Extract diffusion-weighted volumes, b=0 volumes, or certain shells from a DWI dataset


dwiextract [ options ]  input output
  • input: the input DW image.
  • output: the output image (diffusion-weighted volumes by default).


  • -bzero Output b=0 volumes (instead of the diffusion weighted volumes, if -singleshell is not specified).
  • -no_bzero Output only non b=0 volumes (default, if -singleshell is not specified).
  • -singleshell Force a single-shell (single non b=0 shell) output. This will include b=0 volumes, if present. Use with -bzero to enforce presence of b=0 volumes (error if not present) or with -no_bzero to exclude them.

DW gradient table import options

  • -grad file Provide the diffusion-weighted gradient scheme used in the acquisition in a text file. This should be supplied as a 4xN text file with each line is in the format [ X Y Z b ], where [ X Y Z ] describe the direction of the applied gradient, and b gives the b-value in units of s/mm^2. If a diffusion gradient scheme is present in the input image header, the data provided with this option will be instead used.
  • -fslgrad bvecs bvals Provide the diffusion-weighted gradient scheme used in the acquisition in FSL bvecs/bvals format files. If a diffusion gradient scheme is present in the input image header, the data provided with this option will be instead used.
  • -bvalue_scaling mode specifies whether the b-values should be scaled by the square of the corresponding DW gradient norm, as often required for multi-shell or DSI DW acquisition schemes. The default action can also be set in the MRtrix config file, under the BValueScaling entry. Valid choices are yes/no, true/false, 0/1 (default: true).

DW shell selection options

  • -shells b-values specify one or more b-values to use during processing, as a comma-separated list of the desired approximate b-values (b-values are clustered to allow for small deviations). Note that some commands are incompatible with multiple b-values, and will report an error if more than one b-value is provided. WARNING: note that, even though the b=0 volumes are never referred to as shells in the literature, they still have to be explicitly included in the list of b-values as provided to the -shell option! Several algorithms which include the b=0 volumes in their computations may otherwise return an undesired result.

DW gradient table export options

  • -export_grad_mrtrix path export the diffusion-weighted gradient table to file in MRtrix format
  • -export_grad_fsl bvecs_path bvals_path export the diffusion-weighted gradient table to files in FSL (bvecs / bvals) format

Options for importing phase-encode tables

  • -import_pe_table file import a phase-encoding table from file
  • -import_pe_eddy config indices import phase-encoding information from an EDDY-style config / index file pair

Options for selecting volumes based on phase-encoding

  • -pe desc select volumes with a particular phase encoding; this can be three comma-separated values (for i,j,k components of vector direction) or four (direction & total readout time)

Stride options

  • -strides spec specify the strides of the output data in memory; either as a comma-separated list of (signed) integers, or as a template image from which the strides shall be extracted and used. The actual strides produced will depend on whether the output image format can support it.

Standard options

  • -info display information messages.
  • -quiet do not display information messages or progress status. Alternatively, this can be achieved by setting the MRTRIX_QUIET environment variable to a non-empty string.
  • -debug display debugging messages.
  • -force force overwrite of output files. Caution: Using the same file as input and output might cause unexpected behaviour.
  • -nthreads number use this number of threads in multi-threaded applications (set to 0 to disable multi-threading).
  • -help display this information page and exit.
  • -version display version information and exit.

Author: David Raffelt (david.raffelt@florey.edu.au) and Thijs Dhollander (thijs.dhollander@gmail.com) and Robert E. Smith (robert.smith@florey.edu.au)

Copyright: Copyright (c) 2008-2018 the MRtrix3 contributors.

This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, you can obtain one at http://mozilla.org/MPL/2.0/

MRtrix3 is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

For more details, see http://www.mrtrix.org/