code: def ivim_prediction(params, gtab):
      """The Intravoxel incoherent motion (IVIM) model function.
  
      Parameters
      ----------
      params : array
          An array of IVIM parameters - [S0, f, D_star, D].
  
      gtab : GradientTable class instance
          Gradient directions and bvalues.
  
      S0 : float, optional
          This has been added just for consistency with the existing
          API. Unlike other models, IVIM predicts S0 and this is over written
          by the S0 value in params.
  
      Returns
      -------
      S : array
          An array containing the IVIM signal estimated using given parameters.
  
      """
      b = gtab.bvals
      S0, f, D_star, D = params
  
      S = S0 * (f * np.exp(-b * D_star) + (1 - f) * np.exp(-b * D))
  
      return S
  
imports: import warnings
  import numpy
  from scipy.optimize import least_squares, differential_evolution
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.multi_voxel import multi_voxel_fit
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 1 for calling a global function with example usage: ivim_prediction(params, gtab) and returns: S