Vessel size imaging is a new method that is based on simultaneous measurement of the changes ΔR2 and ΔR *2 in relaxation rate constants induced by the injection of an intravascular superparamagnetic contrast agent. Using the static dephasing approximation for ΔR *2 estimation and the slow‐diffusion approximation for ΔR2 estimation, it is shown that the ratio ΔR2/ΔR *2 can be expressed as a function of the susceptibility difference between vessels and brain tissue, the brain water diffusion coefficient, and a weighted mean of vessel sizes. Comparison of the results with 1) the Monte Carlo simulations used to quantify the relationship between tissue parameters and susceptibility contrast, 2) the experimental MRI data in the normal rat brain, and 3) the histologic data establishes the validity of this approach. This technique, which allows images of a weighted mean of the vessel size to be obtained, could be useful for in vivo studies of tumor vascularization. Magn Reson Med 45:397–408, 2001. © 2001 Wiley‐Liss, Inc.

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There has recently been a growing interest in the use of simultaneous electroencephalography (EEG) and functional MRI (fMRI) for evoked activity in cognitive paradigms, thereby obtaining functional datasets with both high spatial and temporal resolution. The simultaneous recording permits obtaining event-related potentials (ERPs) and MR images in the same environment, conditions of stimulation, and subject state; it also enables tracing the joint fluctuations of EEG and fMRI signals. The goal of this study was to investigate the possibility of tracking the trial-to-trial changes in event-related EEG activity, and of using this information as a parameter in fMRI analysis. We used an auditory oddball paradigm and obtained single-trial amplitude and latency features from the EEG acquired during fMRI scanning. The single-trial P300 latency presented significant correlation with parameters external to the EEG (target-to-target interval and reaction time). Moreover, we obtained significant fMRI activations for the modulation by P300 amplitude and latency, both at the single-subject and at the group level. Our results indicate that, in line with other studies, the EEG can bring a new dimension to the field of fMRI analysis by providing fine temporal information on the fluctuations in brain activity.

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We used whole-head magnetoencephalography to study the representation of objects in visual short-term memory (VSTM) in the human brain. Subjects remembered the location and color of either two or four colored disks that were encoded from the left or right visual field (equal number of distractors in the other visual hemifield). The data were analyzed using time-frequency methods, which enabled us to discover a strong oscillatory activity in the 8-15 Hz band during the retention interval. The study of the alpha power variation revealed two types of responses, in different brain regions. The first was a decrease in alpha power in parietal cortex, contralateral to the stimuli, with no load effect. The second was an increase of alpha power in parietal and lateral prefrontal cortex, as memory load increased, but without interaction with the hemifield of the encoded stimuli. The absence of interaction between side of encoded stimuli and memory load suggests that these effects reflect distinct underlying mechanisms. A novel method to localize the neural generators of load-related oscillatory activity was devised, using cortically-constrained distributed source-localization methods. Some activations were found in the inferior intraparietal sulcus (IPS) and intraoccipital sulcus (IOS). Importantly, strong oscillatory activity was also found in dorsolateral prefrontal cortex (DLPFC). Alpha oscillatory activity in DLPFC was synchronized with the activity in parietal regions, suggesting that VSTM functions in the human brain may be implemented via a network that includes bilateral DLPFC and bilateral IOS/IPS as key nodes.

A component of the event-related magnetic field (ERMF) response was observed in magnetoencephalographic signals recorded during the maintenance of information in visual short-term memory (VSTM). This sustained posterior contralateral magnetic (SPCM) field is likely the magnetic equivalent of the sustained posterior contralateral negativity (SPCN) found in electrophysiology. Magnetoencephalography data showed, at the sensor level, a bilateral activation over the parietal cortex that increased in amplitude for higher memory load. Others sensors, also over the parietal cortex, showed an activation pattern similar to the SPCN with higher activation for the hemisphere contralateral to the visual field from which visual information was encoded. These two activation patterns suggest that the SPCN and SPCM are generated by a network of cortical sources that includes bilateral parietal loci, likely intra-parietal/intra-occipital cortex, and contralateral parietal sources.

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