Detection of an Earthworm Axon Current with Simultaneous MRS
Alexander Poplawsky¹, Raymond Dingledine², Xiaoping Hu^3
1Neuroscience, Emory University, Atlanta, GA, United States; ²Pharmacology, Emory University, Atlanta, GA, United States; ³Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, GA, United States

Direct detection of axonal neural magnetic fields (NMFs) by magnetic resonance imaging has met with conflicting evidence. The objective of this study is to demonstrate the temporal signature of axonal NMFs in the free induction decay (FID), which provides the temporal resolution required to capture an axonal event. Simultaneous electrophysiology is used to time-lock earthworm action potentials to FID acquisition. Our data demonstrates clear evidence of a phase change that temporally corresponds to the electrophysiologically recorded action potential and is consistent with theoretical predictions.

Imaging Functional Decrease of the Cerebrospinal Fluid Volume Fraction with a Spin-Locking FMRI Technique
Tao Jin¹, Seong-Gi Kim^1
1Department of Radiology, University of Pittsburgh, Pittsburgh, PA, United States

A voxel of magnetic resonance imaging often contains blood, tissue water, as well as the cerebrospinal fluid (CSF). Recent studies have suggested that brain vascular activation could induce a change in the volume fraction of the CSF compartment that serves as a buffer for the brain cortex. However, current detection of CSF volume fraction and its functional change requires multi-compartment data fitting. In this work we aimed to image the CSF compartment directly using a spin-locking technique at 9.4 T. With a long spin-locking preparation, the parenchyma signal can be suppressed and a functional decrease of CSF volume fraction can be robustly detected during cat visual stimulation.

Time-Course of δR₂ During Visual Stimulation and Hypercapnia Diffusion-Weighted FMRI Experiments
Daigo Kuroiwa¹, Hiroshi Kawaguchi¹, Jeff Kershaw¹, Atsumichi Tachibana¹, Joonas Autio¹, Masaya Hirano², Ichio Aoki¹, Iwao Kanno¹, Takayuki Obata^1
1Department of Biophysics, Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan; ²Advanced Application Center, GE Healthcare Japan, Hino, Tokyo, Japan

It has been suggested that the BOLD effect contributes to heavily diffusion-weighted (DW) fMRI signal changes. The BOLD effect is usually interpreted as a change in transverse relaxation rate (δR₂). In this study, δR₂ during visual stimulation (VS) and hypercapnia (HC) DW fMRI experiments was estimated using a multiple spin-echo EPI acquisitions after motion-probing gradients. δR₂ showed dependence on b-value during VS, but not during HC. The results suggest that δR₂ at high b-value may demonstrate a higher sensitivity to neuronal activation than at lower b-values.

Inter-Areal and Inter-Individual Variations in Diffusion-Weighted FMRI Signal
Toshihiko Aso¹, Shin-ichi Urayama¹, Hidenao Fukuyama¹, Denis Le Bihan,^1,2
1Human Brain Research Center, Kyoto University Graduate School of Medicine, Kyoto, Japan; ²CEA NeuroSpin, Gif-sur-yvette, France

Neuronal activation can be detected with heavily sensitized diffusion-fMRI (DfMRI). The striking temporal precedence of the diffusion response to BOLD in the visual cortex suggests a non-vascular source. A visual working memory task was implemented to investigate DfMRI responses outside visual cortex. We found very similar response patterns between well separated cortices showing temporal precedence over BOLD, while large individual variations were observed with BOLD responses. Discrepancies between DfMRI and BOLD responses were also observed, such as negative BOLD signals accompanying positive DfMRI responses supporting the assumption that the DfMRI and BOLD responses have different origins.

Exploring the Reproducibility and Consistency of Diffusion-Weighted Functional Magnetic Resonance Imaging During Visual Stimulation Using Population-Based Activation Map
Ruiwang Huang¹, Bida Zhang^2
1State Key Lab of Cognitive Neuroscience and Learning, Beijing Normal University, Beijing, China; ²Siemens Mindit Magnetic Resonance, Siemens Healthcare MR Collaboration NE Asia

Human brain functional studies have been generally performed with BOLD-fMRI, but the spatial location and distribution of the activation map is not accurate. Recently, it has been suggested that the diffusion-weighted functional magnetic resonance imaging (dFMRI) may be sensitive to the true neuronal activation. However, the influence of b-value on the activation region is not fully understood. Here we performed a visual stimulation study on twelve subjects with dFMRI (b-value=50/400/800/1200/1600s/mm2) and BOLD-fMRI, and constructed the population-based activation maps. The locations and distributions of dFMRI and BOLD-fMRI measurements were compared, and the consistency of dFMRI study was evaluated.

fMRI Using a Hyperpolarized Tracer Molecule
Ute Goerke¹, Malgorzata Marjanska¹, Manda Vollmers¹, Isabelle Iltis¹, Pierre-Gilles Henry¹, Kamil Ugurbil^1
1Radiology, Center for Magnetic Resonance Research, Minneapolis, MN, United States

For the first time, fMRI utilizing a hyperpolarized tracer ¹³C-labeled urea was performed. Since urea does not cross the blood-brain barrier, it is an ideal marker for perfusion changes caused by neuronal activity. The presented results were obtained in rats with forepaw stimulation. Despite the extremely low tracer concentration in the blood in gray matter, focal activated regions were robustly detected in all ¹³C fMRI experiments.

