Available GUIs

FLIRT comes with a main GUI as well as three supporting guis:

These support GUIs are self-explanatory and perform functions that are detailed in the command line versions. Only the main FLIRT GUI is described in the rest of this page.


Main Options

The simplest use of FLIRT is to register two single volumes together. This is done by choosing the Input image -> Reference image mode in the top box, then filling in the Reference image and Input image boxes with the appropriate images. The result is a registered image which will be saved to the location specified in the Output image box. All other options/boxes can be left at their default values.

The second mode of operation is a two stage registration which takes an input Low res image and two target images. It initially registers the low res image to a High res image and then registers this high res image to the final Reference image. The two resulting transformations are concatenated and then applied to the original low res image to create an Output image that is a version of the low res image transformed (resliced) to the reference image space.

Secondary Images

Apply the estimated transform to other (secondary) images, which were originally aligned with the input/low-res image, in order to align them with the reference image.


Restrict the transformation type. For 3D to 3D mode the DOF can be set to 12 (affine), 9 (traditional), 7 (global rescale) or 6 (rigid body). In 2D to 2D mode only 3 DOF (rigid body) transformations are allowed.

Advanced Options

The four categories of options are:


flirt Command-line Program and Utilities

This document gives a brief description of flirt and the various command-line programs available in the FLIRT component of FSL. A description of the available GUI interfaces is also available.

For each of the programs described here, a usage message which describes the full list of available options can be obtained by running the program with no options.

See also the list of common example usages.


flirt is the main program that performs affine registration. The main options are: an input (-in) and a reference (-ref) volume; the calculated affine transformation that registers the input to the reference which is saved as a 4x4 affine matrix (-omat); and output volume (-out) where the transform is applied to the input volume to align it with the reference volume. In addition, FLIRT can also be used to apply a saved transformation to a volume (-applyxfm, -init and -out) or to apply a transform that aligns the NIFTI mm coordinates (-applyxfm, -usesqform and -out; but not -init). For these usages the reference volume must still be specified as this sets the voxel and image dimensions of the resulting volume.


Cost Function Weighting

Weighting volumes can be specified using -refweight, -inweight (or both). This allows the cost function to have a different weighting at each voxel, which is useful for excluding areas (weight=0) of no interest, or increasing the weighting around important structures such as the ventricles. Note that this is different from masking the original images, as masking introduces artificial boundaries whereas weighting does not.

Degrees of Freedom

Choose from 6,7,9 or 12 Degrees of Freedom (DOF) for full 3D registrations. Also includes a 3DOF 2D-to-2D registration mode which is selected using the -2D option. Note that it does not perform any search in 2D mode, and cannot deal with 2D to 3D registrations. More flexible DOF options are provided by the specific schedule files provided in $FSLDIR/etc/flirtsch.

Interpolation Methods

This includes Nearest Neighbour, a family of Sinc-based methods (three window types - rectangular, Hanning and Blackman) with configurable window width, and spline (a highly efficient method, with similar output characteristics to sinc). The interpolation is only used for the final transformation (and in applyxfm), not in the registration calculations.

Cost Functions

This includes the within-modality functions Least Squares and Normalised Correlation, as well as the between-modality functions Correlation Ratio (the default), Mutual Information and Normalised Mutual Information. In addition, there is the BBR cost function which utilises a segmentation of the reference image to define a boundary, and it is the intensity differences in the input image, across the transformed boundary, that contribute to the cost.


This is a script designed to register EPI images (typically functional or diffusion) to structural (e.g. T1-weighted) images. The pre-requisites to use this method are: (1) a structural image that can be segmented to give a good white matter boundary; and (2) an EPI that contains some intensity contrast between white matter and grey matter (though it does not have to be enough to get a segmentation).

This script will either use an existing white-matter segmentation of the structural image, or create one itself, to define a white-matter boundary. To use an existing white-matter segmentation you can use the --wmseg option. Alternately, there needs to be an image with the same basename as the input t1brain image, but ending with _wmseg. We recommend that the structural image is bias-corrected separately beforehand if there is obvious bias field present.

The script is also capable of using fieldmaps to perform simultaneous registration and EPI distortion-correction. For this it requires extra images (in the format as given by fsl_prepare_fieldmap ) as well as extra information about the EPI sequence (which your operator/radiographer/technician should be able to provide, but make sure you record the values). The inputs echospacing and pedir both refer to the EPI image (not the fieldmap) and are the same as required for FEAT, but be careful to use the correct units.

