The transverse, or spin–spin, relaxation process determines how long an MR signal persists after the application of an RF pulse. It is governed by the interaction of excited spins with their local environment. However, because an MR voxel is large compared to the biological structures within it, that environment is highly complicated. Hence, quantifying T2, the fundamental parameter of transverse relaxation, can give insight into the microstructure contained within a single voxel. This has been widely applied in brain imaging to analyse myelin, a complicated microscopic structure that is key to a healthy nervous system.

This chapter introduces the physical and biological underpinnings of T2 relaxation, details the main sequences (multi-echo T2 and DESPOT2) and analysis methods (both single-component and multi-component), and gives example applications. The effects of B0 and B1 inhomogeneity are detailed and methods to mitigate their impact given. Other confounding issues such as magnetisation transfer effects are discussed, and the different quantification methods are compared to each other.