fmri introduction - newcastle university, newcastle upon tyne
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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 earths 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
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