1
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Naegel S, Zeller J, Hougard A, Weise CM, Zuelow S, Kleinschnitz C, Obermann M, Solbach K, Holle D. No structural brain alterations in new daily persistent headache - a cross sectional VBM/SBM study. Cephalalgia 2021; 42:335-344. [PMID: 34601946 DOI: 10.1177/03331024211045653] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
OBJECTIVE To identify grey matter alterations in patients suffering new daily persistent headache to enrich the pathophysiological concept of this rare headache disorder characterised by a distinct, clearly remembered onset and its instant chronification. METHOD Magnetic resonance-based voxel-based and surface-based morphometry was used to investigate 23 patients suffering from new daily persistent headache and 23 age- and gender-matched healthy controls with 1.5 Tesla MRI.Independent statistical analysis was performed at three sites using statistical parametric mapping, as well as FSL(FMRIB Software Library)-based approaches. RESULTS No grey matter changes were detected using this sophisticated and cross-checked method. CONCLUSION The absence of structural brain changes in patients with new daily persistent headache contribute to the recent discussion regarding structural alterations in primary headache disorders in general and does not provide evidence for grey matter changes being associated with the pathophysiology of new daily persistent headache. Future research will have to determine the underlying pathophysiological mechanisms of this disorder.
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Affiliation(s)
- Steffen Naegel
- Department of Neurology, Martin-Luther-University Halle-Wittenberg, Halle/Saale, Germany.,Department of Neurology, Westgerman Headache Center, University of Duisburg-Essen, University Hospital Essen, Essen, Germany
| | - Julia Zeller
- Department of Neurology, Westgerman Headache Center, University of Duisburg-Essen, University Hospital Essen, Essen, Germany.,Paediatric Haematology and Oncology, Paediatrics III, University of Duisburg-Essen, University Hospital Essen, Essen, Germany
| | - Anders Hougard
- Danish Headache Center, Department of Neurology, Rigshospitalet Glostrup, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
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- Institute for Medical Informatics, Biometry and Epidemiology, Medical Faculty of the University Duisburg-Essen, Essen, Germany
| | - Stefan Zuelow
- Department of Diagnostic and Interventional Radiology and Neuroradiology, University of Duisburg-Essen, University Hospital Essen, Essen, Germany.,Department of Diagnostic and Interventional Radiology and Neuroradiology, Rheinische Friedrich-Wilhelms-Universität, Bonn, Germany
| | - Christoph Kleinschnitz
- Department of Neurology, Martin-Luther-University Halle-Wittenberg, Halle/Saale, Germany
| | - Mark Obermann
- Department of Neurology, Westgerman Headache Center, University of Duisburg-Essen, University Hospital Essen, Essen, Germany.,Department of Neurology, Klinikum Weser-Egge, Höxter, Germany
| | - Kasja Solbach
- Department of Neurology, Westgerman Headache Center, University of Duisburg-Essen, University Hospital Essen, Essen, Germany
| | - Dagny Holle
- Department of Neurology, Westgerman Headache Center, University of Duisburg-Essen, University Hospital Essen, Essen, Germany
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2
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Cheng X, Sie EJ, Naufel S, Boas DA, Marsili F. Measuring neuronal activity with diffuse correlation spectroscopy: a theoretical investigation. NEUROPHOTONICS 2021; 8:035004. [PMID: 34368390 PMCID: PMC8339443 DOI: 10.1117/1.nph.8.3.035004] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2021] [Accepted: 07/16/2021] [Indexed: 05/18/2023]
Abstract
Significance: Diffuse correlation spectroscopy (DCS) measures cerebral blood flow non-invasively. Variations in blood flow can be used to detect neuronal activities, but its peak has a latency of a few seconds, which is slow for real-time monitoring. Neuronal cells also deform during activation, which, in principle, can be utilized to detect neuronal activity on fast timescales (within 100 ms) using DCS. Aims: We aim to characterize DCS signal variation quantified as the change of the decay time of the speckle intensity autocorrelation function during neuronal activation on both fast (within 100 ms) and slow (100 ms to seconds) timescales. Approach: We extensively modeled the variations in the DCS signal that are expected to arise from neuronal activation using Monte Carlo simulations, including the impacts of neuronal cell motion, vessel wall dilation, and blood flow changes. Results: We found that neuronal cell motion induces a DCS signal variation of ∼ 10 - 5 . We also estimated the contrast and number of channels required to detect hemodynamic signals at different time delays. Conclusions: From this extensive analysis, we do not expect to detect neuronal cell motion using DCS in the near future based on current technology trends. However, multi-channel DCS will be able to detect hemodynamic response with sub-second latency, which is interesting for brain-computer interfaces.
