code: def f(t):
      """
      Helper function used to define a mapping time => xyz
      """
      x = np.linspace(-1, 1, len(t))
      y = np.linspace(-1, 1, len(t))
      z = np.linspace(-1, 1, len(t))
      return x, y, z
  
file_path: dipy/dipy/sims/tests/test_phantom.py
imports: import numpy
  from numpy.testing import assert_, assert_array_almost_equal
  from dipy.data import get_fnames
  from dipy.reconst.dti import TensorModel
  from dipy.sims.phantom import orbital_phantom
  from dipy.core.gradients import gradient_table
  from dipy.io.gradients import read_bvals_bvecs
  from dipy.testing.decorators import set_random_number_generator
label_desc: Use image node_id 1 for calling a global function with example usage: f(t) and returns: x,  y,  z