Forward and inverse warps for warping images, pointsets and Jacobians

Quick reference for applying ANTs warps

Applying the deformations computed by ANTs require the user to specify the correct warps, and to specify them in the correct order. Common use cases are explained below.

Terminology

The command

${ANTSPATH}antsRegistrationSyNQuick.sh 
  -d 3 \
  -f fixedImage.nii.gz \
  -m movingImage.nii.gz \
  -o movingToFixed_ \
  -t s 

produces

movingToFixed_1Warp.nii.gz
movingToFixed_1InverseWarp.nii.gz
movingToFixed_0GenericAffine.mat

The forward transforms from moving to fixed space are defined as those we use to deform an image in the moving space and produce output in the fixed space. These are movingToFixed_1Warp.nii.gz and movingToFixed_0GenericAffine.mat.

The inverse transforms are the transforms that are used to perform the opposite operation, deforming an image in the fixed space and producing output in the moving space. This operation uses the file movingToFixed_1InverseWarp.nii.gz and the inverse of the forward affine transform movingToFixed_0GenericAffine.mat. The inverse affine transform is not usually stored because it is easy to invert on demand. However, the inverse warp field cannot be precisely computed from the forward warp field, so the inverse warp field is saved separately.

Deforming an image

Deforming the moving image to fixed space:

${ANTSPATH}antsApplyTransforms \
  -d 3 \
  -i movingImage.nii.gz \
  
  -r fixedImage.nii.gz \
  
  -t movingToFixed_1Warp.nii.gz \
  -t movingToFixed_0GenericAffine.mat \
  
  -o movingToFixedDeformed.nii.gz

Regions of interest are contained in fixedLabels.nii.gz, in the fixed image space. Deforming these to moving space:

${ANTSPATH}antsApplyTransforms \
  -d 3 \
  -i fixedLabels.nii.gz \
  -r movingImage.nii.gz \
  
  -t [movingToFixed_0GenericAffine.mat, 1] \
  
  -t movingToFixed_1InverseWarp.nii.gz \
  -n GenericLabel[Linear] \
  -o movingToFixedDeformed.nii.gz

Transforming a point set

Transform points from fixed to moving space:

${ANTSPATH}antsApplyTransformsToPoints \
  -d 3 \
  -i landmarksInFixedSpace.csv \
  -o landmarksInMovingSpace.csv \
  -t movingToFixed_1Warp.nii.gz \
  -t movingToFixed_0GenericAffine.mat 

The forward warps, which we use to deform an image from moving to fixed space, are the warps that transform points from fixed to moving space. To move points in the opposite direction:

${ANTSPATH}antsApplyTransformsToPoints \
  -d 3 \
  -i landmarksInMovingSpace.csv \
  -o landmarksInFixedSpace.csv \
  -t [movingToFixed_0GenericAffine.mat, 1]
  -t movingToFixed_1InverseWarp.nii.gz \

The input and output to antsApplyTransformsToPoints is in physical space as defined by ITK. This may vary from the coordinates as understood by NIFTI or other file formats. The coordinates need to be carefully validated by users. ANTsR has some capabilities to help with this.

Computing the Jacobian

Warp naming convention in antsCorticalThickness.sh

Warp naming convention in antsLongitudinalCorticalThickness.sh

Details

Internally, deforming an image involves transforming a point set in the opposite direction to the intuitive direction of the warping. The "moving" image appears to be moving, but in reality it's being resampled. The sample points are a regular grid of voxel centers in the fixed space. A point at the center of a voxel is transformed to the corresponding location in moving space, an interpolated intensity value is computed, and the result is placed in the voxel in the output image.

This is why the transform ordering for antsApplyTransforms and antsApplyTransformsToPoints is different.