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Affiliation(s)
- Xiaojun Cheng
- Boston University, Neurophotonics Center, Department of Biomedical Engineering, Massachusetts, United States
- Address all correspondence to Xiaojun Cheng,
| | - Edbert J. Sie
- Facebook Reality Labs Research, Menlo Park, California, United States
| | - Stephanie Naufel
- Facebook Reality Labs Research, Menlo Park, California, United States
| | - David A. Boas
- Boston University, Neurophotonics Center, Department of Biomedical Engineering, Massachusetts, United States
| | - Francesco Marsili
- Facebook Reality Labs Research, Menlo Park, California, United States
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3
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Nunes D, Gil R, Shemesh N. A rapid-onset diffusion functional MRI signal reflects neuromorphological coupling dynamics. Neuroimage 2021; 231:117862. [PMID: 33592243 DOI: 10.1016/j.neuroimage.2021.117862] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2020] [Revised: 01/29/2021] [Accepted: 02/08/2021] [Indexed: 02/07/2023] Open
Abstract
Functional Magnetic Resonance Imaging (fMRI) has transformed our understanding of brain function in-vivo. However, the neurovascular coupling mechanisms underlying fMRI are somewhat "distant" from neural activity. Interestingly, evidence from Intrinsic Optical Signals (IOSs) indicates that neural activity is also coupled to (sub)cellular morphological modulations. Diffusion-weighted functional MRI (dfMRI) experiments have been previously proposed to probe such neuromorphological couplings, but the underlying mechanisms have remained highly contested. Here, we provide the first direct link between in vivo ultrafast dfMRI signals upon rat forepaw stimulation and IOSs in acute slices stimulated optogenetically. We reveal a hitherto unreported rapid onset (<100 ms) dfMRI signal component which (i) agrees with fast-rising IOSs dynamics; (ii) evidences a punctate quantitative correspondence to the stimulation period; and (iii) is rather insensitive to a vascular challenge. Our findings suggest that neuromorphological coupling can be detected via dfMRI signals, auguring well for future mapping of neural activity more directly compared with blood-oxygenation-level-dependent mechanisms.
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Affiliation(s)
- Daniel Nunes
- Champalimaud Research, Champalimaud Centre for the Unknown, Av. Brasilia 1400-038, Lisbon, Portugal
| | - Rita Gil
- Champalimaud Research, Champalimaud Centre for the Unknown, Av. Brasilia 1400-038, Lisbon, Portugal
| | - Noam Shemesh
- Champalimaud Research, Champalimaud Centre for the Unknown, Av. Brasilia 1400-038, Lisbon, Portugal.
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4
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Short Latency Gray Matter Changes in Voxel-Based Morphometry following High Frequent Visual Stimulation. Neural Plast 2017; 2017:1397801. [PMID: 28293437 PMCID: PMC5331306 DOI: 10.1155/2017/1397801] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2016] [Accepted: 01/11/2017] [Indexed: 12/29/2022] Open
Abstract
Magnetic resonance imaging studies using voxel-based morphometry (VBM) detected structural changes in the human brain within periods of months or weeks. The underlying molecular mechanisms of VBM findings remain unresolved. We showed that simple visual stimulation by an alternating checkerboard leads to instant, short-lasting alterations of the primary and secondary visual cortex detected by VBM. The rapidness of occurrence (i.e., within 10 minutes) rather excludes most of the proposed physiological mechanism such as neural or glial cell genesis/degeneration or synapse turnover. We therefore favour cerebral fluid shifts to be the underlying correlate of the here observed VBM gray matter changes. Fast onset gray matter changes might be one important explanation for the inconsistency of VBM study results that often raise concern in regard to the validity of presented data. This study shows that changes detectable by VBM may occur within a few minutes after physiological stimulation and must be considered in future VBM experiments to avoid misinterpretation of results.
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5
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Bai R, Klaus A, Bellay T, Stewart C, Pajevic S, Nevo U, Merkle H, Plenz D, Basser PJ. Simultaneous calcium fluorescence imaging and MR of ex vivo organotypic cortical cultures: a new test bed for functional MRI. NMR IN BIOMEDICINE 2015; 28:1726-1738. [PMID: 26510537 DOI: 10.1002/nbm.3424] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/07/2015] [Revised: 09/03/2015] [Accepted: 09/06/2015] [Indexed: 06/05/2023]
Abstract
Recently, several new functional (f)MRI contrast mechanisms including diffusion, phase imaging, proton density, etc. have been proposed to measure neuronal activity more directly and accurately than blood-oxygen-level dependent (BOLD) fMRI. However, these approaches have proved difficult to reproduce, mainly because of the dearth of reliable and robust test systems to vet and validate them. Here we describe the development and testing of such a test bed for non-BOLD fMRI. Organotypic cortical cultures were used as a stable and reproducible biological model of neuronal activity that shows spontaneous activity similar to that of in vivo brain cortex without any hemodynamic confounds. An open-access, single-sided magnetic resonance (MR) "profiler" consisting of four permanent magnets with magnetic field of 0.32 T was used in this study to perform MR acquisition. A fluorescence microscope with long working distance objective was mounted on the top of a custom-designed chamber that keeps the organotypic culture vital, and the MR system was mounted on the bottom of the chamber to achieve real-time simultaneous calcium fluorescence optical imaging and MR acquisition on the same specimen. In this study, the reliability and performance of the proposed test bed were demonstrated by a conventional CPMG MR sequence acquired simultaneously with calcium imaging, which is a well-characterized measurement of neuronal activity. This experimental design will make it possible to correlate directly the other candidate functional MR signals to the optical indicia of neuronal activity in the future.
