Spin-spin relaxation or T2 Relaxation

Immediately after a 90 degree RF pulse, the individual magnetic moments are all located in the transverse plane ; more important. However , they are coherent ( pointig in the same direction at nearly the same frequency). In other words , all protons have the same rotating phase.
Because the FID (free induction decay) is the sum of voltage from many protons at each spatial location,the protons must precess at the sane frequency and with the same alignment so that their magnetization work together. As the time goes on ,following the RF excitation pulse the individual magnetic moments begin to become spread out in the transverse plane. One cause of this spread is due to small local variation in the otherwise static magnetic field resulting out of random interaction from the magnetic moments of nearby protons. It is the spreading out that causes the vector sum to decrease. With time ,therefore, the individual spins

Spin-lattice relaxation or T1 Relaxation

When a 90 degree RF pulse is applied, it rotates
net magnetization vector M into the x-y or transverse plane , entire vector is located in that plane and its value along the z-axis is zero. Following cessation of the RF pulse , the nuclei returns towards their equilibrium magnetization. M rotates back from the x-y plane to the z-axis and M returns to to the initial magnetization at some finite time latter. The process of returning towards the equilibrium magnetization costitutes T1 rlaxation or longitudinal relaxation. The process is cused by release of energy into the surrounding tissue ( i.e., into the molecular environment or lattice around the proton ). Hence this relaxation is also known as spin-lattice relaxation.