T1 relaxation process. Diagram showing the process of T1 relaxation after a 90° rf pulse is applied at equilibrium. The z component of the net magnetisation, Mz is reduced to zero, but then recovers gradually back to its equilibrium value if no further rf pulses are applied. The recovery of Mz is an exponential process with a time constant T1. This is the time at which the magnetization has recovered to 63% of its value at equilibrium.