code: def perfusion_fraction(self):
      return self.model_params[..., 1]
  
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 4 for calling the IvimFit obj's underlying member method code with example usage: obj.perfusion_fraction() and returns: self