Bundles Evaluation

For each subject and session, the evaluation of the white matter bundles measures is performed using the Tractometry Flow pipeline.

Three types of measurements are extracted for each bundle:

• Bundle-streamline measures

• Bundle-averaged

• Bundle-profile (or track-profile)

Bundle-streamline measures

Three measures are extracted from the streamlines for each bundle (Figure 1):

• The bundle volume, in mm3, is estimated by counting the number of voxels occupied by the bundle and multiplying it by the volume of a single voxel.

• The bundle average length, in mm, is estimated by averaging the length of all the streamlines of the bundle.

• The bundle streamline count, i.e. the number of streamlines in a bundle.

Note

Tractometry Flow pipeline removes any streamline that falls out of the volume, i.e. no negative coordinate or coordinates outside the volume’s dimension are possible.

Bundle-averaged

For each measures, the bundle-averaged mean is computed by averaging a specific measure value for all voxels occupied by the bundle (Figure 1).

Figure 1. Representation of the extraction of Bundle-streamlines and Bundle-average measures.

Bundle-profile

To generate the bundle-profile (also called track-profiles), Tractometry Flow resamples its centroid into 20 equidistant points. Then, each voxel is assigned to the section label (1 to 20) of its closest centroid point (Figure 2).

Figure 2. Representation of white matter major bundle models resampled into 20 segments for illustration. Left and right have been merged. The colors displayed on the bundles represent the section numbers from 1 (blue) to 20 (red).

For each section, mean is computed by averaging the measures value of all voxels occupied by the section (label, Figure 1). Finally, a tract profile is extracted for each combination of measurements and bundles (Figure 3).

Figure 3. Representation of the extraction of Bundle-profile measures.

Note

Mean measurement values can be computed either by session i.e. including all subjects for one session, or by averaging all sessions and all subjects (average).

Visualization

Results are displayed using Plotly’s interactive plots. Click on the legend to select and/or deselect plot elements. Colors represent bundle or MRI measurements.

For most figures, the slider can represent :

• sessions (Session1, Session 2, …) and/or the average of all sessions (average)

• MRI measurements

Streamlines metrics generated

Table describe all streamlines metrics generated for each bundle.

Tools	Streamline metrics
RBXflow	Volume (mm3)
	Count (n)
	Length (mm)

MRI measurements generated

Table describe all measure maps generated.

Tools	MRI measurements (mean)
Tractoflow - DTI	Fractional anisotropy (FA)
	Mean Diffusivity (MD) s/mm2
	Radial Diffusivity (RD) s/mm2
	Axial Diffusivity (AD) s/mm2
Freewater Flow DTI-FW corrected	Fractional anisotropy tissue (FA-FWcorrected)
	Mean Diffusivity tissue (MD-FWcorrected) s/mm2
	Radial Diffusivity tissue (RD-FWcorrected) s/mm2
	Axial Diffusivity tissue (AD-FWcorrected) s/mm2
Freewater Flow - FW	Free water (FW)
Tractoflow - FODF	Total Apparent fiber density (AFD total)
	Number of fober direction (NuFO)
NODDI Flow	Intra-cellular volume fraction (ICvf)
	Extra-cellular volume fraction (ECvf)
	Isotropic volume fraction (ISOvf)
	Orientation direction (OD)
ihMT Flow	ihMT ratio (ihMTR)
	ihMT delta R1 saturation (ihMTsat)
	MT ratio (MTR)
	MT saturation (MTsat)

• Add description of metrics is usefull ?