Transcript Introduction - Department of Psychiatry

The Basics of fMRI
Overview:
• Physiological origins (BOLD)
• Physical origins (MRI)
• Physical measurements of BOLD
• Paradigm (task) design
• Analysis of task-activated fMRI
• Resting fMRI
Physiological origins
stimulus
Change in
oxy:deoxy ratio
Neurophysiological coupling:
neuronal activity  blood oxygenation
Logothetis et al. (2001) Nature 412: 150
LFP – local field potentials
MUA – multi unit activity
SDF – spike density function
Physiological origins
Local hemodynamic changes
[ROY, C. W., SHERINGTON, C. S.
On the regulation of the blood-supply of the brain.
J. Physiol. (Lond.) 11: 85-108, 1890.]
• Increase in local blood flow (+50%)
• Increase in local blood volume
• Small increase in oxygen consumption (+15%)
• Increased flow means reduced O2 extraction ‘hence’
oversupply of blood
• Haemodynamic response function (HRF)
Physical origins
Structural MRI
Proton (1H) has a magnetic moment which can be nontoxically manipulated with transitory magnetic fields (B1) and
RF energy to produce images of its local environment.
Physical origins
Amplitude of signal determined by:
• Proton density: more protons, more signal
• T1 (spin-lattice relaxation): Stimulated by local magnetic
field fluctuations due to magnetic properties of other
molecules.
• T2*: Enhanced dephasing due to inhomogeneities in local
magnetic field (inc. B0) – susceptibility.
• T2 (spin-spin relaxation): residual dephasing during
dephase-rephase period
• Contrast agents: changing local susceptibility
Physical origins
Definitions
• Time to Repetition (TR): The TR is the time between
consecutive sequence initiations
• Time to Echo (TE): TE determines the sensitivity to T2*,
which varies for different tissues
• Acquisition Time (TA): The time between acquisitions
(TR > TE; often TR=TA)
Detecting BOLD
Deoxyhaemoglobin is paramagnetic, positive susceptibility:
Changes local susceptibility and
therefore T2*
MRI Sequence requirements:
• T2* sensitive
• fast
• whole brain
=> Echo planar imaging (EPI)
Thulborn et al., 1982
MRI Safety
• Powerful magnetic field
• Extreme forces during rapidly changing gradients
• RF energy deposition
• Confined environment / restraints
• Need for controlled access and screening
Paradigm Design
Block paradigm
TR
Event Related
Continuous acquisition
…
…
…
…
Continuous acquisition
Event Related (compressed)
…
…
Paradigm Design
Magnitude of the BOLD effect can be modulated
experimentally:
• Task difficulty (set size, encode-retrieve delay, stimulus
frequency, stimulus load, distracters…)
• Learning (decrease in activation)
• Accommodation (decrease in activation)
• Other stimulus (pharmaceutical, mood induction, age…)
[somatosensory internal standard, on-line behavioural data]
Baseline tasks must be appropriate for cognitive subtraction
Data Analysis
EPI MRI data volumes are continuously acquired whilst the
subject performs some cognitive task (paradigm).
Following pre-processing, analysis proceeds as:
1. The within-group activation engendered by the
paradigm on average
2. The between-group difference in activation or the
within-group correlation of activation with some other
variable
Data Analysis: summary
(1) Pre-process data from each individual to correct subject
motion
(2) Estimate response at each voxel (General linear model)
(3) Map subjects into same anatomic space
(4) Statistically infer activation for each group
(5) Statistically infer difference between groups
Data Analysis of individuals
Response estimation (GLM)
Difference between
European/Japanese males
Difference between
male/female Europeans
Subject movement
Spatial normalisation
Real time fMRI
Block paradigm
Two conditions (motor, spatial)
Two distinct regions
Infer responses
Monti et al, 2010
Data Analysis of groups
Differences extend outside “activated” network
Within-group
H0: activation is
uncorrelated to stimulus
Between-group
H0: zero mean difference
between groups (whole brain)
Resting state fMRI
“Doing nothing”
Seed-based correlations
Deactivations: regions
with greater activations
in task-absent conditions
Resting state fMRI
(a) anaesthetized macaque; (b) human
End