Fit a Lorentzian function to the NAA peak at ~ 2 ppm.  Example of
        fitting inverted peak: Foerster et al. 2013, An imbalance between
        excitatory and inhibitory neurotransmitters in amyothrophic lateral
        sclerosis revealed by use of 3T proton MRS

def fit_naa(self, reject_outliers=3.0, fit_lb=1.8, fit_ub=2.4,
                 phase_correct=True):
        """
        Fit a Lorentzian function to the NAA peak at ~ 2 ppm.  Example of
        fitting inverted peak: Foerster et al. 2013, An imbalance between
        excitatory and inhibitory neurotransmitters in amyothrophic lateral
        sclerosis revealed by use of 3T proton MRS
        """
        model, signal, params = ana.fit_lorentzian(self.diff_spectra,
                                                   self.f_ppm,
                                                   lb=fit_lb,
                                                   ub=fit_ub)

        # Store the params:
        self.naa_model = model
        self.naa_signal = signal
        self.naa_params = params
        self.naa_idx = ut.make_idx(self.f_ppm, fit_lb, fit_ub)
        mean_params = stats.nanmean(params, 0)
        self.naa_auc = self._calc_auc(ut.lorentzian, params, self.naa_idx)