Imaging Concepts

Abstract

T he first unit in this chapter (UNIT B4.1), addresses how echoes, including both spin and gradient echoes, are formed in an MR imaging experiment. After the application of an rf pulse, the longitudinal magnetization will be rotated from the direction parallel to the external magnetic field to the transverse plane. Because of the interaction among spins, as well as the potential field variation across the sample of interest, the spins will start to precess at different resonance frequencies, resulting in a signal reduction in MR images. This phenomenon is referred to as dephasing, one of the main topics addressed in this unit. Therefore, in order to obtain the maximal available MR signal, an echo will need to be formed so that all spins will again have the same phase. The basic concept of an echo is introduced and the characteristics as well as the pros and cons of the two different echoes, namely gradient and spin echoes, are also discussed.