Coordinated anatomical and functional variability in the human brain during adolescence.

Adolescence represents a time of unparalleled brain development. In particular, developmental changes in morphometric and cytoarchitectural features are accompanied by maturation in the functional connectivity (FC). Here, we examined how three facets of the brain, including myelination, cortical thickness (CT), and resting-state FC, interact in children between the ages of 10 and 15. We investigated the pattern of coordination in these measures by computing correlation matrices for each measure as well as meta-correlations among them both at the regional and network levels. The results revealed consistently higher meta-correlations among myelin, CT, and FC in the sensory-motor cortical areas than in the association cortical areas. We also found that these meta-correlations were stable and little affected by age-related changes in each measure. In addition, regional variations in the meta-correlations were consistent with the previously identified gradient in the FC and therefore reflected the hierarchy of cortical information processing, and this relationship persists in the adult brain. These results demonstrate that heterogeneity in FC among multiple cortical areas are closely coordinated with the development of cortical myelination and thickness during adolescence.

Age-related topographic map of magnetic resonance diffusion metrics in neonatal brains.

Accelerated maturation of brain parenchyma close to term-equivalent age leads to rapid changes in diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) metrics of neonatal brains, which can complicate the evaluation and interpretation of these scans. In this study, we characterized the topography of age-related evolution of diffusion metrics in neonatal brains. We included 565 neonates who had MRI between 0 and 3 months of age, with no structural or signal abnormality-including 162 who had DTI scans. We analyzed the age-related changes of apparent diffusion coefficient (ADC) values throughout brain and DTI metrics (fractional anisotropy [FA] and mean diffusivity [MD]) along white matter (WM) tracts. Rate of change in ADC, FA, and MD values across 5 mm cubic voxels was calculated. There was significant reduction of ADC and MD values and increase of FA with increasing gestational age (GA) throughout neonates' brain, with the highest temporal rates in subcortical WM, corticospinal tract, cerebellar WM, and vermis. GA at birth had significant effect on ADC values in convexity cortex and corpus callosum as well as FA/MD values in corpus callosum, after correcting for GA at scan. We developed online interactive atlases depicting age-specific normative values of ADC (ages 34-46 weeks), and FA/MD (35-41 weeks). Our results show a rapid decrease in diffusivity metrics of cerebral/cerebellar WM and vermis in the first few weeks of neonatal age, likely attributable to myelination. In addition, prematurity and low GA at birth may result in lasting delay in corpus callosum myelination and cerebral cortex cellularity.

Comparison of regional skeletal muscle tissue oxygenation in college athletes and sedentary control subjects using quantitative BOLD MR imaging.

Blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging permits noninvasive assessment of tissue oxygenation. We hypothesized that BOLD imaging would allow for regional evaluation of differences in skeletal muscle oxygenation between athletes and sedentary control subjects, and dynamic BOLD responses to ischemia (i.e., proximal cuff occlusion) and reactive hyperemia (i.e., rapid cuff deflation) would relate to lower extremity function, as assessed by jumping ability. College football athletes (linemen, defensive backs/wide receivers) were compared to sedentary healthy controls. BOLD signal of the gastrocnemius, soleus, anterior tibialis, and peroneus longus was assessed for peak hyperemic value (PHV), time to peak (TTP), minimum ischemic value (MIV), and time to recovery (TTR). Significantly higher PHVs were identified in athletes versus controls for the gastrocnemius (linemen, 15.8 ± 9.1%; defensive backs/wide receivers, 17.9 ± 5.1%; controls, 7.4 ± 3.5%), soleus (linemen, 25.9 ± 11.5%; backs/receivers, 22.0 ± 9.4%; controls, 12.9 ± 5.8%), and anterior tibialis (linemen, 12.8 ± 5.3%; backs/receivers, 12.6 ± 3.9%; controls, 7.7 ± 4.0%), whereas no differences in PHV were found for the peroneus longus (linemen, 14.1 ± 6.9%; backs/receivers, 11.7 ± 4.6%; controls, 9.0 ± 4.9%). In all subject groups, the gastrocnemius and soleus muscles exhibited the lowest MIVs during cuff occlusion. No differences in TTR were found between muscles for any subject group. PHV of the gastrocnemius muscle was significantly and positively related to maximal vertical (r = 0.56, P = 0.002) and broad jump (r = 0.47, P = 0.01). These results suggest that BOLD MR imaging is a useful noninvasive tool for evaluating differences in tissue oxygenation of specific muscles between active and sedentary individuals, and peak BOLD responses may relate to functional capacity.

Non-invasive quantification of absolute cerebral blood volume during functional activation applicable to the whole human brain.

