Added a volume computation function

Compute volume of mask image.
Equivalent to "fslstats /path/file.nii -V"

Author: JulianKlug

nibabel/funcs.py

@@ -215,3 +215,34 @@ def _aff_is_diag(aff):
     """ Utility function returning True if affine is nearly diagonal """
     rzs_aff = aff[:3, :3]
     return np.allclose(rzs_aff, np.diag(np.diag(rzs_aff)))
+
+
+def mask_volume(img):
+    """ Compute volume of mask image.
+    Equivalent to "fslstats /path/file.nii -V"
+
+    Parameters
+    ----------
+    img : ``SpatialImage``
+        All voxels of the mask should be of value 1, background should have value 0.
+
+    Returns
+    -------
+    mask_volume_mm3: float
+        Volume of mask expressed in mm3.
+
+    Examples
+    --------
+    >>> import nibabel as nf
+    >>> img = nf.load(path) # path is contains a path to an example nifti mask
+    >>> mask_volume(img)
+    50.3021
+    """
+    header = img.header
+    _, vx, vy, vz, _, _, _, _ = header['pixdim']
+    voxel_volume_mm3 = vx * vy * vz
+    mask = img.get_fdata()
+    mask_volume_vx = np.sum(mask)
+    mask_volume_mm3 = mask_volume_vx * voxel_volume_mm3
+
+    return mask_volume_mm3