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Affiliation(s)
- Ruiliang Bai
- Section on Tissue Biophysics and Biomimetics, PPITS, NICHD, National Institutes of Health, Bethesda, Maryland, USA
- Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, USA
| | - Andreas Klaus
- Section on Critical Brain Dynamics, LSN, NIMH, National Institutes of Health, Bethesda, Maryland, USA
| | - Tim Bellay
- Section on Critical Brain Dynamics, LSN, NIMH, National Institutes of Health, Bethesda, Maryland, USA
| | - Craig Stewart
- Section on Critical Brain Dynamics, LSN, NIMH, National Institutes of Health, Bethesda, Maryland, USA
| | - Sinisa Pajevic
- Mathematical and Statistical Computing Laboratory, Division of Computational Bioscience, Center for Information Technology, NIH, Bethesda, Maryland, USA
| | - Uri Nevo
- Department of Biomedical Engineering, Tel-Aviv University, Tel-Aviv, Israel
| | - Hellmut Merkle
- Laboratory for Functional and Molecular Imaging, NINDS, National Institutes of Health, Bethesda, Maryland, USA
| | - Dietmar Plenz
- Section on Critical Brain Dynamics, LSN, NIMH, National Institutes of Health, Bethesda, Maryland, USA
| | - Peter J Basser
- Section on Tissue Biophysics and Biomimetics, PPITS, NICHD, National Institutes of Health, Bethesda, Maryland, USA
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6
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Balasubramanian M, Mulkern RV, Wells WM, Sundaram P, Orbach DB. Magnetic resonance imaging of ionic currents in solution: the effect of magnetohydrodynamic flow. Magn Reson Med 2014; 74:1145-55. [PMID: 25273917 DOI: 10.1002/mrm.25445] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2014] [Revised: 07/25/2014] [Accepted: 08/15/2014] [Indexed: 11/06/2022]
Abstract
PURPOSE Reliably detecting MRI signals in the brain that are more tightly coupled to neural activity than blood-oxygen-level-dependent fMRI signals could not only prove valuable for basic scientific research but could also enhance clinical applications such as epilepsy presurgical mapping. This endeavor will likely benefit from an improved understanding of the behavior of ionic currents, the mediators of neural activity, in the presence of the strong magnetic fields that are typical of modern-day MRI scanners. THEORY Of the various mechanisms that have been proposed to explain the behavior of ionic volume currents in a magnetic field, only one-magnetohydrodynamic flow-predicts a slow evolution of signals, on the order of a minute for normal saline in a typical MRI scanner. METHODS This prediction was tested by scanning a volume-current phantom containing normal saline with gradient-echo-planar imaging at 3 T. RESULTS Greater signal changes were observed in the phase of the images than in the magnitude, with the changes evolving on the order of a minute. CONCLUSION These results provide experimental support for the MHD flow hypothesis. Furthermore, MHD-driven cerebrospinal fluid flow could provide a novel fMRI contrast mechanism.
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Affiliation(s)
- Mukund Balasubramanian
- Department of Radiology, Boston Children's Hospital, Harvard Medical School, Boston, Massachusetts, USA
| | - Robert V Mulkern
- Department of Radiology, Boston Children's Hospital, Harvard Medical School, Boston, Massachusetts, USA
| | - William M Wells
- Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA
| | - Padmavathi Sundaram
- Department of Radiology, Boston Children's Hospital, Harvard Medical School, Boston, Massachusetts, USA
| | - Darren B Orbach
- Department of Radiology, Boston Children's Hospital, Harvard Medical School, Boston, Massachusetts, USA
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7
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Renvall V, Nangini C, Hari R. All that glitters is not BOLD: inconsistencies in functional MRI. Sci Rep 2014; 4:3920. [PMID: 24472878 PMCID: PMC3905278 DOI: 10.1038/srep03920] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2013] [Accepted: 01/13/2014] [Indexed: 12/18/2022] Open
Abstract
The blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal is a widely-accepted marker of brain activity. The acquisition parameters (APs) of fMRI aim at maximizing the signals related to neuronal activity while minimizing unrelated signal fluctuations. Currently, a diverse set of APs is used to acquire BOLD fMRI data. Here we demonstrate that some fMRI responses are alarmingly inconsistent across APs, ranging from positive to negative, or disappearing entirely, under identical stimulus conditions. These discrepancies, resulting from non-BOLD effects masquerading as BOLD signals, have remained largely unnoticed because studies rarely employ more than one set of APs. We identified and characterized non-BOLD responses in several brain areas, including posterior cingulate cortex and precuneus, as well as AP-dependence of both the signal time courses and of seed-based functional networks, noticing that AP manipulation can inform about the origin of the measured signals.
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Affiliation(s)
- Ville Renvall
- Brain Research Unit, O.V. Lounasmaa Laboratory and AMI Centre, Aalto NeuroImaging, Aalto University, FI-00076 AALTO, Espoo, Finland
| | - Cathy Nangini
- Brain Research Unit, O.V. Lounasmaa Laboratory and AMI Centre, Aalto NeuroImaging, Aalto University, FI-00076 AALTO, Espoo, Finland
| | - Riitta Hari
- Brain Research Unit, O.V. Lounasmaa Laboratory and AMI Centre, Aalto NeuroImaging, Aalto University, FI-00076 AALTO, Espoo, Finland
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8
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Mandl RCW, Schnack HG, Zwiers MP, Kahn RS, Hulshoff Pol HE. Functional diffusion tensor imaging at 3 Tesla. Front Hum Neurosci 2013; 7:817. [PMID: 24409133 PMCID: PMC3847896 DOI: 10.3389/fnhum.2013.00817] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2013] [Accepted: 11/11/2013] [Indexed: 11/13/2022] Open
Abstract
In a previous study we reported on a non-invasive functional diffusion tensor imaging (fDTI) method to measure neuronal signals directly from subtle changes in fractional anisotropy along white matter tracts. We hypothesized that these fractional anisotropy changes relate to morphological changes of glial cells induced by axonal activity. In the present study we set out to replicate the results of the previous study with an improved fDTI scan acquisition scheme. A group of twelve healthy human participants were scanned on a 3 Tesla MRI scanner. Activation was revealed in the contralateral thalamo-cortical tract and optic radiations during tactile and visual stimulation, respectively. Mean percent signal change in FA was 3.47% for the tactile task and 3.79% for the visual task, while for the MD the mean percent signal change was only -0.10 and -0.09%. The results support the notion of different response functions for tactile and visual stimuli. With this study we successfully replicated our previous findings using the same types of stimuli but on a different group of healthy participants and at different field-strength. The successful replication of our first fDTI results suggests that the non-invasive fDTI method is robust enough to study the functional neural networks in the human brain within a practically feasible time period.
