# ISMRM tutorial - Structural connectome for Human Connectome Project (HCP)¶

Warning

This page describes an outdated example from 2015, currently retained only for historical reference. The steps described in this page are not up to date with current optimised practices. We do not recommend to use the steps below in connectomics processing for any project, using either HCP or other data.

This document duplicates the information provided during the MRtrix3 demonstration at ISMRM 2015 in Toronto. We will generate a structural connectome for quintessential Human Connectome Project subject 100307. Some of these instructions will be specific to HCP data, others will be more general recommendations.

Note that this page is being retained as a reference of the steps demonstrated during the ISMRM 2015 meeting; it does not constitute an up-to-date ‘recommended’ processing pipeline for HCP data.

## Necessary files¶

To duplicate our methods and results, you will need to download the appropriate files, accessible through the following steps:

• https://db.humanconnectome.org/
• WU-Minn HCP Data - 900 Subjects + 7T
• Session Type: 3T MRI
• Processing level: Preprocessed
• Package Type: MSM-Sulc + MSM-All
• add Structural Preprocessed and Diffusion Preprocessed to queue

The actual files within these compressed downloads that we will make use of are:

• bvals
• bvecs
• data.nii.gz

### Structural preprocessed files¶

• aparc+aseg.nii.gz
• T1w_acpc_dc_restore_brain.nii.gz

## Structural image processing¶

1. Generate a tissue-segmented image appropriate for Anatomically-Constrained Tractography:

5ttgen fsl T1w_acpc_dc_restore_brain.nii.gz 5TT.mif -premasked

Note that it is not necessary to use a tissue-segmented image that has the same resolution as the diffusion images; MRtrix3 will happily acquire interpolated values from each of them separately as tracking is performed. This allows ACT to exploit the higher spatial resolution of the tissue-segmented anatomical image, but still use the diffusion image information at its native resolution also.

1. Collapse the multi-tissue image into a 3D greyscale image for visualisation:

5tt2vis 5TT.mif vis.mif; mrview vis.mif

If the tissue segmentation image contains clearly erroneous tissue labels, you can delineate them manually using the ROI editor tool in mrview, then apply your corrections to the tissue data using the 5ttedit command.

1. Modify the integer values in the parcellated image, such that the numbers in the image no longer correspond to entries in FreeSurfer’s colour lookup table, but rows and columns of the connectome:

labelconvert aparc+aseg.nii.gz FreeSurferColorLUT.txt fs_default.txt nodes.mif

File FreeSurferColorLUT.txt is provided with FreeSurfer in its root directory. The target lookup table file (fs_default.txt in this case) is a handy text file that provides a structure name for every row / column of the connectome matrix: it is provided as part of MRtrix3, and located at shared/mrtrix3/labelconvert/fs_default.txt within the MRtrix3 folder.

1. Replace FreeSurfer’s estimates of sub-cortical grey matter structures with estimates from FSL’s FIRST tool:

labelsgmfix nodes.mif T1w_acpc_dc_restore_brain.nii.gz fs_default.txt nodes_fixSGM.mif -premasked

## Diffusion image processing¶

1. Convert the diffusion images into a non-compressed format (not strictly necessary, but will make subsequent processing faster), embed the diffusion gradient encoding information within the image header, re-arrange the data strides to make volume data contiguous in memory for each voxel, and convert to floating-point representation (makes data access faster in subsequent commands):

mrconvert data.nii.gz DWI.mif -fslgrad bvecs bvals -datatype float32 -strides 0,0,0,1

1. Generate a mean b=0 image (useful for visualisation):

dwiextract DWI.mif - -bzero | mrmath - mean meanb0.mif -axis 3

(If you are not familiar with the ‘|’ piping symbol, read more about it here)

1. Estimate the response function; note that here we are estimating multi-shell, multi-tissue response functions:

dwi2response msmt_5tt DWI.mif 5TT.mif RF_WM.txt RF_GM.txt RF_CSF.txt -voxels RF_voxels.mif

mrview meanb0.mif -overlay.load RF_voxels.mif -overlay.opacity 0.5 (check appropriateness of response function voxel selections)

1. Perform Multi-Shell, Multi-Tissue Constrained Spherical Deconvolution:

dwi2fod msmt_csd DWI.mif RF_WM.txt WM_FODs.mif RF_GM.txt GM.mif RF_CSF.txt CSF.mif -mask nodif_brain_mask.nii.gz

mrconvert WM_FODs.mif - -coord 3 0 | mrcat CSF.mif GM.mif - tissueRGB.mif -axis 3

This generates a 4D image with 3 volumes, corresponding to the tissue densities of CSF, GM and WM, which will then be displayed in mrview as an RGB image with CSF as red, GM as green and WM as blue (as was presented in the MSMT CSD manuscript).

mrview tissueRGB.mif -odf.load_sh WM_FODs.mif (visually make sure that both the tissue segmentations and the white matter FODs are sensible)

## Connectome generation¶

1. Generate the initial tractogram:

tckgen WM_FODs.mif 100M.tck -act 5TT.mif -backtrack -crop_at_gmwmi -seed_dynamic WM_FODs.mif -maxlength 250 -select 100M -cutoff 0.06

Explicitly setting the maximum length is highly recommended for HCP data, as the default heuristic - 100 times the voxel size - would result in a maximum length of 125mm, which would preclude the reconstruction of some longer pathways.

We also suggest a reduced FOD amplitude cutoff threshold for tracking when using the MSMT CSD algorithm in conjunction with ACT; this allows streamlines to reach the GM-WM interface more reliably, and does not result in significant false positives since the MSMT algorithm does not produce many erroneous small FOD lobes.

1. Apply the Spherical-deconvolution Informed Filtering of Tractograms (SIFT) algorithm

This method reduces the overall streamline count, but provides more biologically meaningful estimates of structural connection density:

tcksift 100M.tck WM_FODs.mif 10M_SIFT.tck -act 5TT.mif -term_number 10M

If your system does not have adequate RAM to perform this process, the first recommendation is to reduce the spatial resolution of the FOD image and provide this alternative FOD image to SIFT (this should have little influence on the outcome of the algorithm, but will greatly reduce memory consumption):

mrresize WM_FODs.mif FOD_downsampled.mif -scale 0.5 -interp sinc

If this still does not adequately reduce RAM usage, you will need to reduce the number of input streamlines to a level where your processing hardware can successfully execute the tcksift command, e.g.:

tckedit 100M.tck 50M.tck -number 50M

Alternatively, if you’re feeling brave, you can give SIFT2 a try…

1. Map streamlines to the parcellated image to produce a connectome:

tck2connectome 10M_SIFT.tck nodes_fixSGM.mif connectome.csv

mrview nodes_fixSGM.mif -connectome.init nodes_fixSGM.mif -connectome.load connectome.csv