Usage: epi_reg [options] --epi=<EPI image> --t1=<wholehead T1 image> --t1brain=<brain extracted T1 image> --out=<output name>
Optional arguments
  --fmap=<image>         : fieldmap image (in rad/s)
  --fmapmag=<image>      : fieldmap magnitude image - wholehead extracted
  --fmapmagbrain=<image> : fieldmap magnitude image - brain extracted
  --gdc=<image>          : Gradient-distortion corection warpfield
  --wmseg=<image>        : white matter segmentation of T1 image
  --echospacing=<val>    : Effective EPI echo spacing (sometimes called dwell time) - in seconds
  --pedir=<dir>          : phase encoding direction, dir = x/y/z/-x/-y/-z
  --weight=<image>       : weighting image (in T1 space)
  --nofmapreg            : do not perform registration of fmap to T1 (use if fmap already registered) 
  --noclean              : do not clean up intermediate files
  -v                     : verbose output
  -h                     : display this help message
e.g.:  epi_reg --epi=example_func --t1=struct --t1brain=struct_brain --out=epi2struct --fmap=fmap_rads --fmapmag=fmap_mag --fmapmagbrain=fmap_mag_brain --echospacing=0.0005 --pedir=-y
Note that if parallel acceleration is used in the EPI acquisition then the *effective* echo spacing is the actual echo spacing between acquired lines in k-space divided by the acceleration factor.


convert_xfm is a utility that is used to convert between different transformation file formats. It can read and write ascii 4x4 matrices. In addition, it can be used to concatenate two transforms (using -concat with the second transform) or to find the inverse transformation (using -inverse).


img2imgcoord is a utility that calculates the corresponding coordinate positions (in voxels or mm) within a destination volume given the original coordinates in the source volume and the transformation from the source to the destination volume (either linear/affine transformations or non-linear warps). This is useful for finding corresponding anatomical/functional locations. If the option -mm is used then both input and output coordinates will be in mm coordinates, otherwise (with -vox) both coordinates will be in voxel coordinates. For conversion between voxel and mm coordinates it is necessary to use either img2stdcoord or std2imgcoord (see below).

Note that the source coordinates can either be input via a file or via a pipe (and for the latter the "-" symbol is used as the filename). The format in either case is three numbers per line, space separated.

Example usage:
   cat coordfile.txt | img2imgcoord -src example_func -dest highres -xfm example_func2highres.mat -

Warning: currently there is a buglet in the coordinate conversion tools img2imgcoord, img2stdcoord and std2imgcoord such that the last coordinate is repeated if the input is a file. To avoid this use the pipe input format or suppress the final line:
     cat coordfile.txt | img2imgcoord -src example_func -dest highres -xfm example_func2highres.mat -
     img2imgcoord -src example_func -dest highres -xfm example_func2highres.mat coordfile.txt | sed '$d'


img2stdcoord is a similar utility to img2imgcoord except that the destination volume coordinates are treated as mm (standard space) coordinates. This uses the "standard space" image's coordinate mapping information (qform/sform in the NIfTI format) to convert to mm coordinates. The coordinates for the source image can be either in voxel coordinates (default, or by explicitly using -vox) or in mm coordinates (using -mm). Example usage:
   echo 92 80 59 | img2stdcoord -img highres -std standard -warp highres2standard_warp -

This is useful in conjunction with the tool Atlasquery that can report atlas labels for coordinates in standard space.

It is no longer the case (now that FSL uses the NIfTI format) that the image specified with -std need to be a "standard space" image. Any image can be used, and the mapping to mm coordinates will be done as it is done in FSLView (using the qform/sform information or an FSL default if they are not set). This utility, and std2imgcoord are therefore useful for converting between voxel and mm coordinates within the same image, as well as for mapping coordinates between spaces.

Converting from voxel to mm coordinates within the same image can be done with the command:
   echo X Y Z | img2stdcoord  -img IMAGENAME -std IMAGENAME -vox -
where here we convert just a single coordinate (replace X, Y, Z, and IMAGENAME with appropriate numbers/filename) but multiple coordinates can also be done by replacing the echo X Y Z with cat COORDFILE (again, replacing COORDFILE with the appropriate filename).


std2imgcoord is the complementary utility to img2stdcoord. It works the same way but transfers coordinates from "standard space" to the other image (IMG) space. It can also convert between mm and voxel coordinates within the same image. See the entry on img2stdcoord above.


applyxfm4D is a utility that transforms a 4D time series by applying known affine transformations to them. It can be used with a single transformation applied to all, or using a directory of transformation files in the form MAT_XXXX (where XXXX stands for the volume number, starting with 0000). Sinc interpolation is used internally. Appropriate options (-applyxfm and -init) to FLIRT can be used to apply transformations to single volumes with other interpolation methods. A reference volume is required in order to determine the matrix size and FOV for the final volume. If the required matrices do not start with MAT_ a different prefix can be specified with the -userprefix option.


rmsdiff is a utility that calculates the Root Mean Square deviation (in millimetres) between two transformations. That is, it compares two transformations (normally two possible registrations of the same volume pair) to see how much they differ. This is useful to compare alternative registrations. It calculates the average using an analytic formula applied over an 80mm sphere with the origin at the centre of gravity of the image (for which it requires the input image to be specified).

