Estimate gaba concentration based on equation adapted from Sanacora
        1999, p1045

        Ref: Sanacora, G., Mason, G. F., Rothman, D. L., Behar, K. L., Hyder,
        F., Petroff, O. A., ... & Krystal, J. H. (1999). Reduced cortical
        $\gamma$-aminobutyric acid levels in depressed patients determined by
        proton magnetic resonance spectroscopy. Archives of general psychiatry,
        56(11), 1043.

def est_gaba_conc(self):
        """
        Estimate gaba concentration based on equation adapted from Sanacora
        1999, p1045

        Ref: Sanacora, G., Mason, G. F., Rothman, D. L., Behar, K. L., Hyder,
        F., Petroff, O. A., ... & Krystal, J. H. (1999). Reduced cortical
        $\gamma$-aminobutyric acid levels in depressed patients determined by
        proton magnetic resonance spectroscopy. Archives of general psychiatry,
        56(11), 1043.

        """
        # need gaba_auc and creatine_auc
        if not hasattr(self, 'gaba_params'):
            self.fit_gaba()

        # estimate [GABA] according to equation9
        gaba_conc_est = self.gaba_auc / self.creatine_auc * 1.5 * 9.0
        
        self.gaba_conc_est = gaba_conc_est