Functional MRI

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Functional MRI

• Functional MRI is based on the increase in blood
  flow to the local vasculature that accompanies
  neural activity in the brain.
• Results are functional maps of changes in
  cerebral venous oxygen concentration that
  correlate with neuronal activity.

Picture Naming Task
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More oxygen!

• Neural activity consumes energy
• Energy is provided by ATP when it
• hydrolyses to ADP
• ATP is produced from ADP by
• oxidative phosphorylation
• This metabolism requires oxygen
• Oxygen is supplied in the
• hemoglobin molecule of blood
• Oxyhemoglobin vs. deoxyhemoglobin

BLOOD FLOW
OXYGEN
HEMOGLOBIN
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More oxygen (2)!

• During neural activity the concentration of
  deoxyhemoglobin in the area decreases as blood
  flow increases
• Deoxyhemoglobin is paramagnetic, which causes a
  susceptibility difference between vessel and
  tissue
• This causes dephasing of the MR proton signal
• Deoxyhemoglobin is an endogenous contrast
  enhancing agent, and serves as the source of the
  signal for fMRI
• Blood Oxygen Level Dependent (BOLD) effect

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Functional activity of the brain

• Intensity of fMRI images increases with the
  increase of brain activation
• This increase is small (usually less than 2)
• The increase is easily obscured by noise and
  different artifacts

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Timing diagram for an EPI sequence

• 90 slice selective RF pulse

• slice selection gradient GS

• initial phase encoding gradient pulse G?

• initial frequency encoding gradient pulse Gf

• 180 pulse

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Advantages of fMRI

• Non-invasive
• Does not require injection of radioactive
  isotopes
• Total scan time relatively short
• Good spatial resolution

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Disadvantages of fMRI

• Very sensitive to head movement artefact
• Requires task repetition and image averaging
• Can not directly map neuroreceptors
• Noisy environment for the patient
• Expensive

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Sources of artefacts

• Cardiac pulsation
• Respiration
• Patient movement
• Signal from draining veins

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Future of fMRI

• Neurosurgical planning
• Pain management
• Research of neurological disorders
• Research of cognitive and perceptual events

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References

• Toga Mazziotta (1996) Brain Mapping, the
  Methods Academic Press, Inc.
• Jonathan R. Merrill (1997) Annals of the New
  York Academy of Sciences, vol. 82, New York
  Academy of Sciences
• Shung, Smith, Tsui (1992) Principles of Medical
  Imaging Academic Press, Inc.
• Functional MRI Methods and Applications by
  Stuart Clare http//www.fmrib.ox.ac.uk/stuart/th
  esis/index.html
• The Basics of MRI by Joseph P. Hornak
  http//www.cis.rit.edu/htbooks/mri/
• Memorial Sloan-Kettering Cancer Center, the
  Laboratory of Functional MRI http//www.fmri.org/
  fmri.htm

Brocas Area
Primary Auditory Cortex