ISMRM 23rd Annual Meeting & Exhibition • 30 May - 05 June 2015 • Toronto, Ontario, Canada

Scientific Session • fMRI: Physiology

Monday 1 June 2015

Room 714 A/B

16:30 - 18:30


Richard G. Wise, Ph.D., J. Jean Chen, Ph.D.

16:30 0210.   fMRI post-stimulus undershoots in visual cortex are neuronal in origin
Karen J Mullinger1,2, Matthew Cherukara1, Susan T Francis1, and Stephen D Mayhew2
1SPMIC, School of Physics and Astronomy, University of Nottingham, Nottingham, Nottinghamshire, United Kingdom, 2BUIC, School of Psychology, University of Birmingham, Birmingham, West Midlands, United Kingdom

The fMRI post-stimulus undershoot is widely observed but its origins remain unclear. We recorded EEG-BOLD-ASL responses to 10s-duration flickering and static visual checkerboard stimuli and show that the amplitude of both the BOLD and CBF post-stimulus undershoots in V1 are negatively correlated with the 10-20s post-stimulus power of the occipital EEG alpha (8-13Hz) oscillation. In addition, flicker trials resulted in both significantly larger fMRI undershoots and significantly higher post-stimulus alpha power responses than static trials. These results provide further evidence that the fMRI undershoot arises from a neuronal mechanism and comprises a unique feature of brain activity.

16:42 0211.   
Unravelling the neurochemical mechanism of positive and negative BOLD responses: a combined fMRI-fMRS study
Adam Berrington1, Andre Gouws2, Stuart Clare1, Peter Jezzard1, and Uzay Emir1
1FMRIB Centre, University of Oxford, Oxford, United Kingdom, 2York Neuroimaging Centre, University of York, York, United Kingdom

We investigate neurochemical changes associated with the positive and negative BOLD response. Using a visual stimulus capable of generating PBR and NBR adjacently in visual cortex, along with a two-voxel spectroscopy sequence at 7T, we determine metabolite changes from resting baseline. The relative increase or decrease of certain metabolites e.g. glucose and glutamate, between NBR and PBR, hints at alternate metabolic processes behind either response. Baseline concentrations of GABA and glutamate are also found to correlate to negative BOLD strength suggesting a link between inhibition-excitation balance and BOLD response.

16:54 0212.   Application of quantitative, multimodal fMRI to the estimation of the cerebral metabolic response to CO2 and a visual stimulus in hypoxia
Aaron Benjamin Simon1, Zachary Smith2, Richard Buxton2, and David Dubowitz2
1Bioengineering, University of California San Diego, La Jolla, CA, United States, 2Radiology, University of California San Diego, La Jolla, CA, United States

How does sustained hypoxia affect the brain oxygen metabolism (CMRO2) response to a stimulus? We applied a novel multimodal Bayesian approach for estimating the CMRO2 response to stimuli in human subjects after 6 hrs, 2 days and 7 days of sustained hypoxia (3800m elevation, 12.5% O2). The acute response to a CO2 challenge was a significant reduction in CMRO2, and this did not change with up to a week of acclimatization. In contrast, the CMRO2response to a visual stimulus was significantly reduced for up to 2 days of sustained hypoxia, but recovered to the normoxic response by 7 days.

17:06 0213.   
Multiband BOLD acquisition enhances the sensitivity of cerebrovascular reactivity (CVR) mapping
Harshan Ravi1,2, Peiying Liu1, Shin-Lei Peng1, and Hanzhang Lu1
1Advanced Imaging Research Center, University of Texas at South Western Medical Center, Dallas, Tx, United States, 2Department of Bioengineering, University of Texas at Arlington, Arlington, TX, United States

Cerebrovascular reactivity (CVR) reflects the ability of the brain vasculature to dilate in response to a vasoactive stimulus. CVR is usually measured with CO2 inhalation while continuously acquiring BOLD MRI images. The biggest drawback of CVR is its poor sensitivity and reliability. Multiband EPI is a fast-imaging technology that allows the acquisition of multiple 2D slices simultaneously. In fMRI and DTI applications, it has been shown that multiband EPI provides an SNR advantage over conventional EPI. In this work, we examined the SNR benefit of multiband acquisition in CVR mapping, by comparing the data collected using multiband with those using conventional EPI.

17:18 0214.   
The impact of normoxic and hyperoxic baseline periods in block paradigms of hypercarbic cerebrovascular reactivity studies
Carlos C. Faraco1, Jeroen C.W. Siero2, Megan K. Strother1, Daniel F. Arteaga1, and Manus J. Donahue1
1Radiology and Radiological Sciences, Vanderbilt University School of Medicine, Nashville, TN, United States, 2Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands

Identification of cerebrovascular compromise in the clinic is frequently performed using catheter angiography, which is insensitive to tissue-level hemodynamics and is sub-optimal for longitudinal monitoring. Alternatively, hypercarbic-normoxic BOLD fMRI may be used to assess cerebrovascular reactivity (CVR). As a significant proportion of patients for whom CVR assessment is desirable are hypoxic, and may be operating near reserve capacity, hypercarbic-hyperoxic (HC-HO) BOLD fMRI is advisable. However, HC-HO stimuli introduce several experimental confounds, including non-specific venous O2 rebinding to dHb. Here we demonstrate that administration of baseline HO, before HC-HO, resolves or reduces the drawbacks associated with HC-HO CVR-weighted BOLD imaging.

