fMRI introduction Michael Firbank

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

Brain activation techniques

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

MRI safety Magnetic objects Pacemakers Metal implants

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

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 (*) Time TE

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

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: , (1990).

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

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)

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

BOLD response Reasonably linearly additive Dale & Bucker 1997 HBM

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 ~ % SNR ~ – 5 minutes of acquisition (~200 MRI volumes)

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”

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...”

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

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

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

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

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

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