To analyze the T1-T2 measurement, consider a case with two spin systems labeled  and  . Assume that each spin system has unique relaxation rates in the longitudinal relaxation period, r  . Magnetization can exchange between the spin systems during the experiment. For the magnetization component that begins on spin system  and that is destined to end on spin system  , denoted  → , the behavior during the longitudinal relaxation time period as the magnetization decays towards its equilibrium value is described by