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Affiliation(s)
- René C W Mandl
- 1Department of Psychiatry, Brain Center Rudolf Magnus, University Medical Center Utrecht Utrecht, Netherlands
| | - Hugo G Schnack
- 1Department of Psychiatry, Brain Center Rudolf Magnus, University Medical Center Utrecht Utrecht, Netherlands
| | - Marcel P Zwiers
- 2Radboud University Nijmegen, Donders Institute for Brain, Cognition and Behaviour Centre for Cognitive Neuroimaging Nijmegen, Netherlands
| | - René S Kahn
- 1Department of Psychiatry, Brain Center Rudolf Magnus, University Medical Center Utrecht Utrecht, Netherlands
| | - Hilleke E Hulshoff Pol
- 1Department of Psychiatry, Brain Center Rudolf Magnus, University Medical Center Utrecht Utrecht, Netherlands
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9
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Water diffusion in brain cortex closely tracks underlying neuronal activity. Proc Natl Acad Sci U S A 2013; 110:11636-41. [PMID: 23801756 DOI: 10.1073/pnas.1303178110] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Neuronal activity results in a local increase in blood flow. This concept serves as the basis for functional MRI. Still, this approach remains indirect and may fail in situations interfering with the neurovascular coupling mechanisms (drugs, anesthesia). Here we establish that water molecular diffusion is directly modulated by underlying neuronal activity using a rat forepaw stimulation model under different conditions of neuronal stimulation and neurovascular coupling. Under nitroprusside infusion, a neurovascular-coupling inhibitor, the diffusion response and local field potentials were maintained, whereas the hemodynamic response was abolished. As diffusion MRI reflects interactions of water molecules with obstacles (e.g., cell membranes), the observed changes point to a dynamic modulation of the neural tissue structure upon activation, which remains to be investigated. These findings represent a significant shift in concept from the current electrochemical and neurovascular coupling principles used for brain imaging, and open unique avenues to investigate mechanisms underlying brain function.
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10
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Stroman PW, Wheeler-Kingshott C, Bacon M, Schwab JM, Bosma R, Brooks J, Cadotte D, Carlstedt T, Ciccarelli O, Cohen-Adad J, Curt A, Evangelou N, Fehlings MG, Filippi M, Kelley BJ, Kollias S, Mackay A, Porro CA, Smith S, Strittmatter SM, Summers P, Tracey I. The current state-of-the-art of spinal cord imaging: methods. Neuroimage 2013; 84:1070-81. [PMID: 23685159 DOI: 10.1016/j.neuroimage.2013.04.124] [Citation(s) in RCA: 228] [Impact Index Per Article: 20.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2013] [Revised: 04/08/2013] [Accepted: 04/16/2013] [Indexed: 12/28/2022] Open
Abstract
A first-ever spinal cord imaging meeting was sponsored by the International Spinal Research Trust and the Wings for Life Foundation with the aim of identifying the current state-of-the-art of spinal cord imaging, the current greatest challenges, and greatest needs for future development. This meeting was attended by a small group of invited experts spanning all aspects of spinal cord imaging from basic research to clinical practice. The greatest current challenges for spinal cord imaging were identified as arising from the imaging environment itself; difficult imaging environment created by the bone surrounding the spinal canal, physiological motion of the cord and adjacent tissues, and small cross-sectional dimensions of the spinal cord, exacerbated by metallic implants often present in injured patients. Challenges were also identified as a result of a lack of "critical mass" of researchers taking on the development of spinal cord imaging, affecting both the rate of progress in the field, and the demand for equipment and software to manufacturers to produce the necessary tools. Here we define the current state-of-the-art of spinal cord imaging, discuss the underlying theory and challenges, and present the evidence for the current and potential power of these methods. In two review papers (part I and part II), we propose that the challenges can be overcome with advances in methods, improving availability and effectiveness of methods, and linking existing researchers to create the necessary scientific and clinical network to advance the rate of progress and impact of the research.
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Affiliation(s)
- P W Stroman
- Centre for Neuroscience Studies, Queen's University, Kingston, ON, Canada.
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11
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Tirosh N, Nevo U. Neuronal activity significantly reduces water displacement: DWI of a vital rat spinal cord with no hemodynamic effect. Neuroimage 2013; 76:98-107. [PMID: 23507391 DOI: 10.1016/j.neuroimage.2013.02.065] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2012] [Revised: 02/21/2013] [Accepted: 02/25/2013] [Indexed: 10/27/2022] Open
Abstract
Changes in the diffusion weighted MRI (DWI) signal were observed to be correlated with neuronal activity during chemically induced brain activity, epileptic seizures, or visual stimulation. These changes suggest a possible reduction in water displacement that accompanies neuronal activity, but were possibly affected by other physiological mechanisms such as blood oxygenation level and blood flow. We developed an imaging experiment of an excised and vital newborn rat spinal cord to examine the effect of neuronal function on the displacement of water molecules as measured by DWI signal. This approach provides a DWI experiment of a vital mammalian CNS tissue in the absence of some of the systemic sources of noise. We detected a significant and reproducible drop with an average value of 19.5 ± 1.6% (mean ± SE) upon activation. The drop repeated itself in three orthogonal directions. ADC values corresponded to an oblate anisotropy. This result was validated by high resolution DWI of a fixed tissue, imaged with an ultra-high field MRI. The results support our working hypothesis that water displacement is affected by neuronal activation. These results further imply that water displacement might serve as a potential marker for brain function, and that, although commonly viewed as wholly electrochemical, neuronal activity includes a significant mechanical dimension that affects water displacement.
