code: def est_gaba_conc(self):
          
          # 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
docs: 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.