INTRODUCTION

The previous chapters considered magnetic resonance imaging (MRI) as a sensitive technique for depicting human anatomy. The magnetic resonance (MR) signal is intrinsically sensitive to several properties of tissues, such as relaxation times and diffusion, and the flexibility of pulse sequence design makes possible a variety of imaging techniques. Within the last decade, the field of MRI has expanded to include studies of tissue function in addition to anatomy. The remaining chapters describe how subtle MR effects are exploited to measure different aspects of the perfusion state of tissue. Although the Blood Oxygenation Level Dependent (BOLD) effect is most often used in brain activation studies, a drawback of this technique is that it only provides information on the change in activity between one state and another. For example, measurements made while a subject alternates between a control state and a task state reveal regions of the brain showing a significant signal difference between the two states. But BOLD techniques provide no information on the resting or chronic perfusion state.

In the next three chapters, we describe two classes of MRI techniques that do provide measures of the resting perfusion state. The first class is based on the use of intravascular contrast agents that alter the magnetic susceptibility of blood, and so affect the MR signal. The second class of techniques is arterial spin labeling (ASL), in which arterial blood is magnetically tagged before it arrives in the tissue, and the amount delivered to the tissue is then measured.