An alternative usage is to provide a mask (as the fourth argument) which is then used to specify the ROI, rather than using the sphere. In this mode it gives two outputs: (1) the maximum movement (in mm) over all voxels in the ROI/mask; and (2) the RMS movement (in mm) over all voxels in the ROI/mask. With this calculation it does not require an origin to be set.


avscale is a utility that displays the decomposed elements of an affine matrix. It displays the rotation/translation matrix, the individual axis scalings, the individual skews, the average scaling, and the forward and backward halfway transformations. In order to set the centre of rotation it requires the input volume (also called the reslice volume).

FLIRT Examples

The simplest usage of flirt is to register two images together as:

flirt -in invol -ref refvol -out outvol -omat invol2refvol.mat -dof 6

where invol, refvol, outvol are the input, reference and output volume filenames respectively, invol2refvol.mat is the filename for the saved ascii transformation matrix. Naturally, any filenames you wish to use can be chosen.

Note that -dof was used as the default would otherwise be 12. The default cost function is Correlation Ratio, which normally works well for all images.

Also note that the .mat extension is not compulsory and any filename and extension can be used. The transformation files are simply stored as ascii matrices, and so other conventions can be used (which might be better for MATLAB users to avoid confusion) although the .mat extension is the default within FEAT.

To apply a saved transformation to another image use:

flirt -in newvol -ref refvol -out outvol -init invol2refvol.mat -applyxfm

Note that the previous transformation matrix is used with the -init command and that the size of the otput volume is determined by refvol although its contents are not used.

To perform a 3 DOF 2D (single slice) registration:

flirt -in inslice -ref refslice -out outslice -omat i2r.mat -2D

To perform a 6 DOF 2D registration:

flirt -in inslice -ref refslice -out outslice -omat i2r.mat -2D -schedule ${FSLDIR}/etc/flirtsch/sch2D_6dof

The schedule file specifies what transformations/DOF are allowed and how the optimisation is performed. Note that several other schedule files could be used - including 3D translation only schedules etc. These are all stored in ${FSLDIR}/etc/flirtsch


To invert a saved transformation:

convert_xfm -omat refvol2invol.mat -inverse invol2refvol.mat

To concatenate two transformations:

convert_xfm -omat AtoC.mat -concat BtoC.mat AtoB.mat

Note that the transform after the -concat is treated as the second transformation in the concatenation.

Using FLIRT to Register a Few FMRI Slices


If you need to take an FMRI data set where you only have a few slices (ie the field-of-view - FOV - is small in Z) then it is very hard to get a good registration with either the subject's structural or directly to a standard space image. This page describes our recommendations in such cases.

Now you are ready to register these all together.


flirt -ref whole_func -in example_func -schedule ${FSLDIR}/etc/flirtsch/ztransonly.sch -out example_func2whole_func -omat example_func2whole_func.mat

The custom FLIRT schedule file ztransonly.sch allows only translations in Z (in order for the registration to be robust), so there must be no rotation or within-slice translation between these images.

flirt -ref highres -in whole_func -out whole_func2highres -omat whole_func2highres.mat

flirt -ref standard -in highres -out highres2standard -omat highres2standard.mat

convert_xfm -concat whole_func2highres.mat -omat example_func2highres.mat example_func2whole_func.mat

convert_xfm -concat highres2standard.mat -omat example_func2standard.mat example_func2highres.mat

flirt -ref standard -in thresh_zstat1 -applyxfm -init example_func2standard.mat -out thresh_zstat12standard

Using with FEAT

If you want to put these transforms into a FEAT directory so that running group stats with FEAT will work well, then do the following (instead of using the simple default registration carried out by FEAT): Run a first level FEAT analysis without registration - this creates an output FEAT directory. Place in your FEAT directory appropriate images called whole_func.hdr/.img, highres.hdr/.img and standard.hdr/.img. (highres should probably have been brain-extracted already using BET, and standard should probably be ${FSLDIR}/data/standard/MNI152_T1_2mm_brain) Carry out the above flirt and convert_xfm commands exactly as written. This should leave you with correctly named transform files which FEAT will automatically use when carrying out group statistics.



FLIRT/UserGuide (last edited 22:28:44 04-03-2019 by PaulMcCarthy)