NeuroImage
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Connectivity-based segmentation has been used to identify functional gray matter subregions that are not discernable on conventional magnetic resonance imaging. However, the accuracy and reliability of this technique has only been validated using indirect means. In order to provide direct electrophysiologic validation of connectivity-based thalamic segmentations within human subjects, we assess the correlation of atlas-based thalamic anatomy, connectivity-based thalamic maps, and somatosensory evoked thalamic potentials in two adults with medication-refractory epilepsy who were undergoing intracranial EEG monitoring with intrathalamic depth and subdural cortical strip electrodes. ⋯ This study provides direct electrophysiologic validation of probabilistic tractography-based thalamic segmentation. Importantly, this study provides an electrophysiological basis for using connectivity-based segmentation to further study subcortical anatomy and physiology while also providing the clinical basis for targeting deep brain nuclei with therapeutic stimulation. Finally, these direct recordings from human thalamus confirm early inferences of a sensory thalamic component of the N18 waveform in somatosensory evoked potentials.
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MRI estimates of brain iron concentration in normal aging using quantitative susceptibility mapping.
Quantifying tissue iron concentration in vivo is instrumental for understanding the role of iron in physiology and in neurological diseases associated with abnormal iron distribution. Herein, we use recently-developed Quantitative Susceptibility Mapping (QSM) methodology to estimate the tissue magnetic susceptibility based on MRI signal phase. To investigate the effect of different regularization choices, we implement and compare ℓ1 and ℓ2 norm regularized QSM algorithms. ⋯ However, QSM values are influenced by factors such as the myelin content, whereas FDRI is a more specific indicator of iron content. Hence, FDRI demonstrated higher specificity to iron yet yielded noisier data despite longer scan times and lower spatial resolution than QSM. The robustness, practicality, and demonstrated ability of predicting the change in iron deposition in adult aging suggest that regularized QSM algorithms using single-field-strength data are possible alternatives to tissue iron estimation requiring two field strengths.
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Recent multiple sclerosis (MS) MRI research has highlighted the need to move beyond the lesion-centric view and to develop and validate new MR imaging strategies that quantify the invisible burden of disease in the brain and establish much more sensitive and specific surrogate markers of clinical disability. One of the most promising of such measures is myelin-selective MRI that allows the acquisition of myelin water fraction (MWF) maps, a parameter that is correlated to brain white matter (WM) myelination. The aim of our study was to apply the newest myelin-selective MRI method, multi-component Driven Equilibrium Single Pulse Observation of T1 and T2 (mcDESPOT) in a controlled clinical MS pilot trial. ⋯ Multivariate analysis revealed that global atrophy, mean whole-brain myelin water fraction and white matter atrophy were the three most important image-derived parameters for predicting clinical disability (EDSS). Overall, our results demonstrate that mcDESPOT-defined measurements in NAWM show great promise as imaging markers of global clinical disease activity in MS. Further investigation will determine if this measure can serve as a risk factor for the conversion into definite MS and for the secondary transition into irreversible disease progression.
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Mild traumatic brain injury (mTBI) due to explosive blast is common among military service members and often associated with long term psychological and cognitive disruptions. Little is known about the neurological effects of blast-related mTBI and whether they differ from those of civilian, non-blast mTBI. Given that brain damage from blasts may be diffuse and heterogeneous, we tested the hypothesis that blast mTBI is associated with subtle white matter disruptions in the brain that are spatially inconsistent across individuals. ⋯ Additionally, individuals with more than one blast mTBI tended to have a larger number of low FA voxels than individuals with a single blast injury. These results indicate that blast mTBI is associated with disrupted integrity of several white matter tracts, and that these disruptions are diluted by averaging across the large number of voxels within an ROI. The reported pattern of effects supports the conclusion that the neurological effects of blast mTBI are diffuse, widespread, and spatially variable.
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Hyperoxia or hypercapnia provides a useful experimental tool to systematically alter the blood oxygenation level dependent (BOLD) contrast. Typical applications include calibrated functional magnetic resonance imaging (fMRI), BOLD sensitivity mapping, vessel size imaging or cerebrovascular reactivity mapping. This article describes a novel biophysical model of hyperoxic and hypercapnic BOLD contrast, which accounts for the magnetic susceptibility effects of molecular oxygen that is dissolved in blood and tissue, in addition to the well-established effects caused by the paramagnetic properties of deoxyhaemoglobin. ⋯ Simulations were carried out for different levels of metabolic oxygen extraction fraction (OEF) ranging from 0 to 0.5. The key findings can be summarised as follows: (i) for hyperoxia the susceptibility of dissolved O2 may lead to a significant arterial BOLD contrast; (ii) under normoxic conditions the susceptibility of dissolved O2 is negligible; (iii) an almost complete loss of BOLD sensitivity may occur at lower OEF values in all parts of the vascular tree, whereas hyperoxic BOLD sensitivity is largely maintained; (iv) under hyperoxic conditions, a transition from positive to negative BOLD contrast occurs with decreasing OEF values. These findings have important implications for experimental applications of hyperoxic and hypercapnic BOLD contrast and may enable new clinical applications in ischemic stroke and other forms of acquired brain injury.