INTRODUCTION

Bolus tracking studies with intravascular contrast agents provide a robust measurement of blood volume, but as discussed in Chapter 14 a measurement of cerebral blood flow (CBF) is more difficult. The kinetic curve of an intravascular contrast agent is more sensitive to decreases in CBF than increases, making these techniques a useful tool for clinical studies of ischemia but less useful for measurements of CBF changes with activation in the healthy brain. In recent years, a different class of techniques for measuring local tissue perfusion with MRI has been developed based on arterial spin labeling (ASL) (Alsop and Detre, 1996; Detre et al., 1992; Edelman et al., 1994; Helpern et al., 1997; Kim, 1995; Williams et al., 1992; Wong, Buxton, and Frank, 1998a).

ASL techniques provide noninvasive images of local CBF with better spatial and temporal resolution than any other technique, including nuclear medicine methods. The development of ASL techniques is an active area of research, and although they are not yet widely available on standard MR imagers, ASL applications are steadily growing. The standard technique for mapping patterns of activation in the healthy brain is still Blood Oxygenation Level Dependent (BOLD) imaging, but questions remain about the accuracy of localization of BOLD changes and the quantitative interpretation of the magnitude of BOLD signal changes (see Part IIIB). ASL techniques have already become a standard tool for investigations of the mechanisms underlying the BOLD effect (Buxton, Wong, and Frank, 1998b; Hoge et al., 1999a, 1999b, 1999c; Kim and Ugurbil, 1997), and the applications of ASL to more routine activation studies are likely to continue to expand.