《Principles Of Magnetic Resonance Imaging》教学Chapter 5.pdfVIP

Chapter 5 Signal Localization Zeugmatog raphy P. C. Lauterbur Based on the discussion in Chapters 3 and 4, we now know how to activate an MR signal from an object. In fact, the experimental protocol is rather simple: we place the object in it main, uniform magnetic field and then excite it with another oscillating magnetic field at the resonance frequency. The signal thus generated, however, is a sum of local signals from all parts of the object. For a spatially homogeneous object, this signal is all that we need, since the local signals are the same regardless of their spatial origins. In practice, the objects we deal with are heterogeneous, and it becomes necessary to differentiate local signals from different parts of a given object. This chapter is concerned with spatial localization of these signals. There are basically two types of spatial localization method: selective excita- 1 tion (or reception ) and spatial encoding. Central to localization methods of both types is the use of a gradient field. Modern MRI systems provide three orthogo- nal gradients whose shapes and forms can be adjusted independently. To facilitate the understanding of the role of RF pulses and gradient fields in a general imaging scheme, this chapter first describes the basic concepts of slice-selective excitation and spatial information encoding, and then goes on to discuss multidimensional imaging using a popular mathematical formalism known as k-space. Starting in this chapter and continuing in the rest of the book, we will make use of a graphic tool called pulse sequence diagram, which details the ti