17:30 0215.   
Searching for a truly "iso-metabolic" gas challenge for the use in calibrated fMRI and cerebrovascular reactivity mapping
Shin-Lei Peng1, Harshan Ravi1, Min Sheng1, Binu Thomas1, and Hanzhang Lu1
1Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, Texas, United States

Hypercapnia challenge has been proposed as a method for the calibration experiment because CO2 does not alter brain metabolism. However, the assumption that hypercapnia challenge is iso-CMRO2 has not been validated. This study is to investigate a gas challenge is truly ˇ§iso-metabolicˇ¨ by adding a hypoxic component to the hypercapnic challenge (hypercapnic-hypoxia). We used TRUST MRI to monitor the subjectˇ¦s CMRO2 during hypercapnic-hypoxia challenge. Our data showed the neural suppression effect of hypercapnia appears to be nullified by the metabolic enhancement effect of hypoxia. We therefore propose hypercapnic-hypoxia is an iso-metabolic challenge that may be used for calibrated fMRI studies.

17:42 0216.   
Calibration of BOLD fMRI motor activation maps using BOLD breath hold cerebrovascular reactivity mapping for effective compensation of brain tumor-related neurovascular uncoupling
Shruti Agarwal1, Raag Airan1, Sachin K. Gujar1, Haris I. Sair1, and Jay J. Pillai1
1Division of Neuroradiology, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States

Reduced or impaired cerebrovascular reactivity (CVR) in patients with brain tumors and other cerebral diseases can generate false negative or spuriously decreased BOLD fMRI activation during performance of presurgical mapping of motor and language cortex in patients who are potentially candidates for lesion resection. This phenomenon, referred to as neurovascular uncoupling (NVU) is an under-recognized but nevertheless critical limitation of clinical BOLD fMRI. In this study we demonstrate the feasibility of minimization of false negative motor task-based activation through use of a novel breath-hold CVR-based calibration method to effectively compensate for NVU in patients with perirolandic low grade gliomas.

17:54 0217.   
Task-correlated physiology reveals vascular-neural networks
Molly Gallogly Bright1, Joseph Whittaker1, Ian Driver1, and Kevin Murphy1
1CUBRIC, School of Psychology, Cardiff University, Cardiff, Wales, United Kingdom

We previously identified coupling of vascular-neural networks in BOLD fMRI data using simultaneous neural stimuli and hypercapnia paradigms. We repeat this experiment without a hypercapnic stimulus, observing robust changes in physiology time-locked to the neural tasks. These task-correlated physiologic effects are significantly more associated with the vascular networks identified previously, and identify an additional vascular network in bilateral motor cortices. Task-correlated physiology significantly affects multiple vascular networks and may mimic, spatially and temporally, neural network activations.

18:06 0218.   
Baseline oxygenation in the brain: Correlation with BOLD and comparison between susceptibility and respiratory-calibration methods
Audrey P. Fan1, Andreas Schaefer2, Laurentius Huber2, Steffen N. Krieger2, Harald E. Moeller2, Arno Villringer2, and Claudine J. Gauthier2,3
1Richard M. Lucas Center for Imaging, Stanford University, Stanford, CA, United States, 2Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 3Concordia University, Montreal, Quebec, Canada

This study investigated whether BOLD signal changes during visual stimulus and gas challenge depend on baseline oxygen extraction fraction (OEF0), as measured by quantitative susceptibility mapping (QSM) in cerebral veins. We also directly compared absolute OEF0 values in the visual cortex by QSM and by a novel respiratory-calibration method (QUO2). In eight healthy volunteers, BOLD-ASL scans were acquired during gas breathing and visual stimulus, and gradient echo scans were acquired at rest for QSM reconstruction. Good fidelity was observed between BOLD-ASL signal changes and baseline OEF0 by QSM, as predicted by biophysical models. Our findings also reveal encouraging concordance between absolute OEF0 by QSM (30.6±2%) and by QUO2 (31.5±12%) that warrants examination in a larger cohort.

18:18 0219.   A streamlined approach to mapping the oxygen extraction fraction (OEF) and deoxygenated blood volume (DBV) using the quantitative BOLD technique
Alan J Stone1 and Nicholas P Blockley1
1FMRIB, Nuffield Department of Clinical Neurosciences, Oxford, United Kingdom

There is a need for an easy-to-implement MR method capable of providing quantitative parametric maps of OEF and DBV across the brain in a clinically feasible timescale. In this study a FLAIR GESEPI ASE sequence is used to provide a refined measure of R2′ through optimisation of the data acquisition. The components of the sequence are selected to minimise confounds associated with CSF partial volume effects (FLAIR), macroscopic field inhomogeneities (GESEPI) and R2-weighting (ASE) of the R2′ signal. This affords a simplified application of the qBOLD signal model, allowing parametric maps of OEF and DBV to be produced in a streamlined process.