Following an RF-pulse, both T1 and T2 relaxation occurs. If the repetition time (TR) is on the same time-scale or shorter than the T1-relaxation time, the magnitude of the transversal signal will depend on the T1-relaxation rate, as the magnetization will not have time to return to its equilibrium before the play out of the next RF pulse. Hence regions with long T1-relaxation times will produce a lower signal.