docstr: Total isotropic variance.
  
  Returns
  -------
  v_iso : numpy.ndarray
  
  Notes
  -----
  Total isotropic variance is calculated as
  
      .. math::
  
          V_      ext{iso} = \mathbb{C} : \mathbb{E}_     ext{iso}
docstr_tok: ["Total","isotropic","variance",".","Returns","--","--","--","-","v_iso",":","numpy.ndarray","Notes","--","--","-","Total","isotropic","variance","is","calculated","as","..","math",":",":","V_","ext","{","iso","}","=","\\mathbb","{","C","}",":","\\mathbb","{","E","}","_","ext","{","iso","}"]
code: def v_iso(self):
      """Total isotropic variance.
  
      Returns
      -------
      v_iso : numpy.ndarray
  
      Notes
      -----
      Total isotropic variance is calculated as
  
          .. math::
  
              V_\text{iso} = \mathbb{C} : \mathbb{E}_\text{iso}
      """
      v_iso = np.matmul(
          self.params[..., np.newaxis, 7::], from_6x6_to_21x1(E_iso)
      )[..., 0, 0]
      return v_iso
  
imports: from warnings import warn
  import numpy
  from dipy.reconst.base import ReconstModel
  from dipy.reconst.dti import auto_attr
  from dipy.utils.optpkg import optional_package
label_desc: Use image node_id 7 for calling the QtiFit obj's underlying member method code with example usage: obj.v_iso() and returns: v_iso