2. fMRI introduction Michael Firbank m.j.firbank@ncl.ac.uk

3. Brain activation imaging Functional imaging Used to locate regions of brain activity

4. Brain activation techniques

5. MRI Magnetic resonance imaging (MRI) Person is placed in a large magnet – Approx 20 000 times earth’s magnetic field

6. MRI safety Magnetic objects Pacemakers Metal implants

7. Imaging Techniques Reminder Magnetic Resonance Imaging S N Water RF Energy In MR Signal Out

8. 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 – T 2 (*) 0 0.2 0.4 0.6 0.8 1 Time TE

9. 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

10. 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).

11. Neural activity Increased neuronal activity ► Increased oxygen consumption ► Increased blood flow in excess of oxygen demand ► Decreased deoxy-Haemoglobin concentration

12. 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)

13. fMRI BOLD and Haemodynamic effects Increased energy consumption extracts oxygen Vasodilation and CBF increase oxygen supply Hemodynamic response time of ~3s Initial “dip” (CMRO 2 dominates) Deoxygenates CBF/CBV takes over Oxygenates

14. BOLD response Reasonably linearly additive Dale & Bucker 1997 HBM

15. 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)

16. 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”

17. 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...”

18. Sparse design Imaging BOLD response Stimulus Time (s) Acquire image every 7-14 seconds Makes it easier to hear stimuli But inefficient

19. Analysis All scans spatially aligned together Data spatially smoothed (~5mm) & Temporally filtered Look for correlations between expected response function and data

20. Analysis Brain map showing regions with significant correlation between signal change and task

21. 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

22. fMRI - Summary Factors which modulate blood oxygenation Neuronal activity ? rCBV/CBF Glucose O2O2 CMRO 2 / CMR gl BOLD Effect Altered HbO 2 /Hb Ratio

23. fMRI summary Sensitive to changes in blood oxygenation Spatial resolution of ~2mm Temporal resolution of seconds