Two and three dimensional nonlinear image registration

The niftyreg.nonlinear function performs nonlinear registration for two and three dimensional images. 4D images may also be registered volumewise to a 3D image, or 3D images slicewise to a 2D image. The warping is based on free-form deformations, parameterised using an image of control points.

niftyreg.nonlinear(source, target, init = NULL, sourceMask = NULL,
  targetMask = NULL, symmetric = TRUE, nLevels = 3L,
  maxIterations = 150L, nBins = 64L, bendingEnergyWeight = 0.001,
  linearEnergyWeight = 0.01, jacobianWeight = 0, finalSpacing = c(5, 5,
  5), spacingUnit = c("voxel", "world"), interpolation = 3L,
  verbose = FALSE, estimateOnly = FALSE, sequentialInit = FALSE,
  internal = NA, precision = c("double", "single"),
  threads = getOption("RNiftyReg.threads"))

Arguments

source	The source image, an object of class `"nifti"` or `"internalImage"`, or a plain array, or a NIfTI-1 filename. Must have 2, 3 or 4 dimensions.
target	The target image, an object of class `"nifti"` or `"internalImage"`, or a plain array, or a NIfTI-1 filename. Must have 2 or 3 dimensions.
init	Transformation(s) to be used for initialisation, which may be `NULL`, for no initialisation, or an affine matrix or control point image (nonlinear only). For multiple registration, where the source image has one more dimension than the target, this may also be a list whose components are likewise `NULL` or a suitable initial transform.
sourceMask	An optional mask image in source space, whose nonzero region will be taken as the region of interest for the registration. Ignored when `symmetric` is `FALSE`.
targetMask	An optional mask image in target space, whose nonzero region will be taken as the region of interest for the registration.
symmetric	Logical value. Should forward and reverse transformations be estimated simultaneously?
nLevels	A single integer specifying the number of levels of the algorithm that should be applied. If zero, no optimisation will be performed, and the final control-point image will be the same as its initialisation value.
maxIterations	A single integer specifying the maximum number of iterations to be used within each level. Fewer iterations may be used if a convergence test deems the process to have completed.
nBins	A single integer giving the number of bins to use for the joint histogram created by the algorithm.
bendingEnergyWeight	A numeric value giving the weight of the bending energy term in the cost function.
linearEnergyWeight	A numeric value giving the weight of the linear energy term in the cost function.
jacobianWeight	A numeric value giving the weight of the Jacobian determinant term in the cost function.
finalSpacing	A numeric vector giving the spacing of control points in the final grid, along the X, Y and Z directions respectively. This is set from the initial control point image, if one is supplied.
spacingUnit	A character string giving the units in which the `finalSpacing` is specified: either `"voxel"` for pixels/voxels, or `"world"` for real-world units (see `pixunits`).
interpolation	A single integer specifying the type of interpolation to be applied to the final resampled image. May be 0 (nearest neighbour), 1 (trilinear) or 3 (cubic spline). No other values are valid.
verbose	A single logical value: if `TRUE`, the code will give some feedback on its progress; otherwise, nothing will be output while the algorithm runs. Run time can be seconds or more, depending on the size and dimensionality of the images.
estimateOnly	Logical value: if `TRUE`, transformations will be estimated, but images will not be resampled.
sequentialInit	If `TRUE` and `source` has higher dimensionality than `target`, transformations which are not explicitly initialised will begin from the result of the previous registration.
internal	If `NA`, the default, the final resampled image will be returned as a standard R array, but control point maps will be objects of class `"internalImage"`, containing only basic metadata and a C-level pointer to the full image. (See also `readNifti`.) If `TRUE`, all image-type objects in the result will be internal images; if `FALSE`, they will all be R arrays. The default is fine for most purposes, but using `TRUE` may save memory, while using `FALSE` can be necessary if there is a chance that external pointers will be invalidated, for example when returning from worker threads.
precision	Working precision for the registration. Using single- precision may be desirable to save memory when coregistering large images.
threads	For OpenMP-capable builds of the package, the maximum number of threads to use.

Value

See niftyreg.

Details

This function performs the dual operations of finding a transformation to optimise image alignment, and resampling the source image into the space of the target image (and vice-versa, if symmetric is TRUE). Unlike niftyreg.linear, this transformation is nonlinear, and the degree of deformation may vary across the image.

The nonlinear warping is based on free-form deformations. A lattice of equally-spaced control points is defined over the target image, each of which can be moved to locally modify the mapping to the source image. In order to assess the quality of the warping between the two images, an objective function based on the normalised mutual information is used, with penalty terms based on the bending energy or the squared log of the Jacobian determinant. The objective function value is optimised using a conjugate gradient scheme.

The source image may have 2, 3 or 4 dimensions, and the target 2 or 3. The dimensionality of the target image determines whether 2D or 3D registration is applied, and source images with one more dimension than the target (i.e. 4D to 3D, or 3D to 2D) will be registered volumewise or slicewise, as appropriate. In the latter case the last dimension of the resulting image is taken from the source image, while all other dimensions come from the target. One image of control points is returned for each registration performed.

Note

Performing a linear registration first, and then initialising the nonlinear transformation with the result (via the init parameter), is highly recommended in most circumstances.

References

The algorithm used by this function is described in the following publication.

M. Modat, G.R. Ridgway, Z.A. Taylor, M. Lehmann, J. Barnes, D.J. Hawkes, N.C. Fox & S. Ourselin (2010). Fast free-form deformation using graphics processing units. Computer Methods and Programs in Biomedicine 98(3):278-284.

Author

Jon Clayden <code@clayden.org>