INTRODUCTION

There are many techniques for producing a magnetic resonance (MR) image, and new ones are continuously being developed as the technology improves and the range of applications grows. The variety of techniques available is in part an illustration of the intrinsic flexibility of magnetic resonance imaging (MRI). The MR signal can be manipulated in many ways: radio frequency (RF) pulses as excitation pulses tip magnetization from the longitudinal axis to the transverse plane to generate a detectable MR signal and as refocusing pulses create echoes of previous signals; gradient pulses eliminate unwanted signals when used as spoilers and, in their most important role, serve to encode information about the spatial distribution of the signal for imaging. By manipulating the RF and gradient pulses, many pulse sequences can be constructed.

The large variety of available pulse sequences for imaging also reflects the variety of goals of imaging in different applications. In most clinical imaging applications, the goal is to be able to identify pathological anatomy, and this requires a combination of sufficient spatial resolution to resolve small structures and sufficient signal contrast between pathological and healthy tissue to make the identification. Because the MR signal depends on several properties of the tissue, and the influence of these properties can be manipulated by adjusting the timing parameters of the pulse sequence, MR images can be produced with strong signal contrast between healthy and diseased tissue. For example, in Chapter 4, MRI was introduced with an illustration (Figure 4.1) of the range of tissue contrast that results simply from manipulating the repetition time (TR) and the echo time (TE) of a spin-echo pulse sequence.