fMRI in Anesthesia

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fMRI in Anesthesia

• Katri Suominen
• katri.a.suominen_at_tut.fi

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Introduction

• Functional Magnetic Resonance Imaging is an
  efficient tool to study the human brain
• Blood Oxygen Level Dependent Contrast can detect
  even small blood oxygen level fluctuations

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Outline

• fMRI and BOLD
• Vasomotor fluctuations in fMRI of anesthetized
  child brain
• Methods for detecting BOLD fluctuation

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fMRI

• Technique for determining physiological events
  and changes in the tissues with the help of MRI
• First used to determine which parts of the brain
  are activated by different types of physical
  sensation or activity
• Later on to determine higher brain functions
• In mid 1990 LF fluctuations of non-activated fMRI
  were found

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BOLD-Contrast

• Widely used in fMRI
• BOLD Blood Oxygen Level Dependent
• Oxygenated hemoglobin molecule is diamagnetic and
  deoxygenated hemoglobin is paramagnetic
• Intrinsic contrast molecule for fMRI
• With methods sensitive to susceptibility the
  change in deoxyhemoglobin concentration can be
  detected

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BOLD

• Gradient echo images are sensitive to bigger
  vessels (20µm)
• Spin echo BOLD enhancement is about 2 times
  smaller than gradient echo BOLD enhancement
• Spin echo images are more sensitive to small
  venules (5-10µm)
• Detects BOLD nearer to the actual activation site

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BOLD

• BOLD signal reflects metabolic changes in
  activated areas
• Metabolism is related to neuronal synaptic and
  electrophysiological activity

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Fluctuation in BOLD Signal

• Already first 4T fMRI results showed fluctuation
  in the resting state BOLD signal
• Biswall showed in the mid 1990 that the BOLD
  signal oscillates at 0.1 Hz at resting state
• Hypothesized that these fluctuations exist
  because of the vasomotor activity related to
  functional connectivity of brain areas

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Vasomotor Fluctuation in fMRI of Anesthetized
Child Brain

• Vasomotor fluctuations were investigated in this
  work
• 12 children anesthetized with thiopental
• Hypothesis thiopental reduces blood preasure and
  flow in the cortex and varies the feedback in
  neurovascular coupling
• Reduction of the frequency and increase of
  magnitude of the fluctations

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Materials and Methods

• 12 children anesthetized with thiopental
• Several contiguous spin echo T1 weighted images
  were taken
• The time domain signal from each voxel was
  transferred into frequency domain using FFT
• Selected frequency was color-encoded placed on
  corresponding anatomical image
• To ensure that the fluctuation was not due to
  motion COM was calculated of each image and
  changes in the position as a function of time was
  analyzed

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Results and Conclusions

• The areas presenting fluctuations were related to
  primary sensory areas
• The subject data showed a peak at around 0.033
  Hz, not at 0.1 Hz as previously reported, that is
  in 7 case due to physiological fluctuations
• 5 cases due to subject motion
• In further activation studies, at least in
  thiopental anesthesia, there must be taken care
  to separate stimulus activation out of these
  fluctuations

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Methods for Detecting BOLD Fluctuation in fMRI

• 15 child patients under thiopental anesthesia
• Fast Fourier Transform (FFT)
• Cross Correlation
• Spatial Principal Component Analysis (sPCA)
• Spatial Independent Component Analysis

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ICA

• Data derived method
• Detects domponents of the data that ara maximally
  non-gaussian
• No need for a priori knowledge of the data
• XAS
• Estimates the mixing matrix A using a iterative
  algorithm
• Statistically independent components can be
  solved SA-1X

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FFT vs ICA

• FFT is fast and good for overall screening of
  fluctuations, but has a rather poor accuracy of
  anatomic segmentation
• ICA separates better BOLD signal fluctuation in
  the cortex from other physiological signals
• Random order of components, not enough knowledge
  on the exact physiological events

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References

• Kiviniemi, Vesa Spontaneus Blood Oxygen
  Fluctuation in Awake and Sedated Brain Cortex- A
  BOLD fMRI Study Oulu University Press, Oulu 2004
• http//www.fmrib.ox.ac.uk/fmri_intro/brief.html
• http//www.cs.helsinki.fi/u/ahyvarin/whatisica.sht
  ml

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Thank You