Correlational Tractography

Before practicum on Friday, please complete following:

	Conventional fiber tracking	Differential fiber traacking	Correlational Tractography
seed point	start at any white matter location	start at any white matter location	start at any white matter location
propagation	propagate along fiber orientation	propagate along fiber orientation	propagate along fiber orientation
termination criteria	anisotropy threshold, angular threshold	anisotropy threshold, angular threshold, *threshold for the anisotropy decrease*	anisotropy threshold, angular threshold, *threshold for the t-statistics*

Connectometry: a statistical method using permutation test and bootstrap resampling to test the significance of correlational tractography.

Calculate t-statistics using Spearman rank-based correlation
- permuted versus non-permuted, respectively, both after bootstrap resampling
Fiber tracking based on t-statistics
- Each fiber tracking from a seed is a statistical test.
- The test statisticsis the length of track.
- positive findings: length > L, negative finding: length < L
FDR calculation at length L
- #tracts with length > L after permutation => #false positive
- #tracts with length > L without permutation => #false positive + #true positive
- FDR = (#false positive)/(#false positive + #true positive)
Results
- Fixed L → report FDR
- FDR threshold → estimate L* → showing findings satisfying FDR threshold

cross-sectional versus longitudinal study
correlation with a categorical variable (e.g. control v.s. patient) or continuous variable (e.g. age)

study type	correlation type	examples of null hypothesis
cross-sectional	correlation with a categorical variable	the tractography with decreased metrics in group 1 is the same as those after group permutation.
cross-sectional	correlation with a continuous variable	the tractography with decreased metrics correlated with age is the same as those after age permutation.
longitudinal	increased of decreased	the tractography with decreased metrics is the same as those after random-permuting the sign of change.
longitudinal	associated with a categorical variable	the tractography with decreased metrics in group 1 is the same as those after group permutation.
longitudinal	associated with a continuous variable	the tractography with decreased metrics correlated with age is the same as those after age permutation.

correlational tractography correlated with group
- report FDR given a length threshold
- assign FDR threshold and report findings
- high t-threshold (more localized) versus low t-threshold (less localized)
correlational tractography correlated with group under partial correlation considering age/sex.
stratified analysis using cohort selection (e.g. male and female).
combined ROI/ROA/terminative (wk 3 course)
post-hoc analysis
- identifying pathways using manual virtual dissection and recognition (wk 2 course)
- tract-metric scatter plot using tract-based analysis (week 5 hw).