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  • 1. fMRI introduction
    Michael Firbank
    m.j.firbank@ncl.ac.uk
  • 2. Brain activation imaging
    Functional imaging
    Used to locate regions of brain activity
  • 3. Brain activation techniques
  • 4. MRI
    Magnetic resonance imaging (MRI)
    Person is placed in a large magnet
    Approx 20 000 times earth’s magnetic field
  • 5. MRI safety
    Magnetic objects
    Pacemakers
    Metal implants
  • 6. Imaging Techniques ReminderMagnetic Resonance Imaging
    Water
    RF Energy
    In
    MR Signal
    Out
    N
    S
  • 7. 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
  • 8. 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
  • 9. 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).
  • 10. Neural activity
    Increased neuronal activity
    ► Increased oxygen consumption
    ► Increased blood flow
    in excess of oxygen demand
    ► Decreased deoxy-Haemoglobin concentration
  • 11. 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)
  • 12. 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
  • 13. 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)
  • 14. 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
  • 15. 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...”
  • 16. Sparse design
    Acquire image every 7-14 seconds
    Makes it easier to hear stimuli
    But inefficient
    Imaging
    BOLD
    response
    Stimulus
    Time (s)
  • 17. Analysis
    All scans spatially aligned together
    Data spatially smoothed (~5mm) & Temporally filtered
    Look for correlations between expected response function and data
  • 18. Analysis
    Brain map showing regions with significant correlation between signal change and task
  • 19. 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
  • 20. fMRI - SummaryFactors which modulateblood oxygenation
    rCBV/CBF
    ?
    Altered
    HbO2/Hb
    Ratio
    BOLD Effect
    Glucose
    O2
    CMRO2 /CMRgl
    Neuronal
    activity
  • 21. fMRI summary
    Sensitive to changes in blood oxygenation
    Spatial resolution of ~2mm
    Temporal resolution of seconds