The%20Basics%20of%20fMRI

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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

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Physiological origins
stimulus
Change in oxydeoxy 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
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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)

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Physical origins
Structural MRI
Proton (1H) has a magnetic moment which can be
non-toxically manipulated with transitory
magnetic fields (B1) and RF energy to produce
images of its local environment.
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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

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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 gt TE often TRTA)

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Detecting BOLD
Deoxyhaemoglobin is paramagnetic, positive
susceptibility

• Changes local susceptibility and therefore T2
• MRI Sequence requirements
• T2 sensitive
• fast
• whole brain
• gt Echo planar imaging (EPI)

Thulborn et al., 1982
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MRI Safety

• Powerful magnetic field
• Extreme forces during rapidly changing gradients
• RF energy deposition
• Confined environment / restraints
• Need for controlled access and screening

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Paradigm Design
Block paradigm

Event Related
Event Related (compressed)


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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

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Data Analysis

• EPI MRI data volumes are continuously acquired
  whilst the subject performs some cognitive task
  (paradigm).
• Following pre-processing, analysis proceeds as
• The within-group activation engendered by the
  paradigm on average
• The between-group difference in activation or the
  within-group correlation of activation with some
  other variable

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Data Analysis summary

• Pre-process data from each individual to correct
  subject motion
• Estimate response at each voxel (General linear
  model)
• Map subjects into same anatomic space
• Statistically infer activation for each group
• Statistically infer difference between groups

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Data Analysis of individuals
Response estimation (GLM)
Difference between male/female Europeans
Difference between European/Japanese males
Subject movement
Spatial normalisation
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Real time fMRI
Block paradigm Two conditions (motor,
spatial) Two distinct regions Infer responses
Monti et al, 2010
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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)
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Resting state fMRI
Doing nothing
Seed-based correlations
Deactivations regions with greater activations
in task-absent conditions
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Resting state fMRI
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End