Magnetic resonance imaging
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A quantitative analysis of cerebellar metabolites in normal subjects has been performed by proton MR spectroscopy (MRS) with relaxation time correction. Quantitation was carried out in seven healthy human subjects with the well-established LCModel program. The prior knowledge utilized for quantitation was obtained from solutions containing the major brain metabolites and MRS investigated under the same experimental conditions. ⋯ Both in vitro (for the prior knowledge template) and in vivo data were acquired separately at 1.5 T by PRESS sequence (TR, 1500 ms; TE, 30 ms). The absolute concentration of main cerebellar metabolites was corrected for relaxation time effects. Different noise and line broadening conditions were considered and simulated in the spectral processing in order to evaluate the effect of spectral quality on the concentration estimates.
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The "direct detection" of neuronal activity by MRI could offer improved spatial and temporal resolution compared to the blood oxygenation level-dependent (BOLD) effect. Here we describe initial attempts to use MRI to detect directly the neuronal currents resulting from spontaneous alpha wave activity, which have previously been shown to generate the largest extracranial magnetic fields. Experiments were successfully carried out on four subjects at 3 T. ⋯ It was conservatively assumed that if oscillations occurred at the same frequency in the magnitude signal from the same region or at the same frequency in the phase or magnitude signal from other regions overlying large vessels or cerebrospinal fluid (CSF), then the phase changes were not due to neuronal activity related to alpha waves. Using these criteria the data obtained were consistent with direct detection of alpha wave activity in three of the four volunteers. No significant MR signal fluctuations due to evoked activity were identified.
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Simultaneous EEG-fMRI is a powerful tool to study spontaneous and evoked brain activity because of the complementary advantages of the two techniques in terms of temporal and spatial resolution. In recent years, a significant number of scientific works have been published on this subject. However, many technical problems related to the intrinsic incompatibility of EEG and MRI methods are still not fully solved. ⋯ Thus, timing issue must be carefully considered in order to avoid significant methodological errors. The aim of the present work is to highlight and discuss some of technical and methodological open issues associated with the combined use of EEG and fMRI. These issues are presented in the context of preliminary data regarding simultaneous acquisition of event-related evoked potentials and BOLD images during a visual odd-ball paradigm.
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High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadened in vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral line width are therefore of critical importance. ⋯ Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral line widths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral line width--directly correlated with improved spectral quality--can only be achieved when voxel volumes are significantly reduced. Our line width optimization was sufficient to permit clear glutamate (Glu)-glutamine separation, yielding distinct Glu maps for brain areas including regions of greatly different Glu concentration (e.g., ventricles vs. surrounding tissue).
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In the present paper, for the first time, the feasibility to detect primary magnetic field changes caused by neuronal activity in vivo by spin-echo (SE) magnetic resonance imaging (MRI) is investigated. The detection of effects more directly linked to brain activity than secondary hemodynamic-metabolic changes would enable the study of brain function with improved specificity. However, the detection of neuronal currents by MRI is hampered by such accompanying hemodynamic changes. ⋯ At this aim, we propose the combined use of visual evoked potential (VEP) recordings and BOLD-fMRI measurements prior to SE MRI scanning. Moreover, we exemplify by theory and experimentation how the control of artefactual signal changes due to BOLD and movement effects may be further improved by the experimental design. Finally, results from a pilot study using the proposed combination of VEP recordings and MRI techniques are reported, suggesting the feasibility of this method.