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Affiliation(s)
- Nitzan Tirosh
- Department of Biomedical Engineering, The Iby and Aladar Fleischman Faculty of Engineering, Tel Aviv University, Ramat Aviv, Tel Aviv, Israel
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12
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Cadotte DW, Bosma R, Mikulis D, Nugaeva N, Smith K, Pokrupa R, Islam O, Stroman PW, Fehlings MG. Plasticity of the injured human spinal cord: insights revealed by spinal cord functional MRI. PLoS One 2012; 7:e45560. [PMID: 23029097 PMCID: PMC3446947 DOI: 10.1371/journal.pone.0045560] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2012] [Accepted: 08/20/2012] [Indexed: 12/14/2022] Open
Abstract
Introduction While numerous studies have documented evidence for plasticity of the human brain there is little evidence that the human spinal cord can change after injury. Here, we employ a novel spinal fMRI design where we stimulate normal and abnormal sensory dermatomes in persons with traumatic spinal cord injury and perform a connectivity analysis to understand how spinal networks process information. Methods Spinal fMRI data was collected at 3 Tesla at two institutions from 38 individuals using the standard SEEP functional MR imaging techniques. Thermal stimulation was applied to four dermatomes in an interleaved timing pattern during each fMRI acquisition. SCI patients were stimulated in dermatomes both above (normal sensation) and below the level of their injury. Sub-group analysis was performed on healthy controls (n = 20), complete SCI (n = 3), incomplete SCI (n = 9) and SCI patients who recovered full function (n = 6). Results Patients with chronic incomplete SCI, when stimulated in a dermatome of normal sensation, showed an increased number of active voxels relative to controls (p = 0.025). There was an inverse relationship between the degree of sensory impairment and the number of active voxels in the region of the spinal cord corresponding to that dermatome of abnormal sensation (R2 = 0.93, p<0.001). Lastly, a connectivity analysis demonstrated a significantly increased number of intraspinal connections in incomplete SCI patients relative to controls suggesting altered processing of afferent sensory signals. Conclusions In this work we demonstrate the use of spinal fMRI to investigate changes in spinal processing of somatosensory information in the human spinal cord. We provide evidence for plasticity of the human spinal cord after traumatic injury based on an increase in the average number of active voxels in dermatomes of normal sensation in chronic SCI patients and an increased number of intraspinal connections in incomplete SCI patients relative to healthy controls.
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Affiliation(s)
- David W. Cadotte
- Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
- Department of Surgery, Division of Neurosurgery, Toronto Western Hospital, Toronto, Ontario, Canada
- Krembil Neuroscience Centre, Toronto Western Hospital, University Health Network, Toronto, Ontario, Canada
| | - Rachael Bosma
- Centre for Neuroscience Studies, Queen’s University, Kingston, Ontario, Canada
| | - David Mikulis
- Department of Radiology, Division of Neuroradiology, Toronto Western Hospital, University of Toronto, Toronto, Ontario, Canada
| | - Natalia Nugaeva
- Krembil Neuroscience Centre, Toronto Western Hospital, University Health Network, Toronto, Ontario, Canada
| | - Karen Smith
- Providence Health Care, Spinal Rehabilitation Unit, Kingston, Ontario, Canada
| | - Ronald Pokrupa
- Division of Neurosurgery, Kingston General Hospital, Queen’s University, Kingston, Ontario, Canada
| | - Omar Islam
- Department of Radiology, Division of Neuroradiology and Head & Neck Imaging, Kingston General and Hotel Dieu Hospitals, Queen’s University, Kingston, Ontario, Canada
| | - Patrick W. Stroman
- Centre for Neuroscience Studies, Queen’s University, Kingston, Ontario, Canada
- Departments of Diagnostic Radiology and Physics, Queen’s University, Kingston, Ontario, Canada
| | - Michael G. Fehlings
- Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada
- Department of Surgery, Division of Neurosurgery, Toronto Western Hospital, Toronto, Ontario, Canada
- Krembil Neuroscience Centre, Toronto Western Hospital, University Health Network, Toronto, Ontario, Canada
- Division of Genetics and Development, Toronto Western Research Institute, Toronto Western Hospital, University Health Network, Toronto, Ontario, Canada
- * E-mail:
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Stroman P, Bosma R, Kornelsen J, Lawrence-Dewar J, Wheeler-Kingshott C, Cadotte D, Fehlings M. Advanced MR imaging techniques and characterization of residual anatomy. Clin Neurol Neurosurg 2012; 114:460-70. [DOI: 10.1016/j.clineuro.2012.01.003] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2011] [Accepted: 01/05/2012] [Indexed: 12/28/2022]
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14
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Figley CR, Stroman PW. Measurement and characterization of the human spinal cord SEEP response using event-related spinal fMRI. Magn Reson Imaging 2012; 30:471-84. [PMID: 22285878 DOI: 10.1016/j.mri.2011.12.015] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2011] [Revised: 10/14/2011] [Accepted: 12/04/2011] [Indexed: 01/21/2023]
Abstract
Although event-related fMRI is able to reliably detect brief changes in brain activity and is now widely used throughout systems and cognitive neuroscience, there have been no previous reports of event-related spinal cord fMRI. This is likely attributable to the various technical challenges associated with spinal fMRI (e.g., imaging a suitable length of the cord, reducing image artifacts from the vertebrae and intervertebral discs, and dealing with physiological noise from spinal cord motion). However, with many of these issues now resolved, the largest remaining impediment for event-related spinal fMRI is a deprived understanding of the spinal cord fMRI signal time course. Therefore, in this study, we used a proton density-weighted HASTE sequence, with functional contrast based on signal enhancement by extravascular water protons (SEEP), and a motion-compensating GLM analysis to (i) characterize the SEEP response function in the human cervical spinal cord and (ii) demonstrate the feasibility of event-related spinal fMRI. This was achieved by applying very brief (1 s) epochs of 22°C thermal stimulation to the palm of the hand and measuring the impulse response function. Our results suggest that the spinal cord SEEP response (time to peak ≈8 s; FWHM ≈4 s; and probably lacking pre- and poststimulus undershoots) is slower than previous estimates of SEEP or BOLD responses in the brain, but faster than previously reported spinal cord BOLD responses. Finally, by detecting and mapping consistent signal-intensity changes within and across subjects, and validating these regions with a block-designed experiment, this study represents the first successful demonstration of event-related spinal fMRI.