PURPOSE: Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in response to functional challenges along with changes in blood flow, blood oxygenation, and the cerebral metabolic rate of oxygen. The feasibility of a new method for non-invasive quantification of absolute cerebral blood volume that can be applicable to the whole human brain was investigated. METHODS: Multi-slice data were acquired at 3 T using a novel inversion recovery echo planar imaging (IR-EPI) pulse sequence with varying contrast weightings and an efficient rotating slice acquisition order, at rest and during visual activation. A biophysical model was used to estimate absolute cerebral blood volume at rest and during activation, and oxygenation during activation, on data from 13 normal human subjects. RESULTS: Cerebral blood volume increased by 21.7% from 6.6 ± 0.8 mL/100 mL of brain parenchyma at rest to 8.0 ± 1.3 mL/100 mL of brain parenchyma in the occipital cortex during visual activation, with average blood oxygenation of 84 ± 2.1% during activation, comparing well with literature. CONCLUSION: The method is feasible, and could foster improved understanding of the fundamental physiological relationship between neuronal activity, hemodynamic changes, and metabolism underlying brain activation; complement existing methods for estimating compartmental changes; and potentially find utility in evaluating vascular health.

Potential use and challenges of functional connectivity mapping in intractable epilepsy.

This review focuses on the use of resting-state functional magnetic resonance imaging data to assess functional connectivity in the human brain and its application in intractable epilepsy. This approach has the potential to predict outcomes for a given surgical procedure based on the pre-surgical functional organization of the brain. Functional connectivity can also identify cortical regions that are organized differently in epilepsy patients either as a direct function of the disease or through indirect compensatory responses. Functional connectivity mapping may help identify epileptogenic tissue, whether this is a single focal location or a network of seizure-generating tissues. This review covers the basics of connectivity analysis and discusses particular issues associated with analyzing such data. These issues include how to define nodes, as well as differences between connectivity analyses of individual nodes, groups of nodes, and whole-brain assessment at the voxel level. The need for arbitrary thresholds in some connectivity analyses is discussed and a solution to this problem is reviewed. Overall, functional connectivity analysis is becoming an important tool for assessing functional brain organization in epilepsy.

An EEG (electroencephalogram) recording system with carbon wire electrodes for simultaneous EEG-fMRI (functional magnetic resonance imaging) recording.

Simultaneous EEG-fMRI (Electroencephalography-functional Magnetic Resonance Imaging) recording provides a means for acquiring high temporal resolution electrophysiological data and high spatial resolution metabolic data of the brain in the same experimental runs. Carbon wire electrodes (not metallic EEG electrodes with carbon wire leads) are suitable for simultaneous EEG-fMRI recording, because they cause less RF (radio-frequency) heating and susceptibility artifacts than metallic electrodes. These characteristics are especially desirable for recording the EEG in high field MRI scanners. Carbon wire electrodes are also comfortable to wear during long recording sessions. However, carbon electrodes have high electrode-electrolyte potentials compared to widely used Ag/AgCl (silver/silver chloride) electrodes, which may cause slow voltage drifts. This paper introduces a prototype EEG recording system with carbon wire electrodes and a circuit that suppresses the slow voltage drift. The system was tested for the voltage drift, RF heating, susceptibility artifact, and impedance, and was also evaluated in a simultaneous ERP (event-related potential)-fMRI experiment.

Neural correlates of post-error slowing during a stop signal task: a functional magnetic resonance imaging study.

The ability to detect errors and adjust behavior accordingly is essential for maneuvering in an uncertain environment. Errors are particularly prone to occur when multiple, conflicting responses are registered in a situation that requires flexible behavioral outputs; for instance, when a go signal requires a response and a stop signal requires inhibition of the response during a stop signal task (SST). Previous studies employing the SST have provided ample evidence indicating the importance of the medial cortical brain regions in conflict/error processing. Other studies have also related these regional activations to postconflict/error behavioral adjustment. However, very few studies have directly explored the neural correlates of postconflict/error behavioral adjustment. Here we employed an SST to elicit errors in approximately half of the stop trials despite constant behavioral adjustment of the observers. Using functional magnetic resonance imaging, we showed that prefrontal loci including the ventrolateral prefrontal cortex are involved in post-error slowing in reaction time. These results delineate the neural circuitry specifically involved in error-associated behavioral modifications.

Spatial nonuniformity of the resting CBF and BOLD responses to sevoflurane: in vivo study of normal human subjects with magnetic resonance imaging.

