Ravi S Menon

Publications (138) View all

• Article: A low-cost, mechanically simple apparatus for measuring eddy current-induced magnetic fields in MRI.

  Kyle M Gilbert, L Martyn Klassen, Ravi S Menon
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  ABSTRACT: The fidelity of gradient waveforms in MRI pulse sequences is essential to the acquisition of images and spectra with minimal distortion artefacts. Gradient waveforms can become nonideal when eddy currents are created in nearby conducting structures; however, the resultant magnetic fields can be characterised and compensated for by measuring the spatial and temporal field response following a gradient impulse. This can be accomplished using a grid of radiofrequency (RF) coils. The RF coils must adhere to strict performance requirements: they must achieve a high sensitivity and signal-to-noise ratio (SNR), have minimal susceptibility field gradients between the sample and surrounding material interfaces and be highly decoupled from each other. In this study, an apparatus is presented that accomplishes these tasks with a low-cost, mechanically simple solution. The coil system consists of six transmit/receive RF coils immersed in a high-molarity saline solution. The sensitivity and SNR following an excitation pulse are sufficiently high to allow accurate phase measurements during free-induction decays; the intrinsic susceptibility matching of the materials, because of the unique design of the coil system, results in sufficiently narrow spectral line widths (mean of 19 Hz), and adjacent RF coils are highly decoupled (mean S12 of -47 dB). The temporal and spatial distributions of eddy currents following a gradient pulse are measured to validate the efficacy of the design, and the resultant amplitudes and time constants required for zeroth- and first-order compensation are provided. Copyright © 2013 John Wiley & Sons, Ltd.
  NMR in Biomedicine 03/2013; · 3.21 Impact Factor
• Article: Functional connectivity patterns of medial and lateral macaque frontal eye fields reveal distinct visuomotor networks.

  Sahand Babapoor-Farrokhran, R Matthew Hutchison, Joseph S Gati, Ravi S Menon, Stefan Everling
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  ABSTRACT: It has been previously shown that small and large amplitude saccades have different functions during vision in natural environments. Large saccades are associated with reaching movements towards objects, whereas small saccades facilitate the identification of more detailed object features necessary for successful grasping and manual manipulation. To determine whether these represent dichotomous processing streams, we used resting-state fMRI to examine the functional connectivity patterns of the medial and lateral FEF regions that encode large and small amplitude saccades, respectively. We found that the spontaneous BOLD signals of the medial FEF were functionally correlated with areas known to be involved in reaching movements and executive control processes, whereas lateral FEF was functionally correlated with cortical areas involved in object processing and in grasping, fixation, and manipulation of objects. The results provide strong evidence for two distinct visuomotor network systems in the primate brain that likely reflect the alternating phases of vision for action in natural environments.
  Journal of Neurophysiology 02/2013; · 3.32 Impact Factor
• Article: Information processing architecture of functionally defined clusters in the macaque cortex.

  Kelly Shen, Gleb Bezgin, R Matthew Hutchison, Joseph S Gati, Ravi S Menon, Stefan Everling, Anthony R McIntosh
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  ABSTRACT: Computational and empirical neuroimaging studies have suggested that the anatomical connections between brain regions primarily constrain their functional interactions. Given that the large-scale organization of functional networks is determined by the temporal relationships between brain regions, the structural limitations may extend to the global characteristics of functional networks. Here, we explored the extent to which the functional network community structure is determined by the underlying anatomical architecture. We directly compared macaque (Macaca fascicularis) functional connectivity (FC) assessed using spontaneous blood oxygen level-dependent functional magnetic resonance imaging (BOLD-fMRI) to directed anatomical connectivity derived from macaque axonal tract tracing studies. Consistent with previous reports, FC increased with increasing strength of anatomical connection, and FC was also present between regions that had no direct anatomical connection. We observed moderate similarity between the FC of each region and its anatomical connectivity. Notably, anatomical connectivity patterns, as described by structural motifs, were different within and across functional modules: partitioning of the functional network was supported by dense bidirectional anatomical connections within clusters and unidirectional connections between clusters. Together, our data directly demonstrate that the FC patterns observed in resting-state BOLD-fMRI are dictated by the underlying neuroanatomical architecture. Importantly, we show how this architecture contributes to the global organizational principles of both functional specialization and integration.
  Journal of Neuroscience 11/2012; 32(48):17465-76. · 7.11 Impact Factor
• Article: Simultaneous in vivo pH and temperature mapping using a PARACEST-MRI contrast agent.