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Affiliation(s)
- Chase R Figley
- Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada
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15
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Kozyrev N, Figley CR, Alexander MS, Richards JS, Bosma RL, Stroman PW. Neural correlates of sexual arousal in the spinal cords of able-bodied men: a spinal fMRI investigation. JOURNAL OF SEX & MARITAL THERAPY 2012; 38:418-435. [PMID: 22900624 DOI: 10.1080/0092623x.2011.606887] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The purpose of this study was to determine whether spinal cord functional magnetic resonance imaging could be used to map neural activity throughout the lower thoracic, lumbar, and sacral spinal cord regions during sexual arousal in healthy men. The authors found that viewing erotic films and genital self-stimulation elicited predominantly increased signal, indicative of amplified neuronal input to the dorsal and ventral horns and in the autonomic preganglionic nuclei of the lower thoracic, lumbar, and sacral spinal cord. In addition, linear regression analyses revealed a number of robust correlations (|R| ≥ 0.7) between signal intensity changes in these spinal cord regions and self-reported ratings of mental and physical sexual arousal. Taken together, these results demonstrate that spinal cord functional magnetic resonance imaging is an effective and sensitive technique for mapping the neural correlates of sexual arousal in the spinal cords of able-bodied men. Most important, the results from this study indicate that spinal cord functional magnetic resonance imaging may have important applications as a clinical tool for assessing and mapping the changes that occur in the spinal cords of men suffering from sexual dysfunction as a result of spinal cord trauma.
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Affiliation(s)
- Natalie Kozyrev
- Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada
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16
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Nakamura Y, Suzuki Y, Tsujita M, Huber VJ, Yamada K, Nakada T. Development of a Novel Ligand, [C]TGN-020, for Aquaporin 4 Positron Emission Tomography Imaging. ACS Chem Neurosci 2011; 2:568-571. [PMID: 22022637 PMCID: PMC3198134 DOI: 10.1021/cn2000525] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2011] [Accepted: 06/27/2011] [Indexed: 12/02/2022] Open
Abstract
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Aquaporin 4 (AQP4), the most abundant isozyme of the water specific membrane transporter aquaporin family, has now been implicated to play a significant role in the pathogenesis of various disease processes of the nervous system from epilepsy to Alzheimer’s disease. Considering its clinical relevance, it is highly desirable to develop a noninvasive method for the quantitative analysis of AQP distribution in humans under clinical settings. Currently, the method of choice for such diagnostic examinations continues to be positron emission tomography (PET). Here, we report the successful development of a PET ligand for AQP4 imaging based on TGN-020, a potent AQP4 inhibitor developed previously in our laboratory. Utilizing [11C]-TGN-020, PET images were successfully generated in wild type and AQP4 null mice, providing a basis for future evaluation regarding its suitability for clinical studies.
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Affiliation(s)
- Yukihiro Nakamura
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
| | - Yuji Suzuki
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
| | - Mika Tsujita
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
| | - Vincent J. Huber
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
| | - Kenichi Yamada
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
| | - Tsutomu Nakada
- Center for Integrated Human Brain Science, Brain Research Institute, University of Niigata, 1 Asahimachi, Niigata 951-8585, Japan
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Le Bihan D. Diffusion, confusion and functional MRI. Neuroimage 2011; 62:1131-6. [PMID: 21985905 DOI: 10.1016/j.neuroimage.2011.09.058] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2011] [Revised: 09/21/2011] [Accepted: 09/23/2011] [Indexed: 02/06/2023] Open
Abstract
Diffusion MRI has been introduced in 1985 and has had a very successful life on its own. While it has become a standard for imaging stroke and white matter disorders, the borders between diffusion MRI and the general field of fMRI have always remained fuzzy. First, diffusion MRI has been used to obtain images of brain function, based on the idea that diffusion MRI could also be made sensitive to blood flow, through the intravoxel incoherent motion (IVIM) concept. Second, the IVIM concept helped better understand the contribution from different vasculature components to the BOLD fMRI signal. Third, it has been shown recently that a genuine fMRI signal can be obtained with diffusion MRI. This "DfMRI" signal is notably different from the BOLD fMRI signal, especially for its much faster response to brain activation both at onset and offset, which points out to structural changes in the neural tissues, perhaps such as cell swelling, occurring in activated neural tissue. This short article reviews the major steps which have paved the way for this exciting development, underlying how technical progress with MRI equipment has each time been instrumental to expand the horizon of diffusion MRI toward the field of fMRI.
