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

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

  1. 1. fMRI introduction Michael Firbank m.j.firbank@ncl.ac.uk
  2. 2. Brain activation imaging • Functional imaging • Used to locate regions of brain activity
  3. 3. Brain activation techniques
  4. 4. MRI • Magnetic resonance imaging (MRI) • Person is placed in a large magnet – Approx 20 000 times earth’s magnetic field
  5. 5. MRI safety • Magnetic objects • Pacemakers • Metal implants
  6. 6. Imaging Techniques Reminder Magnetic Resonance Imaging S N Water RF Energy In MR Signal Out
  7. 7. Imaging Techniques Reminder Magnetic Resonance Imaging S N Water RF Energy MR Signal • Signal decays over a few 10s milliseconds – Rate depends on local tissue properties – T2 (*) 0 0.2 0.4 0.6 0.8 1 Time TE
  8. 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. 9. 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 High blood oxygenation Low blood oxygenation Brain magnetic resonance imaging with contrast dependent on blood oxygenation Ogawa et al, Proc Nat Acad Sci, 87:9868-9872, (1990).
  10. 10. Neural activity • Increased neuronal activity ► Increased oxygen consumption ► Increased blood flow in excess of oxygen demand ► Decreased deoxy-Haemoglobin concentration
  11. 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. 12. fMRI BOLD and Haemodynamic effects • Increased energy consumption extracts oxygen • Vasodilation and CBF increase oxygen supply  Hemodynamic response time of ~3s Initial “dip” (CMRO2 dominates) Deoxygenates CBF/CBV takes over Oxygenates
  13. 13. BOLD response • Reasonably linearly additive Dale & Bucker 1997 HBM
  14. 14. 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)
  15. 15. 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) Distortion “Dropout”
  16. 16. 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...”
  17. 17. Sparse design Imaging BOLD response Stimulus Time (s) Acquire image every 7-14 seconds Makes it easier to hear stimuli But inefficient
  18. 18. Analysis • All scans spatially aligned together • Data spatially smoothed (~5mm) & Temporally filtered • Look for correlations between expected response function and data
  19. 19. Analysis Brain map showing regions with significant correlation between signal change and task
  20. 20. 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
  21. 21. fMRI - Summary Factors which modulate blood oxygenation Neuronal activity ? rCBV/CBF GlucoseO2 CMRO2 / CMRgl BOLD Effect Altered HbO2/Hb Ratio
  22. 22. fMRI summary • Sensitive to changes in blood oxygenation • Spatial resolution of ~2mm • Temporal resolution of seconds

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