code: def _isotropic_DTD():
      """Return a distribution of six isotropic diffusion tensors with varying
      sizes."""
      evals = np.linspace(0.1, 3, 6)
      DTD = np.array([np.eye(3) * i for i in evals])
      return DTD
  
imports: import numpy
  import numpy.testing
  import dipy.reconst.qti
  from dipy.core.gradients import gradient_table
  from dipy.sims.voxel import vec2vec_rotmat
  from dipy.core.sphere import disperse_charges, HemiSphere
  from dipy.reconst.dti import fractional_anisotropy
  from dipy.testing.decorators import set_random_number_generator
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 9 for calling a global function with example usage: _isotropic_DTD() and returns: DTD