Arterial input function: A friend or a foe?

The arterial input function (AIF) is essential for perfusion quantification, according to well-established tracer kinetics theory. Accurate measurement of the AIF can, however, be hampered by several factors. In this chapter, the challenges that exist when measuring the AIF are discussed for three different perfusion MRI methods: dynamic susceptibility contrast (DSC) MRI, dynamic contrast-enhanced (DCE) MRI, and arterial spin labeling (ASL). Strategies on how to circumvent the methodological pitfalls are also described. This chapter focuses on AIF measurements in the brain, while AIF selection specifics for other parts of the body are described in the nonneuro applications chapters.