Pulsed arterial spin labeling magnetic resonance imaging (MRI) was performed to investigate the local coupling between resting regional cerebral blood flow (rCBF) and BOLD (blood oxygen level dependent) signal changes in 22 normal human subjects during the administration of 0.25 MAC (minimum alveolar concentration) sevoflurane. Two states were compared with subjects at rest: anesthesia and no-anesthesia. Regions of both significantly increased and decreased resting-state rCBF were observed. Increases were limited primarily to subcortical structures and insula, whereas, decreases were observed primarily in neocortical regions. No significant change was found in global CBF (gCBF). By simultaneously measuring rCBF and BOLD, region-specific anesthetic effects on the coupling between rCBF and BOLD were identified. Multiple comparisons of the agent-induced rCBF and BOLD changes demonstrated significant (P < 0.05) spatial variability in rCBF-BOLD coupling. The slope of the linear regression line for AC, where rCBF was increased by sevoflurane, was markedly smaller than the slope for those ROIs where rCBF was decreased by sevoflurane, indicating a bigger change in BOLD per unit change in rCBF in regions where rCBF was increased by sevoflurane. These results suggest that it would be inaccurate to use a global quantitative model to describe coupling across all brain regions and in all anesthesia conditions. The observed spatial nonuniformity of rCBF and BOLD signal changes suggests that any interpretation of BOLD fMRI data in the presence of an anesthetic requires consideration of these insights.

Sevoflurane 0.25 MAC preferentially affects higher order association areas: a functional magnetic resonance imaging study in volunteers.

BACKGROUND: Functional magnetic resonance imaging (fMRI) can objectively measure the subjective effects of anesthesia. Memory-related regions (association areas) are affected by subanesthetic doses of volatile anesthetics. In this study we measured the regional neuronal effects of 0.25 MAC sevoflurane in healthy volunteers and differentiated the effect between primary cortical regions and association areas. METHODS: The effect of 0.25 MAC sevoflurane on visual, auditory, and motor activation was studied in 16 ASA I volunteers. With fMRI (3 Tesla Siemens magnetom), regional cerebral blood flow (rCBF) was measured by the pulsed arterial spin labeling technique. Subjects inhaled a mixture of O2 and 0.25 MAC sevoflurane and standard ASA monitoring was performed. Visual, auditory, and motor activation tasks were used. rCBF was measured in the awake state and during inhalation of 0.25 MAC sevoflurane, without and with activation. The change in rCBF (deltaCBF) with 0.25 MAC Sevoflurane during baseline state and with activation was calculated in 11 regions of interest related to visual, auditory, and motor activation tasks. RESULTS: The change from baseline rCBF with 0.25 MAC sevoflurane was not statistically significant in the 11 regions of interest. With activation there was a significant increase in CBF in several regions. However, only in the primary and secondary visual cortices (V1, V2), thalamus, hippocampus, and supplementary motor area was the decrease in activation with 0.25 MAC sevoflurane statistically significant (P < 0.05). CONCLUSION: Memory-related regions (association areas) are affected by subanesthetic concentrations of volatile anesthetics. Using fMRI, this study showed that 0.25 MAC sevoflurane predominantly affects the primary visual cortex, the related association cortex, and certain other higher order association cortices.

Emerging vascular applications of magnetic resonance imaging: a picture is worth more than a thousand words.

Vascular applications of magnetic resonance (MR) imaging are reviewed, with emphasis on algorithms that use nonpictorial information contained in the MR data set. Current clinical vascular practice generally limits use of MR angiography and three-dimensional vessel images to qualitative pictorial rendering without routinely using the available quantitative information contained within the MR data. This review is dedicated to recent advances that include characterization of vessel histology, assessment of carotid plaque vulnerability, characterization of blood flow dynamics, quantitative analysis of disease severity, and prediction of vascular intervention outcome. Examples from histologic preparation, in vitro and in vivo experiments, are discussed, with an emphasis on potential clinical applications and advances in acquisition technology.

Removal of time-varying gradient artifacts from EEG data acquired during continuous fMRI.

OBJECTIVE: Recording low amplitude electroencephalography (EEG) signals in the face of large gradient artifacts generated by changing functional magnetic resonance imaging (fMRI) magnetic fields continues to be a challenge. We present a new method of removing gradient artifacts with time-varying waveforms, and evaluate it in continuous (non-interleaved) simultaneous EEG-fMRI experiments. METHODS: The current method consists of an analog filter, an EEG-fMRI timing error correction algorithm, and a temporal principal component analysis based gradient noise removal algorithm. We conducted a phantom experiment and a visual oddball experiment to evaluate the method. RESULTS: The results from the phantom experiment showed that the current method reduced the number of averaged samples required to obtain high correlation between injected and recovered signals, compared to a conventional average waveform subtraction method with adaptive noise cancelling. For the oddball experiment, the results obtained from the two methods were very similar, except that the current method resulted in a higher P300 amplitude when the number of averaged trials was small. CONCLUSIONS: The current method enabled us to obtain high quality EEGs in continuous simultaneous EEG-fMRI experiments. SIGNIFICANCE: Continuous simultaneous EEG-fMRI acquisition enables efficient use of data acquisition time and better monitoring of rare EEG events.