\zeta = 2 \Delta \omega \, (R_2^A - R_2^B +k_{\textrm{AB}} - k_{\textrm{BA}} )
= 2 \Delta \omega \, (R_2^A - R_2^B + p_B k_{\textrm{EX}} - p_A k_{\textrm{EX}} ) = 2 \Delta \omega \alpha_- \\
\Psi = (R_2^A - R_2^B - p_A + p_B k_{\textrm{EX}} + p_B - p_A k_{\textrm{EX}})^2 - \Delta \omega^2 + 4 p_A p_B k_{\textrm{ex}}^2 \\
\eta_+ = \frac{1}{2\sqrt{2} \, \nu_{\textrm{cpmg}}}\sqrt{+\Psi + \sqrt{\Psi^2 + \zeta^2}} \\
\eta_- = \frac{1}{2\sqrt{2} \, \nu_{\textrm{cpmg}}}\sqrt{-\Psi + \sqrt{\Psi^2 + \zeta^2}} \\