  Nevin McVicar, Alex X Li, Mojmír Suchý, Robert H E Hudson, Ravi S Menon, Robert Bartha
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  ABSTRACT: Altered tissue temperature and/or pH is a common feature in pathological conditions, where metabolic demand exceeds oxygen supply such as in tumors and following stroke. Therefore, in vivo tissue temperature and pH may become valuable biomarkers for disease detection and the monitoring of disease progression or treatment response in conditions with altered metabolic demand. In this study, pH is measured using the amide protons of a thulium (Tm(3+) ) complex with a DOTAM-Glycine-Lysine (ligand: Tm(3+) -DOTAM-Gly-Lys). The pH was uniquely determined from the linewidth of the asymmetry curve of the chemical exchange saturation transfer spectrum, independent of contrast agent concentration, or temperature for a given saturation pulse. pH maps with an inter-pixel standard deviation of less than 0.1 pH units were obtained in 10 mM Tm(3+) -DOTAM-Gly-Lys solutions with pH ranging from 6.0 to 8.0 pH units at 37°C. Temperature maps were simultaneously obtained using the chemical shift of the chemical exchange saturation transfer peak. Temperature and pH maps are demonstrated in the mouse leg (N = 3), where the mean and standard deviation for pH was 7.2 ± 0.2 pH unit and temperature was 37.4 ± 0.5°C. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
  Magnetic Resonance in Medicine 11/2012; · 2.96 Impact Factor
• Article: Resting-state networks show dynamic functional connectivity in awake humans and anesthetized macaques.

  R Matthew Hutchison, Thilo Womelsdorf, Joseph S Gati, Stefan Everling, Ravi S Menon
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  ABSTRACT: Characterization of large-scale brain networks using blood-oxygenation-level-dependent functional magnetic resonance imaging is typically based on the assumption of network stationarity across the duration of scan. Recent studies in humans have questioned this assumption by showing that within-network functional connectivity fluctuates on the order of seconds to minutes. Time-varying profiles of resting-state networks (RSNs) may relate to spontaneously shifting, electrophysiological network states and are thus mechanistically of particular importance. However, because these studies acquired data from awake subjects, the fluctuating connectivity could reflect various forms of conscious brain processing such as passive mind wandering, active monitoring, memory formation, or changes in attention and arousal during image acquisition. Here, we characterize RSN dynamics of anesthetized macaques that control for these accounts, and compare them to awake human subjects. We find that functional connectivity among nodes comprising the "oculomotor (OCM) network" strongly fluctuated over time during awake as well as anaesthetized states. For time dependent analysis with short windows (<60 s), periods of positive functional correlations alternated with prominent anticorrelations that were missed when assessed with longer time windows. Similarly, the analysis identified network nodes that transiently link to the OCM network and did not emerge in average RSN analysis. Furthermore, time-dependent analysis reliably revealed transient states of large-scale synchronization that spanned all seeds. The results illustrate that resting-state functional connectivity is not static and that RSNs can exhibit nonstationary, spontaneous relationships irrespective of conscious, cognitive processing. The findings imply that mechanistically important network information can be missed when using average functional connectivity as the single network measure. Hum Brain Mapp, 2012. © 2011 Wiley Periodicals, Inc.
  Human Brain Mapping 03/2012; · 5.88 Impact Factor