code: def test_register_series():
      fdata, fbval, fbvec = dpd.get_fnames("small_64D")
      img = nib.load(fdata)
      gtab = dpg.gradient_table(fbval, fbvec)
      ref_idx = np.where(gtab.b0s_mask)[0][0]
      xformed, affines = register_series(img, ref_idx)
      npt.assert_(np.all(affines[..., ref_idx] == np.eye(4)))
      npt.assert_(
          np.all(xformed[..., ref_idx] == img.get_fdata()[..., ref_idx])
      )
  
imports: import os.path
  import pytest
  from tempfile import TemporaryDirectory
  import numpy
  import numpy.testing
  import nibabel
  import dipy.data
  import dipy.core.gradients
  from dipy.align import syn_registration, register_series, register_dwi_series, center_of_mass, translation, rigid_isoscaling, rigid_scaling, rigid, affine, motion_correction, affine_registration, streamline_registration, write_mapping, read_mapping, register_dwi_to_template
  from dipy.align.imwarp import DiffeomorphicMap
  from dipy.tracking.utils import transform_tracking_output
  from dipy.io.streamline import save_trk
  from dipy.io.stateful_tractogram import StatefulTractogram, Space
  from dipy.io.gradients import read_bvals_bvecs
  from dipy.io.image import load_nifti
  from dipy.testing.decorators import set_random_number_generator
label_desc: Use image node_id 6 for calling a global function with example usage: test_register_series() without return types