Functional Magnetic Resonance Imaging

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Functional Magnetic Resonance Imaging
Albert Parker February 26, 2002
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Sources

• Functional Magnetic Resonance Imaging
• Mark S. Cohen, Ph.D
• Susan Y. Bookheimer, Ph.D.
• UCLA Brain Mapping Division
• Trends in Neurooscience
• http//spinwarp.ucsd.edu/fmri/FMRI-TINS.html)
• Journey to the Center of My Mind
• Stephen S. Hall
• The New York Times, 6/1999
• http//www.fmri.org/jounrey.html
• Basics of NMR
• Joseph P. Hornak
• Department of Chemistry and Imaging Science
• Rochester Institute of Technology
• http//www.cis.rit.edu/htbooks/nmr/bnmr.htm

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• How does fMRI form an image of Neural Activity in
  the brain??
• Overview of the Physics
• Encoding of spatial location
• Encoding of neural activity
• Experimental procedure

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fMRI technology is based on

• Nuclear Magnetic Resonance (NMR) phenomenon
  (1946, 1950)
• high concentration of hydrogen nuclei in
  biological systems and its high sensitivity to
  NMR signals.
• magnetic susceptibility of oxyhemoglobin and
  deoxyhemoglobin (1936)

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The Physics of NMRBloch (1946) and Hahn (1950)

• 1. Atomic nuclei with an odd number of neurons
  and/or protons have
• a small magnetic moment.
• an angular momentum called nuclear spin

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The Physics of NMRBloch (1946) and Hahn (1950)

• 2. Magnetic moments will align (anti) parallel
  to an applied static magnetic fields.
• 3. Random atomic collisions and other
  perturbations allow the system to reach an
  equilibrium with an excess of protons aligned
  with the static magnetic field.

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The Physics of NMRBloch (1946) and Hahn (1950)

• 4. If one applies a static magnetic field to
  nuclei with spin, the magnetization of each
  nucleus has a resonance (or Larmor) frequency f
  defined by
• f ? B0

proportionality constant for specific nuclear
species (MHz/Tesla)
strength of static magnetic field (typically
about 1.5 Tesla)
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The Physics of NMRBloch (1946) and Hahn (1950)

• 5. Excitation If an oscillating magnetic field
  (called a radio frequency pulse) is applied to
  nuclei at their resonance frequency, their spins
  absorb energy, causing the magnetization of the
  nuclei to precess in phase about the static
  magnetic field.

The precessing magnetization can be measured by a
nearby coil.
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The Physics of NMRBloch (1946) and Hahn (1950)

• 6. Decay Following excitation, the precessing
  magnetization returns to equilibrium according to
  exponential decay.
• s(t) be an magnetic resonance (MR) signal
• T2 is the MR signal decay rate (from a few to
  tens of ms)
• The T2 rates are different for different
  biological tissues!
• (in particular, for oxyhemoglobin and
  deoxyhemoglobin!!)

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How does fMRI form an image of neural
activity?How to form an image of neural
activity using NMR? Need to measure1.
Spatial Location2. Neural Activity Magnitude
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Spatial Location in 1-D

• Frequency Encoding (Lauterbur 1973)
• relate frequency linearly to spatial location by
• B(x) B0 Gx x

gradient strength of the static magnetic static
field strength
location in 1-D
base strength of the static magnetic field

• So the resonance frequency of the nuclei at
  location x is given by
• f(x) ? B(x) ?(B0 Gx x)

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Spatial Location in 1-D

• Take the Fourier transform of the MR signal s(t),
  Fs(t), to decompose s(t) into its power
  spectrum (a signals frequencies and associated
  amplitudes).
• In the case of an object composed of a particular
  medium, we get an image of that object at a
  particular instant in time a 1-D distribution of
  the magnetization intensity at location x

measure s(t)
F s(t)
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Spatial Location in 1-D
Frequency Encoding in practice
Phase Encoding
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Spatial Location in 2-D
Echo Planar Imaging (Mansfield 1977) use
frequency encoding to determine the x
direction and echo encoding to determine the y
direction.
sampling windows
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Spatial Location in 3-D

• Since most humans are 3-D, the third dimension is
  incorporated into the procedure by performing
  slice-selective excitation apply the RF pulse
  as a function of position in the 3rd dimension
  (or by moving the static magnetic field along the
  3rd dimension).

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How to measure neural activity?

• The physiology of neural activity involves many
  complex processes.
• MR has the capability to measure parameters
  related to several neural physiological
  functions, including
• changes in phosphorus metabolism and metabolic
  byproducts
• blood flow
• blood volume
• blood oxygenation

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Blood oxygen level dependent contrast
(BOLD)(Ogawa et al. 1990)

• The most common technique used in fMRI.
• Takes advantage of the magnetic susceptibility of
  oxyhemoglobin and deoxyhemoglobin (Pauling 1936).
  Deoxy Hb has a higher precess magnitization
  decay rate than does oxy Hb. (They have
  different T2 rates!)

During periods of neuronal activity, local blood
flow and volume increase with little or no
change in oxygen consumption. As a
consequence, the oxygen content of the venous
blood is elevated, resulting in an increase in
the MR signal.
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Experimental Procedure
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so fMRI works like this

• Use EPI and slice selective excitation
• Apply a static magnetic field with intensity B0.
• Use an RF pulse with frequency matching resonance
  frequency of the desired medium (e.g. hydrogen
  nuclei) for the given static magnetic field
  intensity B0.
• Apply magnetic field gradients in x and y
  directions.
• Use coils to measure the MR signal s(t).
• Calculate Fs(t) over each sampling window to
  determine MR signal location in the 2-D slice we
  have selected.
• Take into account how fast the signal has decayed
  since the RF pulse (T2 decay) when interpreting
  MR signal strength at each location.
• Give image intensity

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Issues!
There is not a one-to-one correspondence between
T2 and the neural activity that we are trying to
measure. There are pathways that might decrease
the decay rate and hence results in a decreased
MR signal!
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Issues!

• Small size in of activation related response
  leaves it susceptible to noise (low SNR) from
• thermal and electromagnetic noise from the
  subject
• reception coil, preamps and other electronics
• quantization noise from analog to digital
  conversion
• cardiac and respiratory cycles
• head movement (problem especially for speech
  tasks)
• uncontrolled neuronal events
• differences in the manner in which a task is
  performed
• neuronal events associated with behavior
  unrelated to the task
• spontaneous firing of networks
• MRI response is delayed and relatively slow
  compared to brain activity

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Dealing with the issues!

• rapid data acquisition techniques
• special reception coils
• increasing static magnetic field intensity
• SNR depends on temporal resolution - lower
  temporal resolution
• post-processing techniques, movement correction
  algorithms,
• different gradient systems
• multi-shot techniques
• head restraints and bite bars
• use cortical landmarks

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Comparison temporal and spatial resolution and
invasiveness
This figure relates the temporal and spatial
resolution of methods for the study of brain
function to the size scale of neuronal features
and to the invasiveness of the methods.
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How fMRI stacks up

• temporal resolution seconds
• spatial resolution cortical columns
• invasiveness apparently completely safe, barring
  pacemakers
• Though few Neuroscientists will be able to
  afford MR devices of their own with fMRI it
  will be possible to perform longitudinal studies
  on individual subjects advancing the practical
  spatial resolution of functional imaging and
  enabling vastly more complex experimental designs
  - Mark Cohen
• fMRI will not map the cortical and subcortical
  functions of the human brain, but it has moved us
  closer to the ideal.