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Lawrence JM, Kornelsen J, Stroman PW. Noninvasive observation of cervical spinal cord activity in children by functional MRI during cold thermal stimulation. Magn Reson Imaging 2011; 29:813-8. [DOI: 10.1016/j.mri.2011.02.008] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2010] [Revised: 01/07/2011] [Accepted: 02/20/2011] [Indexed: 11/24/2022]
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Figley CR, Stroman PW. The role(s) of astrocytes and astrocyte activity in neurometabolism, neurovascular coupling, and the production of functional neuroimaging signals. Eur J Neurosci 2011; 33:577-88. [DOI: 10.1111/j.1460-9568.2010.07584.x] [Citation(s) in RCA: 162] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
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20
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Summers PE, Iannetti GD, Porro CA. Functional exploration of the human spinal cord during voluntary movement and somatosensory stimulation. Magn Reson Imaging 2010; 28:1216-24. [PMID: 20573462 DOI: 10.1016/j.mri.2010.05.001] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2009] [Revised: 04/21/2010] [Accepted: 05/08/2010] [Indexed: 11/30/2022]
Abstract
Demonstrations of the possibility of obtaining functional information from the spinal cord in humans using functional magnetic resonance imaging (fMRI) have been growing in number and sophistication, but the technique and the results that it provides are still perceived by the scientific community with a greater degree of scepticism than fMRI investigations of brain function. Here we review the literature on spinal fMRI in humans during voluntary movements and somatosensory stimulation. Particular attention is given to study design, acquisition and statistical analysis of the images, and to the agreement between the obtained results and existing knowledge regarding spinal cord anatomy and physiology. A striking weakness of many spinal fMRI studies is the use of small numbers of subjects and of time-points in the acquired functional image series. In addition, spinal fMRI is characterised by large physiological noise, while the recorded functional responses are poorly characterised. For all these reasons, spinal fMRI experiments risk having low statistical power, and few spinal fMRI studies have yielded physiologically relevant information. Thus, while available evidence indicates that spinal fMRI is feasible, we are only approaching the stage at which the technique can be considered to have been rigorously established as a viable means of noninvasively investigating spinal cord functioning in humans.
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Affiliation(s)
- Paul E Summers
- Dipartimento di Scienze Biomediche, Univ. Modena e Reggio Emilia, Modena, Italy.
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Leitch JK, Figley CR, Stroman PW. Applying functional MRI to the spinal cord and brainstem. Magn Reson Imaging 2010; 28:1225-33. [PMID: 20409662 DOI: 10.1016/j.mri.2010.03.032] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2009] [Revised: 03/07/2010] [Accepted: 03/11/2010] [Indexed: 12/28/2022]
Abstract
Functional magnetic resonance imaging of the spinal cord (spinal fMRI) has facilitated the noninvasive visualization of neural activity in the spinal cord (SC) and brainstem of both animals and humans. This technique has yet to gain the widespread usage of brain fMRI, due in part to the intrinsic technical challenges spinal fMRI presents and to the narrower scope of applications it fulfills. Nonetheless, methodological progress has been considerable and rapid. To date, spinal fMRI studies have investigated SC function during sensory or motor task paradigms in spinal cord injury (SCI), multiple sclerosis (MS) and neuropathic pain (NP) patient populations, all of which have yielded consistent and sensitive results. The most recent study in our laboratory has successfully used spinal fMRI to examine cervical SC activity in a SCI patient with a metallic fixation device spanning the C(4) to C(6) vertebrae, a critical step in realizing the clinical utility of the technique. The literature reviewed in this article suggests that spinal fMRI is poised for usage in a wide range of patient populations, as multiple groups have observed intriguing, yet consistent, results using standard, readily available MR systems and hardware. The next step is the implementation of this technique in the clinic to supplement standard qualitative behavioral assessments of SCI. Spinal fMRI may offer insight into the subtleties of function in the injured and diseased SC, and support the development of new methods for treatment and monitoring.
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Affiliation(s)
- Jordan K Leitch
- Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada
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22
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Figley CR, Leitch JK, Stroman PW. In contrast to BOLD: signal enhancement by extravascular water protons as an alternative mechanism of endogenous fMRI signal change. Magn Reson Imaging 2010; 28:1234-43. [PMID: 20299173 DOI: 10.1016/j.mri.2010.01.005] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2009] [Revised: 12/23/2009] [Accepted: 01/08/2010] [Indexed: 12/14/2022]
Abstract
Despite the popularity and widespread application of functional magnetic resonance imaging (fMRI) in recent years, the physiological bases of signal change are not yet fully understood. Blood oxygen level-dependant (BOLD) contrast - attributed to local changes in blood flow and oxygenation, and therefore magnetic susceptibility - has become the most prevalent means of functional neuroimaging. However, at short echo times, spin-echo sequences show considerable deviations from the BOLD model, implying a second, non-BOLD component of signal change. This has been dubbed "signal enhancement by extravascular water protons" (SEEP) and is proposed to result from proton-density changes associated with cellular swelling. Given that such changes are independent of magnetic susceptibility, SEEP may offer new and improved opportunities for carrying out fMRI in regions with close proximity to air-tissue and/or bone-tissue interfaces (e.g., the prefrontal cortex and spinal cord), as well as regions close to large blood vessels, which may not be ideally suited for BOLD imaging. However, because of the interdisciplinary nature of the literature, there has yet to be a thorough synthesis, tying together the various and sometimes disparate aspects of SEEP theory. As such, we aim to provide a concise yet comprehensive overview of SEEP, including recent and compelling evidence for its validity, its current applications and its future relevance to the rapidly expanding field of functional neuroimaging. Before presenting the evidence for a non-BOLD component of endogenous functional contrast, and to enable a more critical review for the nonexpert reader, we begin by reviewing the fundamental principles underlying BOLD theory.
