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fMRI introduction - Newcastle University, Newcastle upon Tyne ...

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    fMRI introduction - Newcastle University, Newcastle upon Tyne ... fMRI introduction - Newcastle University, Newcastle upon Tyne ... Presentation Transcript

    • fMRI introduction
      Michael Firbank
      m.j.firbank@ncl.ac.uk
    • Brain activation imaging
      Functional imaging
      Used to locate regions of brain activity
    • Brain activation techniques
    • MRI
      Magnetic resonance imaging (MRI)
      Person is placed in a large magnet
      Approx 20 000 times earth’s magnetic field
    • MRI safety
      Magnetic objects
      Pacemakers
      Metal implants
    • Imaging Techniques ReminderMagnetic Resonance Imaging
      Water
      RF Energy
      In
      MR Signal
      Out
      N
      S
    • Imaging Techniques ReminderMagnetic Resonance Imaging
      1
      0.8
      0.6
      Water
      0.4
      RF Energy
      MR Signal
      0.2
      0
      Time
      TE
      Signal decays over a few 10s milliseconds
      Rate depends on local tissue properties
      T2 (*)
      N
      S
    • Neural activity
      Brain uses ~20% of energy
      Energy use linked to neuronal activity
      Provided through glucose and oxygen
      Oxygen is supplied by haemoglobin in blood
      Oxy haemoglobin
      Deoxy haemoglobin
    • High blood oxygenation
      Low blood oxygenation
      Linking MRI to brain function (fMRI)
      MRI signal can be made sensitive to tissue oxygenation
      Oxygenated haemoglobin is diamagnetic
      No effect on image
      “deoxy-haemoglobin” is paramagnetic
      Locally alters magnetic field
      Intrinsic contrast agent
      Reduces signal amplitude
      Brain magnetic resonance imaging with contrast dependent on blood oxygenation Ogawa et al, Proc Nat AcadSci, 87:9868-9872, (1990).
    • Neural activity
      Increased neuronal activity
      ► Increased oxygen consumption
      ► Increased blood flow
      in excess of oxygen demand
      ► Decreased deoxy-Haemoglobin concentration
    • Neural activity & BOLD
      Deoxy Haemoglobin is paramagnetic
      Causes local variations in magnetic field
      Lower signal on T2* weighted images
      Brain activation ► lower Deoxy Hb in capillaries/venules ► increased MR signal
      Blood oxygenation level dependent signal (BOLD)
    • fMRIBOLD andHaemodynamiceffects
      CBF/CBV takes over
      Oxygenates
      Initial “dip”
      (CMRO2 dominates)
      Deoxygenates
      Increased energy consumption extracts oxygen
      Vasodilation and CBF increase oxygen supply
      • Hemodynamic response time of ~3s
    • BOLD response
      Reasonably linearly additive
      Dale & Bucker 1997 HBM
    • BOLD response
      Relative signal change
      need to compare stimuli / mental tasks
      Slow variations in baseline intensity
      different tasks need to be close together
      Signal change ~ 0.5 - 5 %
      SNR ~ 0.5 - 2
      5 minutes of acquisition (~200 MRI volumes)
    • fMR imaging
      EPI sequence – fast and sensitive to T2*
      Whole brain collection ~2s at ~3 mm resolution
      Some distortion & signal dropout(particularly frontal)
      Noisy (makes auditory tasks difficult)
      “Dropout”
      Distortion
    • fMRI acquisition
      Whole brain image collection every ~2 seconds whilst
      subject does some mental task
      “It is an ancient Mariner, ...”
      “London. Michaelmas term lately over...”
    • Sparse design
      Acquire image every 7-14 seconds
      Makes it easier to hear stimuli
      But inefficient
      Imaging
      BOLD
      response
      Stimulus
      Time (s)
    • Analysis
      All scans spatially aligned together
      Data spatially smoothed (~5mm) & Temporally filtered
      Look for correlations between expected response function and data
    • Analysis
      Brain map showing regions with significant correlation between signal change and task
    • Analysis
      For group studies, individual scans are transformed into a common coordinate space
      Allows generalisations to be made about populations
      Loses individual variations
      Dependent on accuracy of transformation
    • fMRI - SummaryFactors which modulateblood oxygenation
      rCBV/CBF
      ?
      Altered
      HbO2/Hb
      Ratio
      BOLD Effect
      Glucose
      O2
      CMRO2 /CMRgl
      Neuronal
      activity
    • fMRI summary
      Sensitive to changes in blood oxygenation
      Spatial resolution of ~2mm
      Temporal resolution of seconds