Quantitative measure of connectivity : tract density ... connection probability : âconnectivity indexâ or â connectivity strengthâ. BUT it does not quantify ... interface â filter tract ... Classification based on connectivity profie (no hypothesis on target).
Romain Valabregue CENIR ICM heavily inspired from https://fsl.fmrib.ox.ac.uk/fslcourse/lectures/fdt1.pdf and fdt2.pdf
Diffusion applications I Tractography Pitfalls Dissection of major bundles : Tract specific Region of Interest Subregion classification given the connectivity profiles : Cortical or sub-cortical Region subdivision
Quantitative measure
II
Tensor metrics : FA MD L1 Lr Advanced model : Noddi Quantitative measure of connectivity : tract density
Connectomme : Group comparison VBM / TBSS / Fixel based
A connection + a stregth
Tractography
Tractography ●
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connection probability : “connectivity index” or “ connectivity strength” BUT it does not quantify how strong a connection is ! Rather : how robust it is against noies/uncertainty : The dominant path through the diffusion field...
Confounding factor : • • •
Connection length (longer are less probable) Geometric complexity Resolution of the spatial grid
Tractography ●
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Density of tractograms : a quantitative measure ? Sift : Spherical deconvolution Informed filtering of tractograms
(Smith neuroimage 2013)
Tractography
Tract densité estimated from : b fod c streamline d streamline after sift
Tractography
Tractography
Tractography
Tractography
Tractography : disection To find specific track : Add anatomical prior Seed ROI : where the tract start (5000 fiber per voxel) Waipoint ROI : keep only tract that go through Exclusion ROI: Remove tract that goes through Termination ROI : stop the tract when it goad through
Alternative : Seed from the all white matter or gray-white interface → filter tract
Tractography : disection Examle : the Cortico-Spinal Tract
Tractography : disection Examle : the Cortico-Spinal Tract
Tractography : disection Examle : the Cortico-Spinal Tract
Tractography : disection
Tractography : classification ● ● ●
Define different target Seed to target tract Find the biggest
Tractography : classification
Tractography : classification Classification based on connectivity profie (no hypothesis on target) Example with area 6 : separate SMA and pre SMA
Tractography : classification
Diffusion applications I Tractography Pitfalls Dissection of major bundles : Tract specific Region of Interest Subregion classification given the connectivity profiles : Cortical or sub-cortical Region subdivision
Quantitative measure
II
Tensor metrics : FA MD L1 Lr Advanced model : Noddi Quantitative measure of connectivity : tract density
Connectomme : Group comparison VBM / TBSS / Fixel based
A connection + a stregth
Quantitative metrics ●
Tensor metrics
FA ~ eigenvalues Variance MD ~ Eigenvalues mean Longitudinal AD : λ1 Tranverse ADC : (λ2+λ3)/
FA decrease / MD increase → loss of structure → Decrease of longitudinal ADC ~ Axonal breakdow ? → Decrease of transverse ADC ~ Myelin breakdow ?
Quantitative metrics ●
Tensor metrics : BUT
Sensitive but not specific Crossing fiber
Quantitative metrics ●
Tensor metrics :
Sensitive but not specific : FA correlate do axon density
Quantitative metrics ●
Advances model ●
Axon caliber / charmed model –
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Noddi : neurite orientation dispersion and density imaging – – –
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Very long acquisition
Zhang NeuroImage 2012 2 or 3 shell. (64 dir B2000; 32 dir B700; 8 dir b 300) Simplify model : ● 3 compartiment intra-cellular, extra-cellular, CSF ● → Viso Vic + OD (orientation dispersion)
MAP : Mean apparent Propagator – – – –
Ozarslan neuroimage 2013 Extension of the gaussian diffusion process (tensor metric) to characterized the true average propagator ( modecular displacements in “r-space” ) RTOP / RTPP / RTAP Implementation in DIPY.
Quantitative metrics ●
Noddi example
Factors contributing to FA changes, can be disentangled with NODDI OD and Vic measuer
Automatique Sub-cortical segmentation (freesurfer or first from fsl)
Compare mean FA in ROI across subjects
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Group comparison : along tract
Group comparison : TBSS
Group comparison : TBSS
Group comparison : TBSS
Group comparison : TBSS
Group comparison : TBSS
Group comparison
Group comparison : fixel based
AFD Apperent Fiber Density: (Raffelt neuroimage 2012) Tract specific comparison of the FOD amplitude lob. ODF based normalisation Quantitative measure : AFD
Group comparison : fixel based
Differences in a single fibre bundle, in a region containing multiple fibre population