INTRODUCTION

Chapters 7–9 described the enormous flexibility of the MR signal, how it depends on several tissue properties such as relaxation times and diffusion, and how it can be manipulated to emphasize these different properties by adjusting pulse sequence parameters. The sensitivity to the relaxation times is controlled by adjusting timing parameters such as the repetition time TR or the echo time TE, and the magnetic resonance (MR) signal becomes sensitive to the self-diffusion of water by adding additional field gradient pulses. Chapter 10 described how images are made by using gradient fields and exploiting the fact that the nuclear magnetic resonance (NMR) precession frequency is directly proportional to the local magnetic field. The central idea of magnetic resonance imaging (MRI) is that the application of field gradients makes the net signal over time trace out a trajectory in k-space, the spatial Fourier transform (FT) of the distribution of the MR signal. The image is reconstructed by applying the FT to the measured data. Because the gradients are under very flexible control, many trajectories through k-space are possible.

In this chapter we bring together these ideas from the previous chapters to describe several techniques for imaging in terms of how they produce useful contrast and how they scan through k-space. This review is selective, focusing on techniques that illustrate basic concepts of imaging or that are commonly used for functional magnetic resonance imaging (fMRI). Most fMRI work is done with single-shot echo planar imaging (EPI), so this technique is presented in more detail.