exploring magnetoencephalography (meg ) data acquisition and analysis techniques
DESCRIPTION
Exploring Magnetoencephalography (MEG ) Data Acquisition and Analysis Techniques Rosalia F. Tungaraza , Ph.D. Anthony Kelly, B.A. Ajay Niranjan, M.D., MBA July 22, 2010. What does MEG Measure?. Data Acquisition and Analysis Steps. Acquisition. Coregistration. Preprocessing. Averaging. - PowerPoint PPT PresentationTRANSCRIPT
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Exploring Magnetoencephalography (MEG)Data Acquisition and Analysis Techniques
Rosalia F. Tungaraza, Ph.D.
Anthony Kelly, B.A.
Ajay Niranjan, M.D., MBA
July 22, 2010
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What does MEG Measure?
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Data Acquisition and Analysis Steps
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Acquisition
Coregistration
Preprocessing
Averaging
Localization
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Data Acquisition and Analysis Steps
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Fitting isotrack points to structural MRI
Fitting a sphere
Acquisition
Coregistration
Preprocessing
Averaging
Localization
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Data Acquisition and Analysis Steps
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Temporal FilteringBandpass 0.5 – 40Hz
Maxwell’s Filtering
Signal Source Projection (SSP)
Acquisition
Coregistration
Preprocessing
Averaging
Localization
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Raw Continuous Data
Time frame: 10000 ms 6
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Elekta's Maxwell Filtering Method
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After Maxwell Filtering
Time frame: 10000 ms 8
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Is the Filtering Process Worth it?
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Reduction in Covariance of Magnetometer Signals
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Data Acquisition and Analysis Steps
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Selecting a time region and baseline
Averaging by condition
Acquisition
Coregistration
Preprocessing
Averaging
Localization
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Retrieving Trials
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Baseline: 200ms
1000ms :Time-frame
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Median Nerve Stimulation
● Random 10ms long electrical stimulation of same voltage
● Purpose:● localizer● explore effect of number of trials on results
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Median Nerve Stimulation Average
14180 Trials
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Hearing Your Voice in Real Time
Purpose:● explore limitations of MEG (jaw movement, head
motion, activates many simultaneous brain regions e.t.c.) 15
+ house
…+ water
- Press a button- Read word out loud
+ book
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Hearing Your Voice in Real Time Stimulation Average
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- Complex task: elicits response from multiple brain regions- Variations in subject’s response
60 Trials
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Data Acquisition and Analysis Steps
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Dipole Fitting
Acquisition
Coregistration
Preprocessing
Averaging
Localization
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Source Localization
?
ForwardProblem
InverseProblem
Solvable!
Always anestimate
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?
Dipole Fitting Technique
1. Pick a subset of sensors (ROI)
2. Select a time point
3. Dipole fit estimates magnetic field given the two parameters
PROCEDURE
Estimated Signals Measured Signals
4.
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Goodness-of-fit a measure that shows how much the estimated signal matches the measured signal
Confidence Volume the volume within which the dipole fitting method is confident the dipole exists
Measures of Quality
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22ms 52ms 83ms
7 sensors
42 sensors
92 sensors
99.7% 99.2% 98.2%
84.6% 97.6% 85.8%
84.6% 97.6% 85.8%
Median Nerve Dipole Fitting Results
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Hearing Your Voice in Real Time Dipole Fitting Results
• Fits dipole in improbable locations (single dipole insufficient)
• Needs a different source localization technique
Brain Stem Post-central GyrusFace Area
61.0% 80.3% 82.2%
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SummaryWe have explored the following steps in MEG image acquisition and analysis:
• Acquisition of the MEG signals
• Coregistration of MEG isotrack data with structural MRI
• Preprocessing of the MEG signals
• Averaging the preprocessed signals
• Source localization of the averaged waveforms
We found both strengths and weaknesses, which the user must take into account before making inferences from their analyzed data.
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Acknowledgements● MEG
– Erika Laing – Anna Haridis– Dr. Ajay Niranjan
● MNTP– Drs. Seong-Gi Kim and Bill Eddy– Tomika Cohen and Rebecca Clark
● All other MNTP participants
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