Neurovascular Coupling Relationship Between Spontaneous EEG and CBF Responses Is Sensitive to Anesthesia Depth
Xiao Liu^1,2, Xiao-Hong Zhu¹, Yi Zhang¹, Wei Chen^1,2
1CMRR, radiology, University of Minnesota, Minneapolis, MN, United States; ²Biomedical Engineering, University of Minnesota, Minneapolis, MN, United States

In this study, hemodynamic response function (HRF) was estimated by “deconvolution” to describe the neurovascular coupling between spontaneous CBF and EEG signals in the rat brain acquired simultaneously under two anesthesia depths (1.8 and 2.0% isoflurane). We found that a small change in anesthesia depth by increasing 0.2% isoflurane could significantly alter HRF in two aspects: lengthening latency-to-peak and broadening dispersion. This result indicates that the neurovascular coupling quantified by HRF is sensitive to anesthesia depth and this phenomenon should have implication in quantifying the resting brain connectivity and stimulus-evoked BOLD in the anesthetized brains and understanding their underlying neurophysiology basis.

Behavioural Correlate of GABA Concentration in Visual Cortex
Richard A. E. Edden^1,2, Suresh D. Muthukumaraswamy³, Tom Freeman, Krish D. Singh^3
1Russell H Morgan Department of Radiology and Radiological Sciences, The Johns Hopkins University, Baltimore, MD, United States; ²FM Kirby Center for Functional fMRI, Kennedy Krieger Institute, Baltimore, MD, United States; ³CUBRIC, School of Psychology, Cardiff University, United Kingdom

Edited MRS measurements of GABA concentration in visual cortex have recently been shown to correlate with functional metrics: the frequency of gamma ocillations, as measured by MEG; and BOLD signal change in fMRI. This study investigates whether these individual differences have behavioural consequences, using a psychophysical paradigm to measure orientation discrimination thresholds. Orientation discrimination has long been associated with GABAergic neurotransmission at a cellular level; we are able to draw a similar link at the level of individual performance differences.

Cortical Hemodynamics and GABAergic Inhibition. Resting GABA Levels in Human Visual Cortex Correlate with BOLD, ASL-Measured CBF and VASO-Measured CBV Reactivity
Manus Joseph Donahue^1,2, Jamie Near^1,2, Peter Jezzard^1,2
1Clinical Neurology, Oxford University, Oxford, United Kingdom; ²Physics Division, FMRIB Centre, Oxford, United Kingdom

Neurovascular coupling between neuronal activity, energy metabolism and cerebral blood flow (CBF) is supported by synaptic excitation and inhibition. We show inverse correlations between synaptic inhibition (GABA concentration) and BOLD (R=0.68) and cerebral blood volume (CBV)-weighted VASO reactivity (R=0.75) in human visual cortex. A negative correlation between baseline GABA and baseline CBV (R=0.75) is found; however, a positive relationship between GABA and ASL reactivity (R=0.38) and baseline CBF (R=0.67) is found, which we attribute to blood velocity discrepancies. Results provide information on the relationship between cortical activity, GABAergic inhibition, and multimodal fMRI contrast. First two authors are equal contributors.

Hemodynamic and Metabolic Response to Hypoxia
Ashley D. Harris¹, Richard A. E. Edden^2,3, Kevin Murphy¹, C John Evans¹, Chen Y. Poon⁴, Neeraj Saxena⁵, Judith Hall⁵, Thomas T. Liu⁶, Damian M. Bailey⁷, Richard G. Wise^1
1Cardiff University Brain Research Imaging Centre (CUBRIC), Cardiff University, Cardiff, United Kingdom; ²Russell H Morgan Department of Radiology and Radiological Science, The Johns Hopkins University, Baltimore, MD, United States; ³Cardiff University Brain Imaging Research Centre (CUBRIC) and Schools of Chemistry and Biosciences, Cardiff University, Cardiff, United Kingdom; ⁴School of Medicine, Cardiff University, Cardiff, United Kingdom; ⁵Anaesthetics and Intensive Care Medicine, Cardiff University, Cardiff, United Kingdom; ⁶Center for Functional MRI (fMRI), University of California, San Diego, San Diego, CA, United States; ⁷Health, Sport and Science, University of Glamorgan, Mid-Glamorgan, United Kingdom

MR spectroscopy to examine lactate and ASL perfusion imaging are used to study the response to 12% hypoxia in healthy subjects. Lactate and cerebral blood flow increased during hypoxia. Both lactate and blood flow are negatively related to oxygen saturation. The relationship between increased perfusion and lactate accumulation appears to be more complex; however, by understanding these relationships, we may gain insight into cerebral pathologies and conditions that result in hypoxemia.