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