docs: def pick_t1w(mri):
      ''' Pick the MR T1w from the dictionary for MR->PET registration.
      '''
  
      if isinstance(mri, dict):
          # check if NIfTI file is given
          if 'T1N4' in mri and os.path.isfile(mri['T1N4']):
              ft1w = mri['T1N4']
          # or another bias corrected
          elif 'T1bc' in mri and os.path.isfile(mri['T1bc']):
              ft1w = mri['T1bc']
          elif 'T1nii' in mri and os.path.isfile(mri['T1nii']):
              ft1w = mri['T1nii']
          elif 'T1DCM' in mri and os.path.exists(mri['MRT1W']):
              # create file name for the converted NIfTI image
              fnii = 'converted'
              call( [rs.DCM2NIIX, '-f', fnii, mri['T1nii'] ] )
              ft1nii = glob.glob( os.path.join(mri['T1nii'], '*converted*.nii*') )
              ft1w = ft1nii[0]
          else:
              print 'e> disaster: could not find a T1w image!'
              return None
              
      else:
          ('e> no correct input found for the T1w image')
          return None
  
      return ft1w