (rfMRI) for the purpose of mapping the macroscopic functional connectome. After describing MRI acquisition and image-processing methods commonly used to generate data in a form amenable to connectomics network analysis, we discuss different approaches for estimating network structure from that data. Finally, we describe new possibilities resulting from the high-quality rfMRI data being generated by the Human Connectome Project and highlight some upcoming challenges in functional connectomics.
    No preview · Article · Nov 2013 · Trends in Cognitive Sciences
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