By absorbing energy from the RF pulse, the net magnetization rotated through 90º into the X-Y plane, thus the protons are lifted to a higher energy state but they rather prefer to be in a low energy state. Now something happens that is referred to as Relaxation. The relaxation process can be divided into two parts: T1 and T2 relaxation.
T1 relaxation: As the excitation RF pulse stops, the protons tend to return to their lower energy state (and the net magnetization rotates back to align itself along the z- axis) by releasing the absorbed energy in the form of RF wavesto the surrounding tissues. Protons that are tightly bound (for example, in fat tissue) will release their energy much quicker than tissues with loosely bound protons (water); due to the variation, we can differentiate tissues. T1 is defined as the time it takes for longitudinal magnetization to reach 63% of the original magnetization.
T1 is defined as reaching 63 % of the original magnetization
Protons returning to a lower energy state
T2 relaxation: As discussed in the excitation that RF pulse rotates the net magnetization in x-y plane and the protons have been excited to a higher energy state and also starts to spin in-phase. The protons that were originally in phase due to the RF pulse now begin to get out of phase. This process of getting from a total in-phase situation to a total out-of-phase situation is called T2 relaxation. T2 is defined as the time it takes for the spins to de-phase to 37% of the original value.
T2 is defined as the time it takes for the spins to de- phase to 37 % of original value
Protons that are excited spin in-phase
Thus we conclude that T1 and T2 relaxation happen simultaneously as the RF pulse stops and T2 is a much quicker process than T1.When both relaxation processes are over the net magnetization vector aligns itself with the main magnetic field (Bø) again.