Arterial Spin Labeling Measurements of Cerebral Blood Flow: A Review Emphasizing Pulsed versus Continuous Approaches Afonso C. Silva, Bojana Stefanovic and Fernando F. Paiva Cerebral Microcirculation Unit, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda MD 20892-1065 USA ABSTRACT Arterial spin labeling (ASL) refers to the use of endogenous arterial water as the perfusion tracer. To differentiate the tracer from the background tissue, the net magnetization of arterial water flowing proximally to the brain is modified with respect to the net magnetization of brain water. This review is focused on describing two basic approaches to obtain images of cerebral blood flow (CBF): pulsed ASL (PASL) and continuous ASL (CASL). Advantages and disadvantages to either approach are listed, along with potential sources of artifacts and pitfalls. INTRODUCTION Cerebral blood flow (CBF) can be measured by following the kinetics of a diffusible tracer as it perfuses the brain. In methods such as positron emission tomography (PET), single photon emission computerized tomography (SPECT) or quantitative autoradiography (QAR), the tracer is exogenously administered and is imaged against a quiet background, which has the advantages of high sensitivity and robustness of quantification, but also the disadvantages of being invasive and of providing generally poor spatial resolution due to the small amount of tracer used relative to the size of the brain. The same principles of tracer kinetics have laid the foundation for the use of MR-detectable, exogenously administered, diffusible tracers, such as 2 H 2 O, CH 19 F 3 , H 2 17 O or 129 Xe. However, the advantages of using an endogenous perfusion tracer, namely of proportioning a non-invasive method that allows repeated measurements of CBF to be performed indefinitely, motivated the development of arterial spin labeling techniques. Arterial spin labeling refers to the use of endogenous arterial water as the perfusion tracer. Unlike other methods for measuring CBF, in ASL the tracer is imaged against the brain water background. Therefore, the general principle behind the ASL techniques is to differentiate the net magnetization of arterial water flowing proximally to