The detail of perivascular drainage system has been examined mainly by intracranial injection of various tracers, including 125I-labeled albumin (Bradbury et al., 1981; Szentistványi et al., 1984; Yamada et al., 1991), Indian ink (Zhang et al., 1992), and various fluorescent tracers (Carare et al., 2008). Recently, this drainage pathway was also confirmed by multi-photon imaging (Arbel-Ornath et al., 2013).
