add nibabel conversion (lacey import to prevent forced dependency)

ants/utils/nifti_to_ants.py

@@ -1,41 +1,135 @@
-__all__ = ["nifti_to_ants"]
+__all__ = ["nifti_to_ants", "ants_to_nifti", "from_nibabel", "to_nibabel"]
+
+from typing import TYPE_CHECKING
 
-import os
-from tempfile import mkstemp
-import numpy as np
 import ants
+import numpy as np
+
+if TYPE_CHECKING:
+    from nibabel.nifti1 import Nifti1Image
+
 
-def nifti_to_ants( nib_image ):
+def nifti_to_ants(nib_image: "Nifti1Image") -> ants.ANTsImage:
     """
-    Converts a given Nifti image into an ANTsPy image
+    Convert a Nifti image to an ANTsPy image.
 
     Parameters
     ----------
-        img: NiftiImage
+    nib_image : Nifti1Image
+        The Nifti image to be converted.
 
     Returns
     -------
-        ants_image: ANTsImage
+    ants_image : ants.ANTsImage
+        The converted ANTs image.
     """
     ndim = nib_image.ndim
 
     if ndim < 3:
-        print("Dimensionality is less than 3.")
-        return None
+        raise NotImplementedError("Conversion is only implemented for 3D or higher images.")
 
     q_form = nib_image.get_qform()
     spacing = nib_image.header["pixdim"][1 : ndim + 1]
 
-    origin = np.zeros((ndim))
+    origin = np.zeros(ndim)
     origin[:3] = q_form[:3, 3]
 
-    direction = np.diag(np.ones(ndim))
+    direction = np.eye(ndim)
     direction[:3, :3] = q_form[:3, :3] / spacing[:3]
 
-    ants_img = ants.from_numpy(
-        data = nib_image.get_data().astype( np.float ),
-        origin = origin.tolist(),
-        spacing = spacing.tolist(),
-        direction = direction )
-
+    ants_img = ants.from_numpy(data=nib_image.get_fdata(), origin=origin.tolist(), spacing=spacing.tolist(), direction=direction)
+
     return ants_img
+
+
+def get_ras_affine_from_ants(ants_img: ants.ANTsImage) -> np.ndarray:
+    """
+    Convert ANTs image affine to RAS coordinate system.
+
+    Parameters
+    ----------
+    ants_img : ants.ANTsImage
+        The ANTs image whose affine is to be converted.
+
+    Returns
+    -------
+    affine : np.ndarray
+        The affine matrix in RAS coordinates.
+    """
+    spacing = np.array(ants_img.spacing)
+    direction_lps = np.array(ants_img.direction)
+    origin_lps = np.array(ants_img.origin)
+    direction_length = direction_lps.shape[0] * direction_lps.shape[1]
+    if direction_length == 9:
+        rotation_lps = direction_lps.reshape(3, 3)
+    elif direction_length == 4:  # 2D case (1, W, H, 1)
+        rotation_lps_2d = direction_lps.reshape(2, 2)
+        rotation_lps = np.eye(3)
+        rotation_lps[:2, :2] = rotation_lps_2d
+        spacing = np.append(spacing, 1)
+        origin_lps = np.append(origin_lps, 0)
+    elif direction_length == 16:  # Fix potential bad NIfTI
+        rotation_lps = direction_lps.reshape(4, 4)[:3, :3]
+        spacing = spacing[:-1]
+        origin_lps = origin_lps[:-1]
+    else:
+        raise NotImplementedError(f"Unexpected direction length = {direction_length}.")
+
+    rotation_ras = np.dot(np.diag([-1, -1, 1]), rotation_lps)
+    rotation_ras_zoom = rotation_ras * spacing
+    translation_ras = np.dot(np.diag([-1, -1, 1]), origin_lps)
+
+    affine = np.eye(4)
+    affine[:3, :3] = rotation_ras_zoom
+    affine[:3, 3] = translation_ras
+
+    return affine
+
+
+def ants_to_nifti(img: ants.ANTsImage, header=None) -> "Nifti1Image":
+    """
+    Convert an ANTs image to a Nifti image.
+
+    Parameters
+    ----------
+    img : ants.ANTsImage
+        The ANTs image to be converted.
+    header : Nifti1Header, optional
+        Optional header to use for the Nifti image.
+
+    Returns
+    -------
+    img : Nifti1Image
+        The converted Nifti image.
+    """
+    from nibabel.nifti1 import Nifti1Image
+
+    affine = get_ras_affine_from_ants(img)
+    arr = img.numpy()
+
+    if header is not None:
+        header.set_data_dtype(arr.dtype)
+
+    return Nifti1Image(arr, affine, header)
+
+
+# Legacy names for backwards compatibility
+from_nibabel = nifti_to_ants
+to_nibabel = ants_to_nifti
+
+if __name__ == "__main__":
+    import nibabel as nib
+
+    fn = ants.get_ants_data("mni")
+    ants_img = ants.image_read(fn)
+    nii_mni: "Nifti1Image" = nib.load(fn)
+    ants_mni = to_nibabel(ants_img)
+    assert (ants_mni.get_qform() == nii_mni.get_qform()).all()
+    temp = ants.from_nibabel(nii_mni)
+    assert ants.image_physical_space_consistency(ants_img, temp)
+
+    fn = ants.get_data("ch2")
+    ants_mni = ants.image_read(fn)
+    nii_mni = nib.load(fn)
+    ants_mni = to_nibabel(ants_mni)
+    assert (ants_mni.get_qform() == nii_mni.get_qform()).all()