This page discusses the preprocessing steps required to prepare a diffusion-weighted dataset for analysis using TractoR, as well as subsequent operations that can be performed to manipulate diffusion gradient directions and fit models to the data.
The preprocessing steps required to run neighbourhood tractography or other analysis on a diffusion-weighted data set using TractoR are broadly the same as those required by the FSL diffusion toolbox (FDT) for tractography. It is therefore quite possible to perform all of these steps independently of TractoR, and the user may decide to use the relevant FSL tools, or their equivalents from another package. However, reasons to use TractoR might include the following:
- The session hierarchy will be arranged as TractoR expects it, automatically. This saves the user from having to arrange the hierarchy herself. It also means that TractoR can give correct parameters to any external programs without further input from the user.
- The pipeline is run interactively but tries to minimise user input.
- TractoR can establish which stage of the pipeline has been reached, and continue partly completed preprocessing pipelines where necessary.
Whichever method is used to perform them, the requisite steps are as follows.
- Convert DICOM files from an MR scanner into a 4D data set file in Analyze or NIfTI format. The result is an image called
rawdata(with appropriate suffix depending on the file type) in the
diffusionsubdirectory of the TractoR session hierarchy. The file
directions.txt, which describes the diffusion weighting applied to the images, will also be created if possible. If the relevant information is not available, this latter file must be created manually—if this is the case then TractoR will produce a warning.
- Optionally correct for susceptibility-induced distortions with FSL
topup, then identify an image volume with little or no diffusion weighting, to be used as an anatomical reference. By default this file will be called
refb0and stored in the
- Create a mask which covers only that part of the
refb0volume which is within the brain. Skull and other nonbrain tissue is left outside this mask. The image file
maskis created in the
diffusionsubdirectory, as well as a masked version of the anatomical reference image, called
- Correct for eddy current-induced distortion effects in the data, using the anatomical reference volume as a registration target. This step currently uses one of the FSL tools
eddy_correct, or the internal NiftyReg interface, and produces a file called
In addition to these steps, some kind of diffusion model fitting is a prerequisite for most subsequent analysis. Steps for performing such fits, and other common data manipulations, are laid out below.
Sorting DICOM files
A single scan session often comprises a range of image acquisitions of different types, for example including structural and functional imaging as well as a diffusion-weighted sequence. It is therefore a common first step to sort the DICOM files corresponding to the diffusion scan out from everything else. Assuming for present purposes that data sets are stored as subdirectories of
/data, and that each data set has its own
dicom subdirectory containing all associated DICOM files, this sorting process can be achieved by typing something like
cd /data/subject1/dicom tractor dicomsort
This will result in various new subdirectories being created, labelled with their appropriate series number and description. It is up to the user to identify which series corresponds to the diffusion data.
Using the dpreproc script
With DICOM sorting already performed if necessary, running the preprocessing pipeline for a single session directory is a matter of typing something like
cd /data/subject1 tractor dpreproc
By default, TractoR will assume that all DICOM files it finds under the main session directory,
/data/subject1, relate to your DTI acquisition. If in fact your diffusion DICOM files are stored in some subdirectory, perhaps
/data/subject1/dicom/dti, you should tell TractoR this by instead using
tractor dpreproc DicomDirectories:dicom/dti
Notice that the DICOM subdirectory given is relative to the session directory. More than one directory can be specified if multiple acquisition series are relevant.
Alternatively, the newer
divest back-end can be used to select the relevant series interactively, using
tractor dpreproc DicomReader:divest
The preprocessing can be completed noninteractively by setting the “Interactive” option to
tractor dpreproc Interactive:false
(Note, however, that in this case default parameters will be used, without the chance to check that the results are appropriate. Some parameters can, however, be adjusted using other options to the
dpreproc script: run
tractor -o dpreproc for details.) By default, TractoR will run all four stages, but will miss out any stage that has already been successfully completed. To run every stage except the final one, type
tractor dpreproc RunStages:1-3
or to start from the beginning again even if some stages have already been done, use
tractor dpreproc Force:true
If you want to find out which stages have already been run, simply give
tractor dpreproc StatusOnly:true
Checking and rotating gradient directions
The directions of applied diffusion-weighting gradients are determined from the DICOM files if possible, during stage 1 of the
dpreproc script. However, if
dpreproc is not used, or the gradient directions cannot be found, it may be necessary to specify them manually. To do so, the directions should be put into a plain text file, arranged either one-per-column or one-per-row, normalised or unnormalised, and with or without zeroes for b=0 measurements. The
gradread script can then be called, passing the session directory, gradients file and the big and little b-values:
tractor gradread /data/subject1 /data/directions.txt 1000 0
This should normally only be necessary once for a given sequence, since a cache is automatically used to store gradient directions for use with other data sets acquired the same way.
Whichever way the gradients are initially obtained, it is a good idea to check that the signs of the directions are correct. The
gradcheck script assists with this by showing the principal directions of diffusion in three representative slices of the brain, for checking against the user’s expectations. It can be run from the session directory with
Another step which is commonly performed is gradient rotation, to compensate for the registration performed to correct for eddy current effects in
dpreproc stage 4. If required, this should be the last step performed on the directions. It is run with simply
plotcorrections script can be used to see how big the effect of this step will be: run
tractor -o plotcorrections for more information.
Fitting diffusion tensors is a standard processing step for diffusion-weighted data, and results in the creation of a range of derivative images, including maps of fractional anisotropy and mean diffusivity. This fitting can be performed using
There are four alternative approaches to fitting the tensors available, but standard least-squares fitting is the default: see
tractor -o tensorfit for details. The Camino toolkit offers many more methods.
The diffusion tensor is a very limited model, however, particularly for tractography. It is therefore recommended to run FSL BEDPOSTX algorithm to fit a “ball-and-sticks” model and generate samples for probabilistic tractography. This typically takes several hours. The command for running this is
The underlying FSL
bedpostx program takes a parameter which determines the maximum number of fibre populations which may be represented for each voxel. The larger this value, the longer
bedpostx will take to run, but if set to 1 then no crossing fibre information will be available. The default value is 3, and this can be changed using the
tractor bedpost NumberOfFibres:2
It is important to note that the number of fibres fitted is a property of the session, and so once set it cannot be changed without processing the data again. If you wish to try different values of this option on a single data set, you will need to duplicate the session hierarchy, since these would be considered two distinct preprocessing pipelines, producing two different data sets.
The status script
To find out information about a particular session directory and the data stored within it, you can use the
status script, which produces output like the following:
GENERAL: Session directory : /usr/local/tractor/tests/data/session Working directory exists : TRUE DIFFUSION: Preprocessing complete : TRUE Data dimensions : 96 x 96 x 60 x 12 voxels Voxel dimensions : 2.5 x 2.5 x 2.5 mm x 1 s Number of shells : 1 Diffusion b-values : 0, 1000 s/mm^2 Number of gradient directions : 2, 10 Fibre orientation model(s) : DTI, FSL-BEDPOSTX (1 fibre per voxel)