docstr: Mean signal diffusitivity (MSD) calculated from the mean signal
  Diffusion Kurtosis Model.
  
  Returns
  -------
  msd : ndarray
      Calculated signal mean diffusitivity.
  
  References
  ----------
  .. [1] Henriques, R.N., 2018. Advanced Methods for Diffusion MRI Data
         Analysis and their Application to the Healthy Ageing Brain
         (Doctoral thesis). Downing College, University of Cambridge.
         https://doi.org/10.17863/CAM.29356
docstr_tok: ["Mean","signal","diffusitivity","(","MSD",")","calculated","from","the","mean","signal","Diffusion","Kurtosis","Model",".","Returns","--","--","--","-","msd",":","ndarray","Calculated","signal","mean","diffusitivity",".","References","--","--","--","--","--","..","[","1","]","Henriques",",","R.N.",",","2018",".","Advanced","Methods","for","Diffusion","MRI","Data","Analysis","and","their","Application","to","the","Healthy","Ageing","Brain","(","Doctoral","thesis",")",".","Downing","College",",","University","of","Cambridge",".","https",":","\/\/doi.org\/10.17863\/CAM.29356"]
code: def msd(self):
      r"""
      Mean signal diffusitivity (MSD) calculated from the mean signal
      Diffusion Kurtosis Model.
  
      Returns
      -------
      msd : ndarray
          Calculated signal mean diffusitivity.
  
      References
      ----------
      .. [1] Henriques, R.N., 2018. Advanced Methods for Diffusion MRI Data
             Analysis and their Application to the Healthy Ageing Brain
             (Doctoral thesis). Downing College, University of Cambridge.
             https://doi.org/10.17863/CAM.29356
      """
      return self.model_params[..., 0]
  
imports: import numpy
  import scipy.optimize
  from dipy.core.gradients import check_multi_b, unique_bvals_magnitude, round_bvals
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import MIN_POSITIVE_SIGNAL
  from dipy.core.ndindex import ndindex
  from dipy.core.onetime import auto_attr
label_desc: Use image node_id 4 for calling the MeanDiffusionKurtosisFit obj's underlying member method code with example usage: obj.msd() and returns: self