Surface-based Analysis: Intersubject Registration and Smoothing
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Surface-based Analysis:Intersubject Registration and Smoothing
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Outline
• Exploratory Spatial Analysis • Coordinate Systems
• 3D (Volumetric)• 2D (Surface-based)
• Intersubject registration• Volume-based• Surface-based
• Surface-based smoothing• Surface-based clustering
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Exploratory Spatial Analysis
• Don’t know where effect is going to be • vs ROI analysis• Analyze each voxel separately• Create a map• Find clusters
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Aging Exploratory Analysis
Cortical Thickness vs AgingSalat, et al, 2004, Cerebral Cortex
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p<.01
Aging Thickness Study
Positive Age Correlation
Negative Age Correlation
N=40
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Individual Exploratory Analysis
• fMRI Words-vs-Fixation• Single subject (eg, presurgical planning or functional ROI)• Outlines are FreeSurfer cortical ROIs• Yellow and blue blobs are functional activation • Activation does not lie cleanly within a predefined ROI
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Exploratory Spatial Analysis
• Generally requires spatial smoothing of data to increase SNR• For group analysis, requires that subjects’ brains be aligned to each other on a voxelwise basis.• Neither needed for an ROI analysis• Smoothing and intersubject registration can be performed in the volume or surface.
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Why Is a Model of the Cortical Surface Useful?
Local functional organization of cortex is largely 2-dimensional! Eg, functional mapping of primary visual areas:
From (Sereno et al, 1995, Science).
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Coordinate Systems: 3D (Volumetric)
• 3D Coordinate System • XYZ• RAS (Right-Anterior-Superior)• CRS (Column-Row-Slice)• Origin (XYZ=0, eg, AC) • MR Intensity at each XYZ
x
z
y
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Coordinate Systems: 2D (Surface)
superior temporal calcarine
central
sylvian
anterior
posterior
pial inflated
Curvature • SULCUS (+)• GYRUS (-)
Sheet: 2D Coordinate System (X,Y) Sphere: 2D Coordinate System • Latitude and Longitude • Continuous, no cuts• Value at each point (eg, thickness)
x
y
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Intersubject Registration
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Volumetric Intersubject Registration
• Affine/Linear • Translate• Rotate• Stretch• Shear• (12 DOF)
• Match Intensity, Voxel-by-Voxel• Problems• Can use nonlinear volumetric (cf CVS)
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Surface-based Intersubject RegistrationSubject 2
Subject 1
• Translate, Rotate, Stretch, Shear (12 DOF)• Match Curvature, Vertex-by-Vertex• Nonlinear Stretching (“Morphing”) allowed (area regularization)• Actually done on sphere• “Spherical Morph”
Curvature “Intensity”• SULCUS (+)• GYRUS (-)• Codes folding pattern
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A Surface-Based Coordinate System
Common space for group analysis (like Talairach)
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fsaverage
• Has “subject” folder like individual FS subjects • “Buckner 40” subjects • Default registration space• MNI305 coordinates
?h.average.curvature.filled.buckner40.tif
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Surface-based Intersubject Registration
• Gray Matter-to-Gray Matter (it’s all gray matter!)• Gyrus-to-Gyrus and Sulcus-to-Sulcus • Some minor folding patterns won’t line up• Fully automated, no landmarking needed• Atlas registration is probabilistic, most variable regions get less weight.• Done automatically in recon-all • fsaverage
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Spatial Smoothing
Why should you smooth?• Might Improve CNR/SNR• Improve intersubject registration
How much smoothing?• Blob-size• Typically 5-20 mm FWHM • Surface smoothing more forgiving than volume-based
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Volume-based Smoothing
• Smoothing is averaging of “nearby” voxels
7mm FWHM
14mm FWHM
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Volume-based Smoothing
• 5 mm apart in 3D• 25 mm apart on surface!• Kernel much larger• Averaging with other tissue types (WM, CSF)• Averaging with other functional areas
14mm FWHM
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Spatial Smoothing
Full-Width/Half-max
• Spatially convolve image with Gaussian kernel.• Kernel sums to 1• Full-Width/Half-max: FWHM = /sqrt(log(256)) = standard deviation of the Gaussian
0 FWHM 5 FWHM 10 FWHM
2mm FWHM
10mm FWHM
5mm FWHM
Full Max
Half Max
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Effect of Smoothing on Activation
• Working memory paradigm• FWHM: 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20
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Surface-based Smoothing
superior temporal calcarine
central
sylvian
anterior
posterior
Sheet: 2D Coordinate System (X,Y) Sphere: 2D Coordinate System ()
• Smoothing is averaging of nearby vertices
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Group fMRI Analysis: Volume vs Surface
Affine registration to MNI305with volume smoothing
Surface-based Registrationand smoothing
Probe-vs-Fixation. Data from Functional Biomedical Informatics Research Network (fBIRN)
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5HT4 BP Asymmetry Study (N=16)
Surface Smoothing Volume Smoothing
Left > Right
Right > Left
p<10-2
p<10-3
p<10-3
p<10-2
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Surface-based Clustering
• A cluster is a group of connected (neighboring) vertices above threshold
• Neighborhood is 2D, not 3D• Cluster has a size (area in mm2)• Reduced search space (corrections for multiple comparisons)
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Summary
• Why Surface-based Analysis?– Function has surface-based organization
– Inter-subject registration: anatomy, not intensity
– Smoothing
– Clustering
– Like 3D, but 2D
Use FreeSurfer Be Happy