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Affiliation(s)
- Chase R Figley
- Centre for Neuroscience Studies, Queen's University, Kingston, Ontario, Canada
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Aso T, Urayama SI, Poupon C, Sawamoto N, Fukuyama H, Le Bihan D. An intrinsic diffusion response function for analyzing diffusion functional MRI time series. Neuroimage 2009; 47:1487-95. [PMID: 19450693 DOI: 10.1016/j.neuroimage.2009.05.027] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2008] [Revised: 04/19/2009] [Accepted: 05/11/2009] [Indexed: 11/26/2022] Open
Abstract
To disentangle the temporal profiles of the diffusion and BOLD components of diffusion-weighted functional MRI (DfMRI) during visual activation, we extracted the raw signal from an anatomically defined volume of interest encompassing the visual cortex of 16 subjects. Under the assumption of a linear, time invariant system we were able to define an intrinsic diffusion response function (DRF) from neural tissue, as a counterpart to the hemodynamic response function (HRF) commonly used in BOLD-fMRI. The shape of the DRF response was found to be very similar to the time courses of optical imaging transmittance signals, thought to originate from local geometric changes in brain tissue at the microscopic scale. The overall DfMRI signal response was modeled as the convolution of the stimulation paradigm time course with a DhRF, which is the sum of the DRF and a fractional HRF resulting from residual tissue T2-BOLD contrast. The contribution of the HRF to the DfMRI signal was found to be 26% at peak amplitude, but the DRF component which has a much steeper onset contributed solely at beginning of the response onset. The suitability of this model over the canonical HRF to process DfMRI data was then demonstrated on datasets acquired in 5 other subjects using a rapid event-related design. Some non-linearities in the responses were observed, mainly after the end of the stimulation.
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Affiliation(s)
- Toshihiko Aso
- Human Brain Research Center, Kyoto University Graduate School of Medicine, Kyoto, Japan.
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24
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Flint J, Hansen B, Vestergaard-Poulsen P, Blackband SJ. Diffusion weighted magnetic resonance imaging of neuronal activity in the hippocampal slice model. Neuroimage 2009; 46:411-8. [PMID: 19233299 DOI: 10.1016/j.neuroimage.2009.02.003] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2008] [Revised: 01/15/2009] [Accepted: 02/09/2009] [Indexed: 12/20/2022] Open
Abstract
Functional magnetic resonance imaging (fMRI) has become the leading modality for studying the working brain. Being based on measuring the haemodynamic changes after enhanced mass neuronal activity the spatiotemporal resolution of the method is somewhat limited. Alternative MR-based methods for detection of brain activity have been proposed and investigated and studies have reported functional imaging based on diffusion weighted (DW) MRI. The basis for such DW fMRI is believed to be the sensitivity of diffusion weighted MRI to changes in tissue micro-structure. However, it remains unclear whether signal changes observed with these methods reflect cell swelling related to neural activation, residual vascular effects, or a combination of both. Here we present evidence of a detectable, activity-related change in the diffusion weighted MR-signal from the cellular level in live hippocampal slices in the absence of vasculature. Slices are exposed to substances which evoke or inhibit neural activity and the effects are evaluated and compared. The results are also compared to earlier DW fMRI studies in humans.
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Affiliation(s)
- Jeremy Flint
- Department of Neuroscience, University of Florida, 32610 Gainesville, Florida, USA
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Mandl RCW, Schnack HG, Zwiers MP, van der Schaaf A, Kahn RS, Hulshoff Pol HE. Functional diffusion tensor imaging: measuring task-related fractional anisotropy changes in the human brain along white matter tracts. PLoS One 2008; 3:e3631. [PMID: 18982065 PMCID: PMC2574009 DOI: 10.1371/journal.pone.0003631] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2008] [Accepted: 10/10/2008] [Indexed: 11/19/2022] Open
Abstract
Background Functional neural networks in the human brain can be studied from correlations between activated gray matter regions measured with fMRI. However, while providing important information on gray matter activation, no information is gathered on the co-activity along white matter tracts in neural networks. Methodology/Principal Findings We report on a functional diffusion tensor imaging (fDTI) method that measures task-related changes in fractional anisotropy (FA) along white matter tracts. We hypothesize that these fractional anisotropy changes relate to morphological changes of glial cells induced by axonal activity although the exact physiological underpinnings of the measured FA changes remain to be elucidated. As expected, these changes are very small as compared to the physiological noise and a reliable detection of the signal change would require a large number of measurements. However, a substantial increase in signal-to-noise ratio was achieved by pooling the signal over the complete fiber tract. Adopting such a tract-based statistics enabled us to measure the signal within a practically feasible time period. Activation in the sensory thalamocortical tract and optic radiation in eight healthy human subjects was found during tactile and visual stimulation, respectively. Conclusions/Significance The results of our experiments indicate that these FA changes may serve as a functional contrast mechanism for white matter. This noninvasive fDTI method may provide a new approach to study functional neural networks in the human brain.
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Affiliation(s)
- René C W Mandl
- Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Utrecht, The Netherlands.
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