NeuroImage
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Carbon dioxide is a potent cerebral vasodilator. We have identified a significant source of low-frequency variation in blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal at 3 T arising from spontaneous fluctuations in arterial carbon dioxide level in volunteers at rest. Fluctuations in the partial pressure of end-tidal carbon dioxide (Pet(CO(2))) of +/-1.1 mm Hg in the frequency range 0-0.05 Hz were observed in a cohort of nine volunteers. ⋯ Doppler ultrasound suggests that a component of low-frequency BOLD signal fluctuations is mediated by CO(2)-induced changes in cerebral blood flow (CBF). These fluctuations are a source of physiological noise and a potentially important confounding factor in fMRI paradigms that modify breathing. However, they can also be used for mapping regional vascular responsiveness to CO(2).
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FMRI modelling requires flexible haemodynamic response function (HRF) modelling, with the HRF being allowed to vary spatially and between subjects. To achieve this flexibility, voxelwise parameterised HRFs have been proposed; however, inference on such models is very slow. An alternative approach is to use basis functions allowing inference to proceed in the more manageable General Linear Model (GLM) framework. ⋯ Here we extend the work of Penny et al. to give inference on the GLM with constrained HRF basis functions and with spatial Markov Random Fields on the autoregressive noise parameters. Constraining the subspace spanned by the basis set allows for far superior separation of activating voxels from nonactivating voxels in FMRI data. We use spatial mixture modelling to produce final probabilities of activation and demonstrate increased sensitivity on an FMRI dataset.
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A method for quantitative determination of the glutamate (Glu) concentration in human brain using PRESS-based single voxel MR spectroscopy (MRS) at 3 T has been developed and validated by repeatedly analyzing voxels comprising the anterior cingulate cortex (acc) and the left hippocampus (hc) in 40 healthy volunteer brains. At an optimum echo time of 80 ms, the C4 resonance of Glu appears well resolved and separated from major interferents, that is, glutamine and N-acetylaspartate. As a complementary method, a multiple quantum coherence filter sequence for Glu was employed. ⋯ When the concentrations were corrected by individual cerebrospinal fluid fractions obtained by segmentation using spm, CVs tended to increase and the correlation coefficients for the two MRS sessions tended to decrease, indicating that this type of correction adds uncertainty to the data. The concentrations of Glu in the two voxels studied were found to be significantly different (11.6 mmol/l in acc, 10.9 mmol/l in hc, P = 0.023) and decrease with age (P < 0.04). These concentrations agreed well with those determined using the quantum coherence filter method although the uncertainty of the latter limits reliable analysis.
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White matter hyperintensities (WMHs) are commonly seen on brain magnetic resonance imaging (MRI) scans of elderly individuals, but their functional significance remains controversial. We used perfusion-weighted MRI to determine the impact of WMHs on cortical regional cerebral blood volume (rCBV). We studied 24 elderly stroke patients and 27 control subjects with conventional MRI which included T2-weighted FLAIR coronal slices through whole brain and gadolinium-DTPA (0.2 mmol/kg)-based perfusion MRI (pMRI) with echo planar imaging. ⋯ Although the trends of correlation still existed when the two groups were analyzed separately, they were not significant. The correlations between cortical rCBV and WMHs in the same lobe were significant for subjects with more severe hyperintensities irrespective of the group. In conclusion, T2-weighted WMHs are associated with reduced rCBV in the cerebral cortex, particularly in individuals with extensive hyperintensities.
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Clinical Trial
fMRI evidence of brain reorganization during attention and memory tasks in multiple sclerosis.
Functional magnetic resonance imaging (fMRI) data on motor function have shown adaptive functional changes related to brain injury in multiple sclerosis (MS). We investigated whether patients with MS have altered fMRI activation patterns during attention and memory tasks, and whether functional changes in the brain correlate with the extent of overall tissue damage on conventional MRI. Twenty-two right-handed patients with relapsing-remitting MS (RRMS) and no or only mild deficits at neuropsychological testing and 22 matched healthy subjects were scanned during the Paced Auditory Serial Addition Test (PASAT) and a recall task. fMRI data were analyzed using Statistical Parametric Mapping (SPM99). ⋯ Patients with RRMS exhibit altered patterns of activation during tasks exploring sustained attention, information processing and memory. During these tasks, fMRI activity is greater in patients with better cognitive function than in those with lower cognitive function. Functional changes in specific brain areas increase with increasing tissue damage suggesting that they may also represent adaptive mechanisms that reflect underlying neural disorganization or disinhibition, possibly associated with MS.