Relaxation Times, Gradient Echoes, and Spin Echoes

Abstract

Immediately after the rf pulse, the spin system starts to return back to its original state, that is, equilibrium. This process is known as relaxation. In fact, there are two distinct relaxation processes that relate to the two components of the Net Magnetization, the longitudinal (z) and transverse (xy) components. The first longitudinal relaxation process, commonly referred to as T1 relaxation, is responsible for the regrowth of the z component along the longitudinal (z) axis to its original value at equilibrium. This is explained in Sect.5.1.1. The second transverse relaxation process is responsible for the decay of the xy component as it rotates about the z axis and hence the observed decay of the MR signal. There are two types of transverse relaxation, known as T2 relaxation and T2* relaxation and these are explained in Sects. 5.1.2–5.1.4. Longitudinal and transverse relaxation both occur at the same time; however, transverse relaxation is typically a much faster process in human tissue, that is, the signal decays away long before the spin system returns to equilibrium.